Population growth could grind to a halt by 2050, before decreasing to as little as 6 billion humans on Earth in 2100, a new analysis of birth trends has revealed.

The study, commissioned by the nonprofit organization The Club of Rome, predicts that if current trends continue, the world's population, which is currently 7.96 billion, will peak at 8.6 billion in the middle of the century before declining by nearly 2 billion before the century's end. 

The forecast is both good and bad news for humanity: A plummeting human population will slightly alleviate Earth's environmental problems, but it is far from being the most important factor in solving them. 

And falling populations will make humanity older as a whole and lower the proportion of working-age people, placing an even greater burden on the young to finance health care and pensions. The researchers — members of the Earth4All collective, which is made up of environmental scientists and economists — published their findings March 27 in a working paper.

Related: Why global population growth will grind to a halt by 2100

"We know rapid economic development in low-income countries has a huge impact on fertility rates," Per Espen Stoknes, director of the Centre for Sustainability at Norwegian Business School and the project lead of Earth4All, said in a statement. "Fertility rates fall as girls get access to education and women are economically empowered and have access to better healthcare." 

The study is a follow-up to The Club of Rome's 1972 Limits to Growth study, which warned the world of an imminent "population bomb." The new result diverges from other recent population forecasts. For instance, in 2022, the United Nations estimated that the world population would reach 9.7 billion by 2050 and rise to 10.4 billion by 2100. U.N. estimates from a decade ago suggested the population would reach 11 billion.

Other models forecast population growth based on factors that affect women's social independence and bodily autonomy, such as access to education and contraception. Earth4All's model is slightly more complex, integrating variables connected to the environment and the economy. These include energy abundance, inequality, food production, income levels and the impacts of future global warming.

The model predicted two possible outcomes for the future human population. The first, "business-as-usual" case — in which governments continue on their current trajectories of inaction, creating ecologically fragile communities vulnerable to regional collapses — would see populations rise to 9 billion people by 2050 and decline to 7.3 billion in 2100. The second, more optimistic scenario — in which governments invest in education, improved equality and green transitions — would result in 8.5 billion people on the planet by the century's halfway point and 6 billion by 2100.

The team also investigated the connection between population sizes and the planet's ability to sustain human populations. They found that, contrary to popular Malthusian narratives, population size is not the key factor driving climate change. Instead, they pinned the blame on high levels of consumption by the world's richest individuals, which they say must be reduced.

"Humanity's main problem is luxury carbon and biosphere consumption, not population," Jorgen Randers, one of the modelers at the Norwegian School of Business and a member of Earth4All, said in the statement. "The places where population is rising fastest have extremely small environmental footprints per person compared with the places that reached peak population many decades ago."

Global population growth will nearly grind to a stop by the end of the century, a new analysis by the Pew Research Center suggests.

Right now, the world's population is over 7.7 billion people, and it has been growing between 1% and 2% every year since 1950, according to the Pew Research Center. By 2100, the center projects the population will reach around 10.9 billion people and grow by less than 0.1% a year, the center wrote.

This is mostly due to a decreasing number of children born worldwide, the analysis said, based on data from the United Nation's report "World Population Prospects 2019."

The U.N.'s report found that global fertility rates will be less than the "replacement fertility rate," or the number of births per woman that would keep the population the same size, replacing people as they die. The current replacement fertility rate is 2.1 births per woman, which is less than the current global fertility rate of 2.5 births per woman. By 2100, the global fertility rate is expected to dip to 1.9 births per woman. [5 Ways the World Will Change Radically This Century]

What's more, the U.N. report found that the global median age to which people live will increase from 31 to 42 by 2100. Between 2020 and 2100, people 80 and over will increase from the current 146 million to 881 million. Latin America and the Caribbean will have the oldest people in the world by 2100.

Only Africa is expected to have a strong population growth by the end of the century, increasing from 1.3 billion people in 2020 to 4.3 billion people in 2100. Meanwhile, Europe's population is expected to peak in 2021, and both Europe and Latin America will be declining in population by 2100. Asia will increase in population by 2055, then decline and North America's population will continue to increase, mostly because of migration to the area, according to the U.N. report.

• 10 Species Our Population Explosion Will Likely Kill Off
• 7 Population Milestones for 7 Billion People
• Countdown to 7 Billion: Should World Adopt 'One-Child' Policy?

Editor's note: This article was updated to correct an error. The U.N. report found the global median age, not the average age, that people will live to will increase from 31 to 42 by 2100.

Originally published on Live Science.

As Pope Francis addresses a joint meeting of Congress today (Sept. 24), scientists are praising his encyclical on climate change — with a few caveats about population control.

A series of editorials published today in the journal Nature Climate Change applaud the pope's in-depth missive for his calls for collective action on warming temperatures, which are driven by fossil-fuel combustion. The encyclical was a "decisive democratic act," wrote Anabela Carvalho, a communication sciences professor at the University of Minho in Portugal. It was "passionate and compelling," added Stanford University ecologist Paul Ehrlich and University of California, Berkeley environmental scientist John Harte in their co-authored editorial.

But the researchers warned that change would be difficult in the face of an entrenched status quo. And it might not be possible, some said, to save the world without contraception, which the Catholic Church opposes. [Catholics in America: Views on Contraception & Other Social Issues (Infographic)]

"Pope Francis needs to heed his own comments on the church's 'obsession' with contraception and abortion, and assume a leadership position in support of women’s rights and family planning," Ehrlich and Harte wrote.

The population question

Contraception is linked to climate change, because peoplethrough their activities and patterns of consumption, produce greenhouse gases. A 2009 cost-benefit analysis funded by the U.K. charity Optimum Population Trust makes this connection clear. That research estimated the cost of providing contraception to women who want to use birth control but don't have the opportunity. Meeting this need between 2010 and 2050 would decrease projected carbon dioxide emissions by 34 gigatonnes (34 billion tonnes), the study found. This translated to a cost of $6.46 per tonne of emissions reduction. For comparison, the use of low-carbon technologies such as wind power and carbon capture for coal plants would cost $32 per tonne, according to the same study. [What 11 Billion People Mean for Climate Change]

Pope Francis, a chemist by training, drew heavily from the realm of science in his encyclical, which is a letter to the bishops of the church. He condemned the doubting of climate change, cited concerns about diminishing biodiversity and even called for more green space in cities, which has been shown to boost health and happiness.

The pope also signaled, however, that he would not be reversing Catholic teachings on contraception, arguing against population control as a major solution for climate and environmental woes.

"Instead of resolving the problems of the poor and thinking of how the world can be different, some can only propose a reduction in the birth rate," Francis wrote. "At times, developing countries face forms of international pressure which make economic assistance contingent on certain policies of 'reproductive health.'"

Quoting Pope John Paul II, Francis went on to say that demographic growth is compatible with development, calling for an equitable distribution of resources rather than a focus on birth rates. [11 Billion People: 7 Ways Population Impacts the Planet]

Here, the pope and many scientists part ways. Ehrlich, in particular, has been warning of the dangers of overpopulation since his 1968 book "The Population Bomb." Though dire effects like global famine have not come to pass thanks to advances in agriculture, the growing population has produced problems, Harte told Live Science.

"The world hasn't collapsed, but we have on the order of a billion people who are underfed and undernourished in the world, and that's a serious problem," he said.

The United Nations projected in 2013 that the world population will reach 9.6 billion by 2015. While fertility rates have fallen in most developed countries, fertility has remained higher than previously estimated in sub-Saharan Africa.

A place for family planning

The pope said that if developed countries consume less, and resources are shared more, population growth can continue. However, most environmental scientists or development experts don't agree with this view. The United Nations puts family planning front and center in its quest for sustainable development.

"Policies to increase the availability of safe and effective contraceptives and accessibility to family planning programs and reproductive health care have been instrumental in facilitating reductions in fertility," according to the agency's 2014 World Population Report. A 2005 report by the World Health Organization (WHO) found that contraceptive use went up from about 10 percent of women in the early 1960s to nearly 60 percent by 2000. In the same time period, the fertility rate dropped from 4.97 children per women to 2.69 children per women.

In 2014, the U.N. reported, 60 percent or more of married or coupled women used contraceptives across the globe, with the exception of fast-growing Africa.

There, contraceptive usage by coupled women hovers just over 30 percent. The continent also has a high level of "unmet need," defined as the gap between those who use contraception and those who say they want to, but don't. In Africa, the percentage of women reporting unmet need for contraceptives clusters around 30 percent or more. In Europe and North American countries, by comparison, it's more like 10 percent.

Other population influencers

Although contraception plays a large role in birth rates, birth control availability isn't the only factor that determines family size. Culture, religion and the economy all play roles in whether people use birth control, even if it's available.

Researchers found that in Bangladesh, for example, economic factors, particularly education of women and a shift to urban dwelling, were the strongest drivers of family size changes. Health care access and infant mortality rates had effects, but they were smaller, the researchers reported in their 2013 study. Culture played a small role and influenced contraception access, the study found. The take-home message, the researchers told Live Science at the time, was that educating women is the best way to decrease population growth.

Some experts have argued that, in the grand scheme of things, the overconsumption of developed nations matters more than the population growth in poor countries. Most of the fastest-growing nations have low per capita greenhouse gas emissions, David Satterthwaite, who researches climate change adaptation at the International Institute for Environment and Development in the United Kingdom, told Live Science in 2011. In that sense, Pope Francis' call for more equitable distribution of resources makes sense: If developed countries cut back on emissions enough, they could make up for the lower-emitting but fast-growing underdeveloped nations.

But sub-Saharan Africa wants — and deserves — to develop, too, Harte said. How the continent ends up affecting climate change depends on whether those nations choose sustainable, green energy or "make the same mistakes we did in rich countries and burn coal and oil," he said.

Sustainable and equitable development is less likely to occur against a backdrop of population growth, Harte said.

"All of the evidence at hand is that in parts of the world that are growing the most rapidly … the plight of the people is worse, and the amount of time and energy they have to deal with issues like more fair and just government is reduced," he said.

Thus, Harte said, it's unrealistic to call for fair distribution without also working to slow population growth.

"We feel very positive about what the pope has done, but we could have given it three cheers instead of two if he had only dealt with women's rights and population," he said. "We understand that this goes against a lot of church dogma, yet I think it's incumbent on us to call the church to task for not revising and updating its ideas about what constitutes personal freedom for women and families."

Follow Stephanie Pappas on Twitterand Google+. Follow us @livescience, Facebook& Google+. Original article on Live Science.

If you're in perfect health, you're in the minority: Less than 5 percent of people worldwide had no health problems in 2013, a new study finds.

Researchers analyzed information on about 300 diseases and conditions — everything from acne and PMS to chronic conditions such as heart disease and diabetes — and more than 2,300 disease-related consequences, in people in 188 countries.

Overall, just 4.3 percent of people had no health problems, the researchers found. The likelihood of having any disease or condition increased with age: In developed countries, about 64 percent of kids under age 5 had a health problem in 2013, compared with 99.97 percent of adults ages 80 and older.

Often, people had more than one health condition, and  about 2.3 billion people worldwide had more than five health ailments. Among people ages 80 and over in developed countries, about 65 percent had five to nine health ailments, and a quarter had 10 or more. [8 Tips for Healthy Aging]

Some conditions affected more than 10 percent of the world's population, including cavities in permanent teeth, tension headaches, iron-deficiency anemia, hearing loss and genital herpes. In 2013, there were also more than 18 billion cases of upper respiratory infections (a category that includes sinus infections and the common cold), and more than 2 billion cases of diarrhea diseases.

The study also found that, over the last two decades, the total number of years that people in the world are living in less than perfect health increased. This increase was mainly due to the population growing larger and getting older, the researchers said.

Some of the leading causes of disability worldwide were low back pain, major depression, neck pain, migraine, diabetes and anxiety disorders. People are also living longer with conditions such diabetes, Alzheimer's disease and arthritis, the researchers said.

The researchers noted that although public health efforts have focused on reducing deaths, it is important to pay attention to the increasing health toll from disability.

"Large, preventable causes of health loss, particularly serious musculoskeletal disorders and mental and behavioral disorders, have not received the attention that they deserve," study co-authorTheo Vos, a professor of global health at the University of Washington's Institute of Health Metrics and Evaluation in Seattle, said in a statement. "Addressing these issues will require a shift in health priorities around the world, not just to keep people alive into old age, but also to keep them healthy."

The study was funded by the Bill & Melinda Gates Foundation.

Follow Rachael Rettner @RachaelRettner. Follow Live Science @livescience, Facebook & Google+. Original article on Live Science.

The U.S. birth rate reached an all-time low in 2013, as the number of babies born in the country declined for the sixth straight year since the peak in 2007, a new report finds.

The country's birth rate dipped to 62.5 births per 1,000 women between ages 15 and 44, according to the report from the Centers for Disease Control and Prevention (CDC). That is 10 percent lower than the birth rate in 2007, which was 69.3 per 1,000 women, and a record low since the government started tracking birth rates in 1909, when birth rate was 126.8.

In 2013, there were 3.93 million babies born in the U.S., down less than 1 percent from 2012, and down 9 percent from 2007, when a record-breaking 4.32 million babies were born in the U.S. [10 Scientific Tips for Raising Happy Kids]

But trends in the birth rate varied among age groups. Among women under age 30, childbearing is on the decline, whereas it continues to rise among women older than 30, in line with the general trend over the last three decades. In fact, 2013 birth rates for women ages 35 and over was "at the highest levels seen in approximately 50 years," the researchers wrote in their report.

In 2013, the birth rate for women ages 35-39 reached 49.3 births per 1,000, and for women ages 30–34, the rate reached 98 births per 1,000, both small increases from the previous year.

The rate for women ages 40 to 44 was unchanged from the previous year, at 10.4 births per 1,000, whereas for women ages 45 to 49 the birth rate increased from 0.7 to 0.8 per 1,000.

In contrast, among teenagers the birth rate fell 10 percent from 2012, dropping to 26.5 births per 1,000.

Rates also declined by 3 percent for women ages 20 to 24, continuing the general decline of birth rates for women under age 25 over the last two decades, the researchers said.

The report is published today (Dec. 4) by the CDC's National Center for Health Statistics.

Email Bahar Gholipour. Follow Live Science @livescience, Facebook & Google+. Originally published on Live Science.

Editor's note: By the end of this century, Earth may be home to 11 billion people, the United Nations has estimated, earlier than previously expected. As part of a week-long series, LiveScience is exploring what reaching this population milestone might mean for our planet, from our ability to feed that many people to our impact on the other species that call Earth home to our efforts to land on other planets. Check back here each day for the next installment.

Robert Zubrin wants humanity to put down roots on Mars, and he thinks he knows how to make it happen at a reasonable price.

The key is to travel light and live off the land, accessing the plentiful resources available in the Red Planet's dirt and air, says Zubrin, president and founder of the nonprofit Mars Society, which advocates manned exploration of the Red Planet. For example, pioneers could generate oxygen and rocket fuel by pulling feedstock out of the thin Martian atmosphere, and they could get all the water they need from the soil under their boots.

That Martian dirt is also rich in iron oxide and silicon oxide, so explorers could make their own iron, steel and glass. They could manufacture plastics, too, using soil water and carbon dioxide, which dominates the Red Planet's atmosphere.

Becoming as self-sufficent as possible is the ultimate goal, so that the continued existence of the outpost does not rely on frequent (and expensive) resupply trips from Earth.

Whenever the first manned outpost springs up on the Red Planet, its solitary existence on a previously virgin world will inspire a lot of soul-searching here on our increasingly crowded Earth.

And in an interesting twist, the more crowded our planet gets, the more likely we may be to start setting up shop on Mars and other locations beyond Earth.

The number of people who call Earth home is slated to jump from the current 7.1 billion to potentially 11 billion by the end of the century, according to a recent United Nations report that used a more sophisticated statistical analysis than previous reports and that upped the expected population rise by the end of the century. This boom could accelerate the pace of climate change, strain the availability of key resources such as fresh water and threaten biodiversity around the globe, experts say.

But the ongoing population rise may have an impact beyond Earth as well, making humanity more able and more willing to leave our home planet and begin settling the solar system, according to Zubrin.

"Technology is going to advance, the wealth of society is going to increase and the number of people is going to increase, which means the number of outliers is going to increase — the number of people who think differently," Zubrin told LiveScience. [What 11 Billion People Means for the Planet]

"Most billionaires are going to continue to donate to the opera," he added. "But since there are going to be more of them, there's a greater probability that some of them are going to say, 'What I want to do with my life is open space to humanity.'"

Increased wealth and knowledge

The field of private spaceflight is beginning to take off now, thanks in no small part to the efforts of a few extremely wealthy individuals with big space dreams — billionaires like Elon Musk (founder of SpaceX), Sir Richard Branson (Virgin Galactic), Jeff Bezos (Blue Origin) and Paul Allen (Stratolaunch Systems).

SpaceX has already made history by becoming the first private company to make cargo deliveries to the International Space Station, and Musk has bigger dreams for the firm. He has said he founded SpaceX primarily to help humanity become a multiplanet species, and late last year the entrepeneur laid out his vision for establishing a Mars colony that could support perhaps 80,000 people. [Red Planet or Bust: 5 Manned Mars Mission Ideas]

The burgeoning global population should help humanity achieve such ambitious goals, Zubrin said. For one thing, there will be more rich space enthusiasts like Musk willing to help initiate or bankroll manned exploration efforts — perhaps many more by 2100, if per capita income continues to rise as population increases, as occurred throughout the 20th century.

A higher population will also help humanity acquire the knowledge and skills needed to explore and settle a world such as Mars, Zubrin added.

"Inventions are cumulative," he said. "The technological base is going to be expanding at an accelerating rate, because there are going to be more people contributing inventions."

But such reasoning only applies up to a point, before Earth is too jam-packed with people, other experts say.

"Getting into space means you have to have a basic infrastructure, and if the Earth's population gets so enormous that we're all reduced to poverty, we won't be able to afford to move into space," award-winning sci-fi author Ben Bova told LiveScience. "There's an old saying in Washington: 'I'm so busy doing what's urgent, I can't do what's important.'"

The riches of space

If the march toward 11 billion does indeed help push humanity off the planet, then population growth may have some self-correcting potential. Expanding our species' footprint out into the solar system could help deal with the boom and perhaps even slow it down, Bova said.

"As population continues to grow, we are going to need more and more natural resources to support that population," said Bova, whose "Grand Tour" novel series explores this issue in depth. "And we have seen in space natural resources in larger amounts than the whole Earth can provide."

One of those resources is energy, Bova said. By looking up and out, humanity could generate huge amounts of power while simultaneously reducing or even ending its dependence on fossil fuels, which contribute to climate change by spewing heat-trapping carbon dioxide into Earth's atmosphere.

"The Earth intercepts less than 1 percent of the amount of energy that the sun is beaming out," Bova said. "We can build solar power satellites and greatly increase the amount of energy available to the human race."

The moon and asteroids are also packed with riches. For example, some space rocks contain large amounts of platinum-group metals — platinum, ruthenium, rhodium, palladium, osmium and iridium — which are key components of virtually all electronic devices.

The platinum-group elements represent "one opportunity to get a material in space that could be of great benefit back here on Earth," said Chris Lewicki, president of the billionaire-backed asteroid-mining firm Planetary Resources. [How Asteroid Mining Could Work (Infographic)]

The enormous wealth created by extracting such resources could help put the brakes on our species' exponential growth, Bova said.

"My reading of history is that the best way to reduce population growth is to make people richer; rich people don't have as many children as poor people," he said. (Indeed, social scientists have found that fecundity tends to drop as per capita income rises, both within a society and across different countries.)

A relatively small number of people working in space, Bova added, "can return the resources needed to support a huge population and make it wealthy enough so that the population growth slows down."

Changing hearts and minds

Bova doesn't see the colonization of other worlds such as Mars as a viable safety valve for an overcrowded Earth, though, saying that it would be impractical to ship enough people off the planet to make an appreciable dent in the global population anytime soon.

However, that doesn't mean that off-world settlement can't lighten humanity's footprint on Earth, others say.

"Colonizing Mars will have a positive effect on achieving a sustainable human population on Earth because of the impact that it will have on Earth, not because of the number of Mars migrants," said Bas Lansdorp, CEO and co-founder of the Netherlands-based nonprofit Mars One, which aims to land four people on the Red Planet in 2023 as the vanguard of a permanent human settlement.

This plan is quite bold, as NASA — the world's premier space agency — is hoping to get people to the vicinity of Mars by the mid-2030s.

"Having humanity experience setting foot on another planet will make people aware that the Earth is our spaceship and that we need to treat it well," Lansdorp told LiveScience via email. "This happened because of the moon landings too, but the effect will be larger when it concerns colonization instead of a visit, and because of the larger reach of video images in 2023 as compared to 1969."

The rise of the space tourism industry — which will likely be aided by population growth, since more people should be able to afford flights to space — could have similar effects, helping more and more people see Earth as it really is: a tiny outpost of life, alone and adrift in the vast blackness of space, experts say.

"Those of us who have had the luxury of having had that view — I think you become a better steward of your own planet," said Commercial Spaceflight Federation president Michael Lopez-Alegria, a former NASA astronaut with four space missions under his belt.

"So the more people who are part of that club, so to speak, I think the better for us all," Lopez-Alegria told LiveScience.

Follow Mike Wall on Twitter @michaeldwall and Google+. Follow us @livescience, Facebook & Google+. Original article on LiveScience.

Editor's note: By the end of this century, Earth may be home to 11 billion people, the United Nations has estimated, earlier than previously expected. As part of a week-long series, LiveScience is exploring what reaching this population milestone might mean for our planet, from our ability to feed that many people to our impact on the other species that call Earth home to our efforts to land on other planets. Check back here each day for the next installment.

Hong Kong, a city of 7 million inhabitants, faces a major garbage crisis. The region's three landfills are expected to fill up completely by 2020, and even if recycling increases, the country will have to expand its landfills to deal with the thousands of tons of waste generated every day, officials say.

"Hong Kong currently solely relies on landfills to dispose of its municipal solid waste, which is not a sustainable way to treat waste," said a spokesman for Hong Kong's Environmental Protection Department.

Hong Kong and its overflowing landfills are not alone. In fact, the planet as a whole faces a serious problem: what to do with the tons and tons of garbage, poop and other waste humans generate, especially with the population set to grow considerably this century.

A recent statistical analysis predicts the world population will hit 11 billion by the year 2100, outpacing United Nations estimates. By then, these piles of rubbish and other waste may become insurmountable.

Today, as an example, the world has about 3 billion urban residents, who generate 2.6 lbs. (1.2 kilograms) of municipal solid waste per person per day, a World Bank report estimates. That adds up to about 1.4 billion tons per year. [What 11 Billion People Means for the Planet]

By 2025, as a result of economic development and urbanization, that number will climb to 4.3 billion urban residents generating 2.4 billion tons of waste per year, the report estimates. Where will it all go?

Some countries may run out of space to put all that waste, meaning the rubbish could end up in the oceans. Experts say people will have to find ways to recycle more and to generate energy from these waste streams if they're to have any hope of managing the issue.

"Even if the population were stable, we already have a serious problem," said Barbara Evans, a civil and environmental engineer at the University of Leeds, in England.

Dealing with the world's trash

Waste management varies widely from country to country. Larger countries can afford to create more landfills, but must also find ways to recycle more. Meanwhile, smaller countries face a more urgent risk of running out of landfill space.

"The current situation with solid waste is a bit of a checkerboard," said Sara Bixby, deputy executive director of the Solid Waste Association of North America. Europe, the United States and Australia are focusing on managing waste and lessening its environmental impact, but in many developing nations, rapid urbanization is outstripping waste management infrastructure, she said.

In 2011, the United States generated about 250 million tons (227 million metric tons) of trash, according to the Environmental Protection Agency. Just over half of that garbage went to landfills, about a third was recycled, and the remainder was burned to generate energy.

While the average size of U.S. landfills has increased over the years, their number has decreased. From 1990 to 2011, the total amount of waste going to landfills dropped by more than 11 million tons — from 145.3 million to 134.2 million tons. Despite producing one of the highest levels of solid waste per capita, the continental United States has enough open space for landfills in the foreseeable future.

The situation in smaller countries and areas is strikingly different. In Hong Kong, where landfills could fill up by 2020, the government is pursuing an aggressive recycling goal. But Hong Kong Environmental Protection Department officials told LiveScience that "landfills are an essential part of the waste management chain, as even with the best efforts in waste reduction and recycling, there is still a need to landfill wastes that cannot be recycled or treated."

And not all waste is created equal: Plastic is one of the worst offenders, because it takes so long to degrade. An unsettling amount winds up in the ocean, contributing to the Great Pacific Garbage Patch, a region of swirling marine debris in the central North Pacific Ocean. "We can't look at the ocean as just a liquid landfill," Bixby told LiveScience. Marine animals can ingest or become tangled in trash, and toxic waste can poison ecosystems.

Rather than simply finding more places to dump trash, the world should look for a way to decrease its need for landfills, Bixby said. But trash isn't the only kind of waste humans generate — there's human waste, also. And many parts of the developing world don't have basic sanitation facilities, making the issue even more urgent in those locations.

Human waste and public health

West Africa had an unusually severe rainy season in 2012, causing flooding in the slums of Sierra Leone and Guinea. The latrines in those countries weren't built to withstand so much water and overflowed, bringing on an epidemic of cholera, a diarrheal disease that spreads through contact with contaminated feces. The news agency IRIN News reported that the disease killed about 400 people and sickened more than 25,000 others.

Eleven billion people will produce a heap of human waste.  Even now, rather than being treated and sent out into the ecosystem as environmentally safe waste fluid, much of the poop just piles up in inhabited areas. That's because, currently, about 2.6 billion people (35 percent of the world's population) live without access to basic sanitation. They don't have working toilets, or even a pit latrine, let alone sewage treatment plants. Many of these people live in developing countries, where most of the population growth is expected to occur. [5 Ways Toilets Change the World]

As in the case of Sierra Leone and Guinea, insufficient sanitation poses a serious threat to public health. Without proper toilet facilities, people are forced to defecate in the open, near rivers or living areas. In India, 290,000 gallons (1.1 million liters) of raw sewage get dumped into the Ganges River every minute, according to the World Health Organization.

Currently, poor sanitation "contributes to two of the three leading causes for preventable death for children under five," said Lisa Schectman, head of policy at the NGO WaterAid America. Human waste can contaminate the water supply, leading to diarrheal diseases like cholera, which many people suffer from chronically, Schectman said. These diseases cause malnourishment, low birth weight and cognitive problems. Poor sanitation also increases the risk of ingesting fecal matter, which can lead to stunted growth.

A fly that breeds exclusively on human excrement carries the disease called trachoma, the leading cause of preventable blindness. Feces can also contaminate soil, breeding parasitic worms like roundworm and hookworm that cause delays in brain development in children and serious intestinal blockages or even death in adults.

In the course of her work, Schectman has visited places with dire sanitation needs. "You would see ramshackle buildings. You'd see a concentration of flies. It might smell, particularly in hot climates. Sometimes you would see an open pit, especially in rural areas. In Bangladesh, during the rainy season, you may see sludge running down the streets," she told LiveScience.

Aside from health, lack of sanitation facilities is complicated by cultural mores and limits access to education. And the problems will only become more widespread as the population grows.

"An increasing population means increased human waste, and government provisions are not keeping up," Schectman said.

Culture and urbanization

Complicating the world's ability to deal with physical waste itself, discussing toilet matters is culturally taboo in some places, especially among women. "For women to discuss their bodily functions is considered completely out of the realm of possibility in many cultures," Schectman said. Women who lack toilet facilities must travel farther away to relieve themselves, putting them at risk for sexual violence. And many girls in developing countries drop out of school because they have no private restroom.

Public toilet blocks were advocated as a solution to the problem, but a study of such blocks in Bhopal, India, in November 2008 found that men were twice as likely as women to use them. Many women prefer to use "flying toilets" — basically plastic bags inside their homes — so they don't have to go outside at night.

The population uptick means more and more women will face these cultural dilemmas. A lack of sanitation facilities will keep more young women out of school, creating a wider education gap, Schectman said.

Further, the world population is not only growing, it's becoming more urbanized, placing a larger load on the systems that do exist. Cities need networks for removing waste, whether these are pipes or trucks or even handcarts. In places like Africa, a growing population means many people will live on the margins of formal cities, ignored by the political system.

"Sanitation is a highly politicized topic," said Evans, of Leeds University. "If you want systems to work well, you need to plan them in advance." Marginalized people lack the money and clout to build these systems for themselves. They need politicians to lobby for better infrastructure. If there's no economic development, a growing population could make sanitation problems exponentially worse, Evans said.

First-world problems

Developed countries have sanitation problems of their own. In Europe, most of the sewage treatment works were built before people understood biology. These systems were designed to remove organic matter, not microbes, said Sandy Cairncross, a public health engineer at the London School of Hygiene & Tropical Medicine in England. (Organic matter consists of anything containing carbon, whereas microbes are living organisms that can make people sick.) [Managing the World's Waste (Infographic)]

It wasn't until about 10 years ago that the European Union introduced standards of sanitation in terms of bacteria. Until recently, the city of Brussels dumped all of its sewage into the river Senne (or Zenne). But starting in 2000, the city began building water treatment plants to limit the amount of sewage that goes into the river.

Then there's the problem of expense. In the developed world, installing plumbing and sewers costs hundreds, if not thousands, of dollars, Cairncross said. As the urban population grows, it "means spending a lot of money in a hurry that isn't there," he said. 

Climate change is further exacerbating the problems of managing a growing population's waste. Global warming is decreasing the snow and ice cover on mountains, which feeds rivers that supply cities with water for sewers. "Diminishing snow cover means many rivers that feed cities to flush sewers are not going to have much water in the dry season," Cairncross said.

Over the past 50 years, rainfall has been declining and becoming more seasonal in many cities, he said. At least one city in Australia has a desalination plant to get fresh water, and London is talking about putting one in, he said.

On the other hand, extreme weather, partly linked to climate change, will cause a greater risk of flooding in coastal cities. Flooding can wreak havoc on sanitation systems, like that seen during the 2012 cholera epidemic in Sierra Leone and Guinea.

Reduce, reuse, recycle

One promising option for dealing with the world's waste problems involves recovering materials or energy from all that refuse.

Some European countries have already stepped up recycling efforts. The United Kingdom has a landfill tax on waste that must be buried in the ground, and the country's total amount of landfill waste has dropped from about 110 million tons in 1997, a year after the tax was implemented, to about 45 million tons in 2012. Germany requires its citizens to separate all their waste and recyclables. And in the United States, about a third of waste is recycled, much of which is processed in China.

There's also potential to generate energy and resources from waste. Sweden, for instance, has launched a successful program for converting garbage into renewable energy. Only 4 percent of Swedish household waste gets landfilled. The country created its first waste incineration plants in the 1940s, and today the process is so efficient that Sweden has begun importing trash from other countries that pay for the service.

But solid waste isn't the only valuable kind of waste.

Wastewater contains about 10 times the amount of energy — in biochemical form — as that needed to treat the water, according to Barry Liner, director of the Water Science & Engineering Center at the not-for-profit Water Environment Federation. The excess energy in biological waste could be fed back to the power grid, and some companies are already doing this.

Microbes can digest biowaste and turn it into natural gas. In addition, water can be reclaimed for drinking, and nutrients such as nitrogen and phosphorus can be recovered for agricultural use. These systems are easier to implement in developed countries, Liner said, but they could also exist on a smaller scale elsewhere.

"If we're truly going to stay ahead of population growth, we're going to have to change the way we think," Bixby said.

Future of waste

There are reasons to be optimistic that the world can make the changes it needs to, based on changes that are already occurring.

One of the Millennium Development Goals, a set of eight international development goals established following the Millennium Summit of the United Nations in 2000, was to provide sanitation to 75 percent of the world by 2015. Currently, the world has reached 67 percent, said Liner, who is cautiously optimistic. "While we're not meeting the Millennium Development Goals in total, there is a lot of hope," he said.

The world partially achieved these goals by implementing decentralized systems such as shared latrines in urban areas. Small-scale systems are much less expensive than sewers and centralized wastewater facilities, Liner said.

Some organizations are trying to find ways to make sanitation facilities available off the grid. For example, the Bill and Melinda Gates Foundation's Reinvent the Toilet Challenge aims to develop sanitary waterless toilets that don't need a sewer connection or electricity, and cost less than five cents per user per day. The foundation has awarded grants to researchers worldwide to develop engineering solutions for human waste management, from solar-powered toilets to ones that recover and purify wastewater.

These kinds of efforts must be expanded if the world is to meet remaining sanitation needs, Liner and others say.

As Evans said, "We've got the opportunity today, if we take it, to put ourselves on the right path."

Follow Tanya Lewis on Twitter and Google+. Follow us @livescience, Facebook & Google+. Original article on LiveScience.

Editor's note: By the end of this century, Earth may be home to 11 billion people, the United Nations has estimated, earlier than previously expected. As part of a week-long series, LiveScience is exploring what reaching this population milestone might mean for our planet, from our ability to feed that many people to our impact on the other species that call Earth home to our efforts to land on other planets. Check back here each day for the next installment.

In mid-April 2009, samples from two California children suffering from the flu arrived at the Centers for Disease Control and Prevention in Atlanta for further investigation; something didn't seem normal about the particular flu strains they had. Local clinics and flu surveillance staff had detected a virus that had a unique genetic makeup, different from any known human flu virus. It was entirely new to science.

That was the beginning of the 2009 swine flu pandemic. Countries around the world took notice and prepared for possible outbreaks, the World Health Organization sent out guidelines to ministries of health and vaccines were developed in a matter of months. The virus, which may have started infecting people first in Mexico, spread across the globe, infecting millions of people and killing thousands before running its course, with the pandemic coming to an end in August 2010.

The virus was a new strain of H1N1, the influenza virus involved in the devastating 1918 Spanish flu pandemic, which killed between 30 million and 50 million people worldwide, according to the U.S. Department of Health and Human Services, more than died during World War I. The emergence of the new H1N1 in 2009 was a reminder that despite the unprecedented progress in treating infectious disease in the past decades, the looming shadow of a deadly pandemic still persists.

In fact, with every mysterious virus that surfaces, be it the 2009 swine flu, the 2002 SARS coronavirus, or most recently, MERS (Middle East Respiratory Syndrome, a viral respiratory illness that has emerged around the Arabian Peninsula and killed half of the people who have had it), the same questions come to the minds of researchers and health authorities: Is this the virus that's going to cause the next pandemic? And will humanity be able to stop it?

And now, new challenges are being added to existing ones: The latest population projections from the United Nations, announced in a new report last summer, estimate that the world's population will reach 9.6 billion people by mid-century, and 11 billion by 2100.

The sheer number of people, their interactions with animals and ecosystems, and the increase in international trade and travel are all factors that will likely change the way humanity deals with preventing and treating epidemics, experts say. In fact, the unprecedented growth of the human population in the second half of the last century — growing from 2.5 billion to 6 billion — may have already started changing how infectious diseases emerge. [What 11 Billion People Means for the Planet]

"There's a strong correlation between the risk of pandemic and human population density. We've done the math and we've proved it," said Dr. Peter Daszak, a disease ecologist and the president of Eco Health Alliance, who examined the link in a 2008 study published in the journal Nature.

Looking at contemporary outbreaks since the mid-20th century, Daszak and colleagues found that the rate of emergent diseases caused by pathogens new to humans has increased significantly with time, even when controlling for progress in diagnosis techniques and surveillance, which could make it only seem like diseases were on the rise. More than 300 new infectious diseases emerged between 1940 and 2004, the study found.

Some of these diseases were caused by pathogens that have hopped across species and finally into humans — for example, the West Nile virus, the SARS coronavirus and HIV. Others were caused by a new variant of a pathogen that evolved to thwart available drugs, such as drug-resistant tuberculosis and malaria.

Certain pathogens, such as the bacteria that cause Lyme disease, are not new to humans, but their incidence increased dramatically, perhaps due to changes that newly arrived humans made to the environment inhabited by animals carrying these pathogens.

In light of the continuous population growth, health authorities are calling for strengthening public health organizations, and giving more resources to systems that would protect people. Researchers are studying ways to identify viruses faster, so that vaccines could be developed early in the process, and scientists are trying to understand the complicated interactions between humans and the surrounding ecosystem, so that they could identify emerging disease hotspots and find the next emerging virus before it finds humans. All of these are done in an effort to have the new creative solutions that preventing pandemics on a populated planet would require.

"You can predict very confidently as each year moves forward, we're going to see more and more diseases emerge," Daszak said. "It's a little abstract to most people. And to be fair, it's new for scientists too."  

Diseases of the future are already in nature

When Daszak and his colleagues analyzed the characteristics of emerging diseases, they found some similarities between them. All known emerging diseases were linked to sudden human population growth, new human activity in the environment and high wildlife diversity in the area where the pathogen originated.

About two-thirds of new diseases were transmitted to humans from animals, the researchers found.

More than 70 percent of these diseases, known as zoonotic diseases, were caused by pathogens originating in wildlife — for example, the Nipah virus that causes inflammation of the brain and first surfaced in 1999 in Perak, Malaysia, or the SARS coronavirus that first infected a farmer, are both traced back to viruses in bats. [Video: Time-lapse of new infectious diseases worldwide during 1944-2004] 

As humans do not often come into contact with wildlife, such pathogens should theoretically not pose much danger to people. But the pathogens can make the leap to humans by first infecting other animals that humans do come into contact with, such as domestic pigs. The animals serving as the middle link of this disease chain, however, have to be in places in some overlapping territory, which occurs when burgeoning populations push people into wild areas where humans once rarely, if ever, ventured.

"Each wildlife species carries a bunch of microbes, most of them we've never known about," Daszak said. "When you build a road into a new patch of rainforest, you put a pig farm in there, people move in and come into contact with these pathogens."

The number of pathogens originating in wildlife and infecting humans has increased with time, too, Daszak's research shows. In the last decade of the 20th century, such pathogens were responsible for more than half of the new infectious diseases that cropped up in that time period.

Human contact with wildlife species that facilitate the transmission of novel viruses may increase in the future, as the population grows and humans searching for places to live and farm fan out to areas inhabited by or closer to wildlife.

Predicting the future

Stephen Morse, an epidemiologist at Columbia University, was in the early years of his career when the first case of HIV/AIDS was detected in the United States in 1981. In a pandemic that continues to this day, HIV, believed to have originated in chimpanzees, has infected 60 million people and caused an estimated 30 million deaths. [5 Scariest Disease Outbreaks of the Past Century]

"For many years, there was complacency, thinking that infectious diseases were pretty much becoming ancient history," said Morse, who studies how pathogens develop the ability to infect humans.

The kind of complacency present in those pre-HIV years largely no longer exists. Scientists are constantly on the lookout for the next pathogen that may cause an epidemic. One of the viruses that scientists have thought posed the greatest pandemic threat is the bird flu, or H5N1, a strain of influenza virus that has been circulating in birds and killing them. Resources devoted to prepare for and combat a bird flu pandemic in humans were shifted and applied to the swine flu pandemic in 2009.

Another worrisome influenza virus on the watch list is H7N9, an avian flu first detected in China in 2013 that has infected a number of people who had come in contact with infected birds. As viruses constantly change, it is also possible for them to mutate in a way that allows them to easily spread among people. In fact, one of the hardest questions for the scientists to solve is not just how viruses living in animals become able to infect humans, but also what makes them able to move from person to person, Morse said.

For H5N1, scientists have shown that the virus needs only four mutations to be able to transmit via air among mammals.

"With H5N1 and H7N9, we do a lot of worrying and watching, because we really don't know what to look for until it begins taking off in people," Morse said. "And at that point, it's already too late."

Morse and his colleagues are working on a project called PREDICT, part of the Emerging Pandemic Threat program run by the U.S. Agency for International Development, to help anticipate the next big disease threat.

"The idea is to see how early we can identify potential infections that could be serious, like the next SARS," Morse told LiveScience, calling from Uganda, one of the focus countries of the PREDICT program, where scientists monitor wildlife and people in contact with wildlife to discover novel pathogens.

"We are trying to understand more about the ecology of these infections, and what pathogens that wildlife species carry are likely to come into contact with human," he said.

Scientists have found that new viruses are more likely to surface in some parts of the world than others. Tropical Africa, Latin America and Asia are the disease emergence hotspots, and their high biodiversity and increasing human interaction with the environment may be helping viruses to make the leap into humans. And from there, they can go anywhere on the globe.

Epidemics may grow faster and cost more

Today, travelers are just a few hours' flight away from places that would have taken months to travel to by ground or sea in the past. This is a boon not just to humans, but to the microbes they carry. Sick travelers can introduce pathogens to new people as they travel, and at their destination, before they even realize they are sick. With future population growth, simple math suggests that there's going to be more travelers, potentially helping epidemics grow by quickly spreading the contagion.

"We're going to see connectivity between people increase, so there's more risk of a disease emerging in remote parts of the Amazon, and actually getting into our global travel network and affecting those in London, Moscow and Delhi," Daszak said.

The emergence of SARS in 2002 in China painted a picture of what it would be like when a virus finds its way into the travel network: The virus rapidly propagated around the world in just a few weeks, infecting more than 8,000 people and killing about 800 before it was brought under control by limiting unnecessary travel and quarantining those affected.

A traveling virus may also cause economic damage, beyond even the costs associated with disease treatment and control. SARS cost billions of dollars by cutting international travel by 50 to 70 percent, and affected businesses in several sectors. Growth of the Chinese GDP fell by 2 percentage points in one quarter, and half a percentage point in annual growth, according to the World Bank and the Chinese government’s estimations. Is humanity prepared to face the future?

The movement of the world's population from sparsely populated rural areas to dense cities may also impact the spread of pathogens. By the year 2050, 85 percent of people in the developed world and 54 percent of those in the developing world are expected to have left rural areas for cities, according to United Nations' estimates.

From a global disease-fighting perspective, urbanization can have some positive effects. Better communication systems can help spread early warnings and other critical information at times of outbreaks. Moreover, better disease surveillance systems can be set up in urban settings compared with remote rural areas.

However, concentrated populations in cities may need a stronger public health sector to protect them.

People in crowded cities are often more vulnerable to infectious disease, especially in the face of natural disasters such as hurricanes and floods, which have particular public health problems associated with them, said Dr. Ali S. Khan, director of the CDC's Office of Public Health Preparedness.

"We are going to need a robust public health system to respond to population increase, urbanization, the aging population and increased travel, increased interaction between humans and animals that give rise to new diseases," Khan said.

But instead, "we've hollowed out public health, and I think this poses a great threat to the health security of our nation and global communities," he said.

The U.S. public health sector is suffering budget cuts at both the state and federal levels. Khan said the CDC's $1 billion program supporting disease identification and emergency operations now runs on $600 million, and with 45,700 fewer public health workers in the field, functioning as the eyes and ears of the agency.

However, the news isn't all gloomy, Khan said. "It is pretty clear that as we concentrate people in the city, there's an increase in creativity. So I'm optimistic that this increased creativity will lead to novel solutions that will help us identify disease, prevent and track better than we've ever been able to do in the past," he told LiveScience.

And there have already been glimpses of progress nowadays, Khan said, noting the rapid response by the CDC, WHO and other public health organizations to recently emerging viruses such as MERS, as well as the agencies' enhanced communication with the public.

"So think about all this great work where we look at social media to try to understand when a disease is emerging in the community, and we use social media to communicate with people in a way we were never able before," Khan said.

There has also been progress in developing novel diagnostics that can quickly detect infection before a person starts showing symptoms, and in sequencing genetic material of a pathogen to understand what it is and how it works, Khan said.

"Immense progress" has also been made in reducing the amount of time it takes to make a vaccine, Khan said. About two months after the 2009 swine flu pandemic was announced by the WHO, vaccines had been developed and production of enormous quantities of them were underway.

"We should expect to see a continuous acceleration of progresses, but this is not a given," Khan said. "I think people nowadays have a false sense of security, and I think part of this is that public health is working,” but that can only last so long if public health resources keep decreasing instead of strengthening, he said.

"We have eradicated and eliminated some diseases from our community, but the honest truth is most diseases don't get eliminated," Khan said. "Most diseases come home to stay."

Email Bahar Gholipour. Follow us @livescience, Facebook & Google+. Original article on Live Science.

Editor's note: By the end of this century, Earth may be home to 11 billion people, the United Nations has estimated, earlier than previously expected. As part of a week-long series, LiveScience is exploring what reaching this population milestone might mean for our planet, from our ability to feed that many people to our impact on the other species that call Earth home to our efforts to land on other planets. Check back here each day for the next installment.

On the western coast of Alaska, nestled against the Bering Sea, residents of the remote village of Newtok may soon become the country's first climate refugees.

Like many Alaskan villages, Newtok sits atop permanently frozen soil called permafrost. In recent years, however, warming oceans and milder surface temperatures have melted the icy subsoil, causing the ground beneath Newtok to erode and sink. In 2007, the village already sat below sea level, and studies warned that the subarctic outpost could be completely washed away within a decade.

Now, despite political and financial hurdles, the community is looking to relocate its roughly 350 residents. With climate change rapidly altering human ecosystems around the globe, Newtok may not be alone in its fight against warming temperatures, melting ice and rising seas.

For the roughly 7.2 billion people who live on Earth today, the impacts of a changing climate may be taking different forms, but the consequences are already being felt across the globe — from severe monsoons in Southeast Asia, to the increasing pace of melting ice at the poles, to hotter-than-average temperatures throughout the contiguous United States.

Over the course of the next century, if the levels of greenhouse gas emissions are not reduced, and nations have failed to address the myriad challenges of climate change, scientists say Earth's fragile ecosystem could be in serious jeopardy. But, what if in those same 100 years, nearly 4 billion people are added to the world's population? Could this type of rapid growth overwhelm the carrying capacity of our "Pale Blue Dot" and our ability to mitigate and cope with climate change?

A recent United Nations analysis of world population trends indicates global population growth shows no signs of slowing, with current projections estimating a staggering 11 billion people could inhabit the planet by the year 2100, faster growth than previously anticipated. The majority of this surge in population is likely to occur in sub-Saharan Africa, with the population of Nigeria expected to surpass that of theUnited States before 2050, according to the statistical analysis.

The new report also suggests India will eventually become the world's largest country, matching China's estimated population of 1.45 billion people in 2028, and continuing to swell beyond that point, even as China's population begins to decrease.

Some scientists say rapid population growth could be catastrophic for the planet, because it will likely lead to overcrowding in cities, add stress to Earth's already dwindling resources, and worsen the effects of climate change. But within the scientific community, a debate is brewing, and there is little consensus about how — or even if — population growth is linked to global warming.

Assessing the impact of population growth on climate change has been tricky. Most scientists agree that humans are to blame for most of the planet's warmingsince 1950, but precisely which events were aggravated by human activities (and how much) are unknown. [What 11 Billion People Mean for the Planet]

"It's a question that's really hard to answer, because climate science is not to the point of being able to identify specific impacts, or changes that have occurred so far, as being directly caused by climate change," said Amy Snover, co-director of the Climate Impacts Group and a researcher at the Center for Science in the Earth System at the University of Washington in Seattle. "What we can do is look at the many things that have happened recently that are similar, and what we expect to happen, and see that these things are problematic and will certainly raise concerns for the future."

Furthermore, scientists on both sides of the equation — those who study demographics and those who study climate science — do not necessarily agree on how, or even if, population growth and climate change are connected.

A growing debate

Increasing the number of people on the planet does not, in itself, intensify climate change, said David Satterthwaite, a senior fellow studying climate change adaptation and human settlements at the International Institute for Environment and Development, in the United Kingdom. Rather, changes in consumption are the key drivers of global warming, he explained.

"Higher consumption is what drives anthropogenic climate change," Satterthwaite told LiveScience. "The high-consumption lifestyles of the richest half-billion people scare me much more than the growth in population in low-income nations."

This is because developing nations, where the U.N. estimates most of the next century's surge in population will occur, have much smaller carbon footprints than developed countries, such as the United States, Canada and the United Kingdom.

"If you think of population as the driving force, it makes sense to look at the fast-growing nations and say: 'We have to slow that population growth,'" Satterthwaite said. "But most of the nations with the fastest growing populations have far lower per capita greenhouse gas emissions."

During the Industrial Revolution, which began in the mid-1700s in England and later spread across the Atlantic Ocean to the United States, emissions of carbon dioxide and other heat-trapping gases soared as manufacturing and transportation boomed. The technologies used during the Industrial Revolution were also inefficient and largely based on coal and fossil fuels, which emit large amounts of greenhouse gases that linger in the atmosphere.

This flurry of activity has taken a toll on the planet. Since the start of the Industrial Revolution, human activities have increased the concentration of atmospheric carbon dioxide by a third, according to NASA.

Now, as developing countries seek their own industrial revolution, there are concerns that too much damage has already been done.

"There are opinions that we're already past a sustainable population now, in terms of being able to provide a high quality of life for every citizen on the planet," said David Griggs, a climatologist and director of the Monash Sustainability Institute at Monash University in Melbourne, Australia, and a former head of the Intergovernmental Panel on Climate Change (IPCC), an international body jointly established by the United Nations Environment Programme and the World Meteorological Organization to assess the environmental and socioeconomic impacts of climate change.

Others say improvements in technology will yield better crop production and distribution, enabling cities and towns to accommodate more people, he added. But more is not necessarily better.

"I'm not a fan of thinking about this as a tipping point — there isn't a point where we just go over the edge," said Griggs, who was previously the deputy chief scientist of the United Kingdom's national weather service. "It's a slow deterioration, and the more people there are, the more challenging it is for those people to have their basic needs met."

Population versus consumption

To understand the potential environmental impacts, it is important to consider both population growth and trends in consumption, said Robert Engelman, president of the Worldwatch Institute, an environment and sustainability think tank based in Washington, D.C.

"Some people will say one matters more than the other, but they multiply each other," Engelman said. "It would be dangerous to ignore population as a major factor."

In 2008, China, the United States, the European Union (excluding Estonia, Latvia and Lithuania), India, the Russian Federation, Japan and Canada were among the top emitters of carbon dioxide. Combined, these nations contributed more than 70 percent of the global carbon dioxide emissions from fossil fuel combustion and industrial processes. In contrast, the rest of the world represented only 28 percent of carbon dioxide emissions.

"In some of the poorest countries in the world, emissions are very low, but the idea is we want these countries to develop," Engelman said. "As we've seen happen in India and China as they've industrialized, countries that are populous and poor can experience a rapid rise in greenhouse gas emissions. We can't just consider how much the average person is emitting in these populous countries now. We have to think about what will be happening to the people in these countries over the next 70 years."

Beginning in the 1960s, China embarked on a rapid path toward industrialization. By the end of the century, the country had secured its place as a manufacturing powerhouse and a veritable economic superpower. But, China's speedy industrialization has come at an environmental cost.

Within 20 years, China more than tripled its emissions of carbon dioxide — from 2.46 million tons of carbon dioxide in 1990 to 8.29 million tons in 2010, according to United Nations estimates.

Since 2000, China's energy-related greenhouse gas emissions have increased at an average rate of more than 10 percent each year, according to the Harvard Project on International Climate Agreements, which is designed to identify "scientifically sound, economically rational, and politically pragmatic post-2012 international policy architecture for global climate change."

Adding politics to the mix

But, developing climate policies has been a challenging, and an often fruitless, process.

Jerry Karnas, the Center for Biological Diversity's population campaign director in Miami, is all too familiar with these political pitfalls, particularly in addressing the impact of population growth on climate change.

In 2008, Karnas was appointed to a statewide commission to help design a plan for Florida to reduce its carbon dioxide emissions to 80 percent of 1990 levels by the year 2050. The final report was more than 1,000 pages and comprehensively tackled every sector of Florida's economy, except population.

"Population was the only thing not on the table," Karnas said. "We had to take growth as a given, and not challenge the notion that for Florida to succeed, it had to grow."

One of the reasons the state government accepts rapid population growth has to do with the way Florida's economy is set up, Karnas said.

"Florida is a sales tax state. We have no income tax, but much of the state is also funded by the documentary stamp tax," he said. "Documentary stamps are real estate transactions, so every time a real estate transaction occurs, it gets taxed, and that goes into the state coffer. So, the two major funding sources for Florida are dependent on increasing the population numbers in the state."

While the population of the United States is not expected to leap significantly in the next century, diminishing natural resources are already adding stress to the country's food and water supplies, and the availability of future energy resources.

In regions of the world where vast population growth is projected, such as sub-Saharan Africa, the issue of dwindling natural resources will likely be magnified. [5 Places Already Feeling the Effects of Climate Change]

Feeding a hungry planet

If the global population increases by 3 billion people, food production will also need to rise to meet these growing demands. Finding adequate agricultural land, however, will be a challenge, as soil erosion and more frequent droughts related to climate change render larger tracts of land unusable, Griggs, the Monash University climatologist, said.

"If we look at the next 50 years, we'd need to grow more food than we have in the whole of human history to date to feed those 9 billion people," Griggs said. "But since we have no more agricultural land, we'll have to produce all this food on the same land we're producing food on at the moment."

In particular, southern Asia, western Asia and northern Africa have virtually no spare land available to expand agricultural practices, according to the 2013 Statistical Yearbook of the Food and Agricultural Organization for the United Nations, published in June.

More people on Earth also means more competition for water, Griggs added. Currently, one of the main uses for water is in agriculture, and ensuring that populations have access to clean drinking water will be another significant challenge, he said, since global warming may cause arid regions of the planet to become even more parched.

In the United States, the Bureau of Reclamation released a report on the status of the Colorado River Basin in December 2012. The study concluded that over the next 50 years, water supply from the Colorado River will be insufficient to meet the demands of its adjacent states, including Arizona, New Mexico and California.

"The U.S. government was effectively saying, there will be no way to completely satisfy the water needs of the population that is currently projected in that part of the country," Engelman said.

Worldwide, the situation is not much better. A 2011 report on the state of the world's land and water resources, released by the Food and Agriculture Organization of the United Nations, established that more than 40 percent of the world's rural population lives in water-scarce regions.

Ways to mitigate the impacts

While the impact of population growth on climate change remains a topic of debate, experts agree that finding ways to mitigate the effects of climate change will be critical for the sustainability of the planet.

For one, nations need to address climate change issues now, in order to make communities more resilient in the future, said Declan Conway, a professor of water resources and climate change at the University of East Anglia in the United Kingdom. This includes investing in renewable energy alternatives, such as technologies to efficiently harness solar and wind energy, he added.

As part of his work at the Worldwatch Institute, Engelman also promotes the idea of carbon taxes, which would introduce fees based on the carbon content of fuels. While these types of resource taxes have been suggested as an incentivized way to reduce greenhouse gas emissions, they remain politically divisive.

Still, others see positive changes on the horizon.

"Twenty years ago, climate change wasn't seen as an issue at all, but since then, technology has improved rapidly," Griggs said. "We don't have to hang around and wait for something bad to happen. There's no question that we can deal with all of these climate change issues now, if we want to. The real issue is: Will we? Will there be the political will and the leadership to take on these things?"

As for whether he remains optimistic overall, Griggs is a little more hesitant. "I'm schizophrenic about it," he said. "[At] times, I look at what's happening in the world and the lack of progress, and I say, we're stuffed. On my good days, I'm optimistic and I see us moving in a direction that will allow us to solve these problems."

Follow Denise Chow on Twitter @denisechow. Follow LiveScience @livescience, Facebook & Google+. Original article on Live Science.

Editor's note: By the end of this century, Earth may be home to 11 billion people, the United Nations has estimated, earlier than previously expected. As part of a week-long series, LiveScience is exploring what reaching this population milestone might mean for our planet, from our ability to feed that many people to our impact on the other species that call Earth home to our efforts to land on other planets. Check back here each day for the next installment.

After 14 years of drought, Lake Powell is less than half full.

Water flows into Lake Powell, nestled between Utah and Arizona, from high in the Rocky Mountains via the Colorado River. More than 30 million people in seven states depend on the mighty Colorado for water to grow crops, fuel power plants and keep cities such as Las Vegas alive. But this year, the worst drought in a century has slowed the flow to a trickle.

In August, the federal Bureau of Reclamation cut, by 9 percent, the amount of water people in the southwestern United States could draw from Lake Powell. As states and counties squabble over their allotment of water in the coming years, hydroelectric plants (including the one on the Hoover Dam) could idle, and farmers are bracing for reduced crop production.

In western Colorado, water is fed to farms through a network of ditches. Because water is allocated based on seniority, some of the newest farmers saw their water turned off in July, before the end of harvest, said Kate Greenberg, the Western organizer for the National Young Farmers Coalition, a group that supports young and independent farmers. Greenberg is also part of a working group looking for agricultural solutions to water shortages along the Colorado River.

While small farms managed to keep going by using private water supplies, some of the alfalfa farmers have been hard-hit, Greenberg said. Alfalfa requires a plentiful, steady supply of water, and is one of the most prevalent cash crops in Colorado, she said.

"If you have an alfalfa crop, it's ideal to get three cuttings a year, but in a dry year like this year, a lot of farmers have gotten one," Greenberg told LiveScience. "They get a third of their crop that they can bring to harvest."

The water woes plaguing the Southwest foreshadow a worldwide problem to come. Already, 2.7 billion people globally face at least some water scarcity, according to a 2012 study detailed in the journal PLOS ONE. Fights over water rights are causing political conflicts and instability in such places as the Nile valley and the Indian subcontinent. As population sizes rise, those conflicts will get more intense, according to a report by the National Intelligence Council, which advises the director of national intelligence for the United States about national security issues.

And the latest population models predict that 11 billion people will live on Earth by 2100, according to a United Nations report released last summer. Given that the existing population is already taxing water supplies in many regions, how will the planet provide for all the new people who will be here next century? [What 11 Billion People Means for the Planet]

"Water is the new oil," said Bill Davies, a plant biologist at the Sustainable Agriculture Center at Lancaster University in England. "People will be scrambling for water."

To provide for the planet, it's critical to understand the available water supply by creating detailed maps of where water is scarce or abundant, and improving water infrastructure, experts say. Making agriculture more efficient is also key. But even those measures may not be enough to provide for everybody. The global economy also needs to account for the true costs of water so that water-demanding products are made in water-rich areas and imported into more parched regions.

Measuring a finite resource

Right now, nobody even knows how much water is really in the ground. There are estimates at the global or the regional level — for instance, Californians pump about 14.5 billion gallons (54.9 billion liters) of groundwater a year, according to the National Groundwater Association (NGA).

But an individual farmer or people drawing from private wells may not know how much water is in their well until it runs dry or becomes contaminated with arsenic or nitrogen. In the United States, there are about 15.9 million water wells, and about 500,000 new wells are drilled every year for residential purposes, according to the NGA.

In most parts of the world (including much of the United States), individual well usage isn't metered, and anyone can pump groundwater without notifying an authority. Few places measure agricultural water usage.

Many places also rely on distant locales for their water sources, making sensible conservation policies tricky to enact on a local level. For example the Tigris River flows from Turkey to Iraq, so ensuring Iraq's supply requires conservation from Turkey — a political problem that requires international negotiations.

Because groundwater is the single biggest thing that moves around on the Earth's surface, new satellite systems can detect changes in Earth's gravity due to groundwater depletion. But because measurements rely on distant satellites in space, they have relatively poor spatial resolution, said David Maidment, a hydrologist at the University of Texas at Austin.

"It's a big regional measure. You can't say, 'This farmer here is stealing the water,'" Maidment told LiveScience.

Maidment and his colleagues are creating a way for local government, cities, states and countries around the world to share their water data. The goal is to get a detailed picture of the world's freshwater resources. Once that happens, officials will be able to decide how to allocate resources efficiently, he said.

Water-sparing plants

Of course, using more water efficiently also means not squandering it, particularly through wasteful agricultural practices.

Agriculture uses about 70 percent of the freshwater on the planet, said Giulio Boccaletti, managing director of the Global Freshwater Program at the Nature Conservancy.

Flood irrigation, a practice in which farmers drench fields with water from hoses or other sources, creates runoff that carries pesticides into local rivers and often out of the local watershed, Boccaletti told LiveScience. Other water evaporates into the atmosphere and is then carried away to distant parts of the globe. [5 Ways We Waste Water]

To stem those wasteful processes, farmers would need to cover crops with plastic to prevent evaporation and use drip irrigation to target water directly to a plant's roots.

Conversion to drip irrigation is happening, albeit slowly. In the United States, the National Resources Conservation Service offsets some costs of implementing water-sparing irrigation. However, many farmers don't know about these programs, and installing drip irrigation requires placing irrigation tape underground, which is labor-intensive and costly. This method may not work with crops in which tilling could frequently cut the underground infrastructure, Greenberg said. Only a fraction of farm acreage in Colorado is drip irrigated, although Greenberg said she sees more and more sprinkler systems replacing wasteful flood-irrigation systems.

In another solution, seawater or treated wastewater could replace freshwater for crops, Davies said.

Currently, the United States treats 70 percent of its wastewater but uses only about 4 percent, mostly for applications such as farming, according to a study in the September issue of the journal of Agricultural Water Management. But that number could increase as water becomes scarcer, the study cautioned. And Australia, the driest continent, has already commissioned several desalination plants along its eastern coastline.    

Drought-prone regions will also have to shift crop production, relying on less-thirsty plants for agriculture, Davies said.

Tweaking plant growth

But changing how water is used may not be sufficient: Many climate-change models predict that some regions, such as the Southwest, may face more frequent droughts. Even today, water scarcity is a threat farmers routinely face. To provide for 11 billion people, farmers will have to know how to manipulate plants' own systems for dealing with drought.

For instance, slightly water-stressed plants redirect their sugar formation into seeds and fruits at the expense of leaves and branches, which lose water easily. So, for example, farmers who grow wheat or grapes could increase their yield by actually watering their crops less at key times in the growing season, Davies said. [Dry and Dying: Images of Drought]

And cities could rely on massive glass towers to grow their food. Those futuristic buildings would lose no water to evaporation, would recycle nutrients from fertilizer and crops, and could rely, in part, on treated wastewater from a city, Davies said. Indoor farming is starting to transition from sci-fi to reality. At one of the first commercial vertical farms, a Singapore-based company called Sky Greens grows about a half-ton of bok choy and cabbage in three-story greenhouse towers. 

Good news, bad news

In theory, there could be enough water for everyone on the planet. The trick is to use it intelligently and get it to the people in most dire need, Boccaletti said.

Ideally, water-rich areas, such as Argentina, should export items that require lots of water to produce (such as beef), while parched areas should devote their efforts to more water-sparing products, said Nico Grove, a researcher at the Institute for Infrastructure Economics and Management in Germany. Beef — which requires roughly 4,000 gallons (15,000 liters) of water for every 2.2 lbs. (1 kilogram) produced, according to the United Nations' UN-Water program — could be produced in the Amazon River Basin, the largest watershed in the world. In contrast, drier regions, such as the Middle East — which, from 2002 to 2009, pumped enough groundwater from the ground to fill the Dead Sea, according to a 2013 study in the journal Water Resources Research — could harvest fruits from drought-resistant crops such as xerophyte plants, cactus-related plants that are water-sparing.

For this water-transfer idea to work, an index of global water usage is needed, Grove said. One way to do this is by measuring virtual water, or how much water went into the production of an item. Using that metric in the economy could help countries shift their manufacturing and agricultural priorities to keep their production in line with their water resources. Some regions, such as the Middle East, could wind up importing most of their food based on this metric, he said.

But improving the water situation takes money, political will and good governance. The richest countries may be able to spend their way out of water shortages, Boccaletti said, but the places that need water the most, such as sub-Saharan Africa, are least equipped to fix the problem. Those regions are caught in a bind: They need cheap, safe water to grow economically, but they lack the money to create new infrastructure — such as reservoirs, canals and dams — that would help them get that water, he said. 

"Water is nature's currency for the entire economy," Boccaletti said. "Economic growth depends on us getting water for the right purposes."

People who rely on the Colorado River are facing that problem now. A June 2013 study in the journal Bulletin of the American Meteorological Society suggests that the low flows will worsen in the future. To protect the resource, everyone who relies on the river must agree on a new way to allocate its waters, Greenberg said.

Politicians, conservation groups and grassroots organizations have proposed several ideas, from far-fetched schemes, such as moving cellophane-wrapped icebergs to the coasts or diverting water from the Mississippi River, to politically unpopular solutions, such as decommissioning water-based power plants and limiting population growth.

Greenberg said that improving crop efficiency and creating positive incentives for farmers to save water are key. One option is water leases, in which farmers, anticipating a dry year, could temporarily fallow their land and lease their water to municipalities that need it. The laws should be changed as well, she said. Currently, the legal regime that governs water use for the Colorado River States, the "Laws of the Rivers," make water a use-it-or-lose it commodity, in which farmers risk losing water rights if they use less. Those rules discourage water conservation, she said.

But only time will tell whether measures like these will be enough, and whether people and governments will be willing to act.

Follow Tia Ghose on Twitter and Google+. Follow LiveScience @livescience, Facebook & Google+. Original article on LiveScience.

Water is a resource that much of the developed world takes for granted, but that many in the developing world struggle to find enough of every day.

That struggle could spread as climate change and other manmade pressures change the availability of water around the globe, and as Earth's population grows ever larger, making the need for that resource even more acute.

The number of humans on the planet could reach 11 billion people by the end of the century, the United Nations projects, up from just over 7 billion people now. Already, more than 2 billion people face a water scarcity each month, but tremendous amounts of water are still wasted. [What 11 Billion People Mean for Water Scarcity]

From lawns to flood irrigation, here are five ways that people waste water and some ways to reduce that waste.

Irrigation

Agriculture uses about 70 percent of the available freshwater on the planet. Around the world, most farming relies on flood irrigation — where fields are drenched with water and the excess runs off into nearby streams and rivers.

But flood irrigation wastes tons of water and can pollute waterways with fertilizers, creating dead zones in the ocean (where oxygen is used up and not available for marine creatures) and contributing to algal blooms, which can be toxic to marine life.

Some regions, such as Israel, have moved to highly efficient drip irrigation, which directs water right onto the roots of the plant. But such systems are expensive to implement and don't work for all crops, so many regions will probably shift toward intermediate solutions such as sprinklers, which produce less waste runoff, and covering crops to prevent water evaporation.

Lawns

Lawns are one of the thirstiest water hogs in cities and towns. While lawns may be appropriate in some areas, most green expanses aren't made of local grasses adapted to grow in the area. And the vast majority of manicured front yards require hefty watering to flourish.

As cities tighten their belts, some areas may require residents to water lawns less frequently or forgo lawn-watering altogether. In particularly arid regions, that may mean a lawn of cacti or rocks, whereas other areas may rip out the water-hungry grass species, such as St. Augustine, and replace them with mixtures of native grasses that guzzle less water. As a bonus, many of these native grasses are softer and less itchy than the old standbys.

Poor crop choice

As the population grows, it doesn't make sense for desert-dwellers to grow thirsty crops such as cotton or raise cattle, which requires much more water than producing an equivalent weight of wheat or potatoes.

As the planet becomes drier, countries will have to shift their economies, so that drier regions produce less thirsty products and wetter regions make water-hungry products such as beef.

Newer plants

But simply switching which crops are produced may not be enough for some regions of the world. Instead, they may need to manipulate the plants own systems' for dealing with drought to increase production.

One way to do that is to water crops less during certain parts of the harvest. The plants then direct more growth into the fruit, away from leaves and stems. That means farmers can grow more crops with less water. 

Flushed down the toilet

One of the biggest sources of usable water is treated wastewater. After people brush their teeth, wash their vegetables or flush the toilet, most of that water is treated and sanitized.

While that water isn't really suitable for a big glass of water (unless you're on the International Space Station), much of it could be put to use watering crops, freeing up freshwater for drinking. Currently, the United States treats 70 percent of its wastewater, but only uses 4 percent of that amount. Increasing the wastewater usage would provide more water for everyone.

Follow Tia Ghose on Twitter and Google+. Follow LiveScience @livescience, Facebook & Google+. Original article on LiveScience.

The loo, the W.C., the lavatory, the privy, the porcelain god — while it goes by many names, the toilet — one of life's most mundane objects — plays a fundamental role in society.

Yet more than a third of the world's population lacks access to even a basic pit latrine, and the problem may get worse. A recent statistical analysis predicts the world population will hit 11 billion by 2100. From preventing illness to fostering education, here are five ways toilets change the world:

1. Keeping people healthy

Improper disposal of human waste can cause devastating illness. When people don't have toilets, they defecate in the open, often near living areas or the rivers that supply water for drinking or bathing. For example, about 290,000 gallons (1.1 million liters) of raw sewage goes into the Ganges River in India every minute, according to the World Health Organization. [What 11 Billion People Mean for Sanitation]

Contaminated water causes diarrheal diseases such as cholera, which afflict many people on a chronic basis. In 2012, heavy rains in Sierra Leone and Guinea caused latrines to flood, bringing on a deadly cholera outbreak that killed more than 392 people and sickened more than 25,000 others, according to news reports.

Diseases caused by fecal contamination also lead to malnourishment, low birth weight, cognitive problems and stunted growth. Poor sanitation contributes to two of the three leading causes for preventable death among children under five years old.

2. Preventing blindness

Trachoma, the leading cause of preventable blindness, is carried by a fly that breeds exclusively on human excrement. The disease is caused by Chlamydia trachomatis, a bacterium that also causes the sexually transmitted disease Chlamydia. Flies and contact with eye discharge from an infected individual can both spread the disease.

Trachoma affects about 21.4 million people, according to the World Health Organization. Of these, about 2.2 million are visually impaired and 1.2 million are blind.

3. Keeping women safe

In places without toilets, women must travel farther away to relieve themselves, which places them at risk of sexual violence. To avoid that danger, many women use so-called "flying toilets" — basically plastic bags that they keep in their houses. Flying toilets are a breeding ground for nasty microbes, such as the bacterium responsible for the blindness-causing disease trachoma.

4. Promoting school attendance

Talking about toilet matters is taboo in many places, particularly among women. Young girls may stop attending school if the building lacks private toilet facilities, which ultimately limits these girls' access to education.

But the solution isn't always straightforward. For instance, some aid workers have suggested installing public toilet blocks. However, when toilet blocks were installed in Bhopal, India, as part of a study in November 2008, men were twice as likely as women to use them.

5. Saving energy

Wastewater from toilets contains about 10 times the amount of energy, in biochemical form, as that needed to treat it. Scientists and engineers are developing ways of processing wastewater to save energy and reclaim drinking water. 

For instance, the Bill and Melinda Gates Foundation started the Reinvent the Toilet Challenge to develop sanitary, waterless toilets that don't require a sewer connection or electricity, and would cost less than five cents per user per day.

Clearly, a toilet is far more than a place to store waste.

Follow Tanya Lewis on Twitter and Google+. Follow us @livescience, Facebook & Google+. Original article on LiveScience.

Editor's note: By the end of this century, Earth may be home to 11 billion people, the United Nations has estimated, earlier than previously expected. As part of a week-long series, LiveScience is exploring what reaching this population milestone might mean for our planet, from our ability to feed that many people to our impact on the other species that call Earth home to our efforts to land on other planets. Check back here each day for the next installment.

Beetles, scorpions and other insects may not be found on most restaurant menus — at least in the Western world — but they may need to find a place in human diets, if society is to feed the world's booming population.

At least that's one solution, albeit an unconventional one, offered up in a 200-page-plus report released in May by the United Nations' Food and Agriculture Organization, in which the group outlined the potential of edible insects to help alleviate food insecurity in the present and future.

"To meet the food and nutrition challenges of today — there are nearly 1 billion chronically hungry people worldwide — and tomorrow, what we eat and how we produce it needs to be re-evaluated," the report reads. "We need to find new ways of growing food."

Although eating insects may sound like a strange prospect to some people, such broad-minded thinking may be necessary at a time when human population growth shows no signs of slowing down.

The world's population is projected to hit 11 billion by 2100, and exactly how the planet will feed this growing population is one of the biggest questions society faces in the coming years, experts say. The new 2100 population estimate, released in a new U.N. report in June, is 800 million more people than previously predicted by that time. Much of the increase is due to birthrates in Africa not falling as quickly as expected. [What 11 Billion People Mean for the Planet]

However, the world's future food security is not a simple matter of producing more food. Rather, food security relies upon a number of intertwining factors, including population size, climate change, food production, food utilization (for things like animal feed and biofuels) and prices, experts say. Humans also have to pay close attention to their use of the Earth's resources, or risk making the situation worse, according to the World Resource Institute, a nonprofit organization that aims to protect the Earth for current and future generations.  

Experts agree the planet can definitely produce enough food for 11 billion people, but whether humans can do it sustainably, and whether consumers will ultimately be able to afford that food, are not guarantees. Feeding the growing population will likely require a number of different strategies — from creating new crop varieties and reducing food waste to, yes, eating insects — with efforts from governments, farmers, the private sector and consumers themselves.

"The world's facing a great balancing act," said Craig Hanson, director of the People & Ecosystems Program at WRI. "On the one hand, the world needs to feed more people," Hanson said. "At the same time, you want agriculture to continue to advance economic and social development. And we've got to reduce agriculture's impact on the environment." There's no easy way to meet all of those demands, Hanson added.

Challenges

To feed just 9 billion people (the estimated population in 2050) would require a 60 percent increase in the number of food calories available for human consumption, according to WRI, based in Washington, D.C. When taking into account food needed to feed livestock, the world needs to increase crop production by 103 percent, or 6,000 trillion calories per year, according to WRI, which released a series of reports this year about the world's food security future.

One obstacle to increasing food production will be climate change, which is predicted to reduce crop yields in certain parts of the world. A 2009 study published in the journal Science found that, in 2100, regions in the tropics and subtropics are very likely to experience unprecedentedly warm temperatures during the growing season, reducing crop yields in the tropics by 20 to 40 percent. About 3 billion people, or nearly half the world's population, live in the tropics and subtropics, and the population in these regions is growing faster than anywhere else, the researchers said.

Extreme weather events, such as heavy rains and flooding, as well as drastic changes in weather in a short period will also pose challenges for crop production, said Walter Falcon, deputy director of the Center on Food Security and the Environment at Stanford University.

Falcon pointed out that while U.S. agriculture was affected by drought in 2012 — the most extensive drought since the 1950s — farmers had to contend with the opposite, heavy rains, this year. Rains can prevent farmers from planting their crops at the optimal time, or prevent them from planting altogether in certain areas that are flooded, said Falcon, who owns a farm in Iowa that was hit by the drought.

Changes to the food supply — which can occur when crop production is reduced by extreme weather events or when countries designate a portion of food crops to be turned into fuel, like the United States does with 40 percent of its non-exported corn crop — can push up food prices and affect people's ability to afford food. Using corn to produce ethanol has caused corn prices to increase, Falcon said.

In the midst of last year's drought, corn prices rose 50 percent, to $8 a bushel. Because corn is also used for animal feed, an increase in corn prices can affect the cost of other foods. "Corn is kind of a linchpin commodity," Falcon said. Most experts don't think the United States will increase the amount of corn that goes to ethanol in the near future, but over the course of the century, that could change, Falcon said.

Improving trade cooperation

To continue to feed a growing population in light of the food shortages that are likely to occur with climate change, global crop production in the future will have to be much more coordinated than it is today, said Jason Clay, an expert in natural resources management at the World Wildlife Fund (WWF).

"We're going to have to work to make sure that we have a global food system that takes care of everybody," Clay said. Because extreme weather events may cause crop yields to be destroyed in certain parts of the world in a given year, such a system should be able to shift food from areas that have plenty to those that have less, Clay said. [Can the World Feed 11 Billion People? (Infographic)]

Falcon agreed. Currently, certain restrictions on trade exist that may prove problematic in the future, such as when countries ban exports if their crop production is down. The idea that each country should be self-sufficient in food production is not the answer, Falcon said.

"In a world of lots of [climate] variation, there is a lot of work to be done in getting trade flows straightened out," Falcon said.

Reduce food waste

Another strategy to help ensure food security in a world with so many hungry mouths to feed is to simply reduce food waste. One out of every four calories that's produced for human consumption today is lost or wasted, according to WRI. (Food loss refers to food that spoils, spills, etc., before it reaches the consumer, while food waste refers to food that is discarded by the consumer, either when it is still edible, or after it spoils due to negligence, according to WRI.) The average American household loses $1,600 a year on wasted food, Hanson said.

Some 56 percent of global food loss and waste occurs in the developed world — particularly in North America and Oceania, where about 1,500 calories are lost or wasted per person per day, WRI reported. In developed countries, the majority of food is wasted at the consumption stage, whereas in developing countries, most food loss occurs during production, handling and storage.

A number of changes could reduce food loss and waste around the world. For example, better storage facilities on farms in Africa — and even putting harvested crops in plastic storage bags — would reduce the amount of food that falls victim to pests there, Hanson said.

And using simple plastic crates — instead of bags and sacks — to transport food to market can reduce food damage, such as bruising and smashing, that would otherwise cause goods to be inedible. Introducing plastic crates to farmers in a town in Afghanistan — a $60,000 project sponsored by the nonprofit international development organization CNFA — reduced tomato losses from 50 percent to 5 percent, according to WRI.

At home, Americans can reduce the amount of food they throw away, perhaps by eating leftovers, or not preparing more food than they'll need for a given meal, Hanson said.

Americans also commonly have misperceptions about the meaning of labels with dates on foods, and may throw away food before it has really "gone bad," according to a WRI report. These labels, which typically read "sell-by," "best if used by" or "use-by," refer to the quality or flavor of the food, but not to food safety (whether the food would cause someone to be sick). "So while food that has passed its 'sell-by' date might be less desirable than newly purchased food, it is often still entirely safe to eat," WRI reported. Governments may be able to help by creating guidelines on what types of labels should appear on packages, and then explaining to consumers what the labels mean, according to WRI.

Eat differently

Even with less wasted food, the world could not support 11 billion people who eat the way Americans do today, said Jamais Cascio, a distinguished fellow at the Institute for the Future, a think tank in Palo Alto, Calif. Feeding 11 billion people would require a different diet, which may involve eating less meat, or consumers growing more of their own food, Cascio said.

Beef, in particular, is a very unsustainable food to eat, Cascio said. "If we get away from thinking that feeding 11 billion people means giving them all Big Macs and steak sandwiches, then we're at a better starting point," Cascio said. According to an analysis by Cascio, the greenhouse gas emissions generated by the production of cheeseburgers in the United States each year is about equal to the greenhouse gas emissions from 6.5 million to 19.6 million SUVs over a year (There are about 16 million SUVs on U.S. roads, Cascio said.) [6 Ways to Feed 11 Billion People]

Scientists have been working on developing cultured meat, or meat grown in a lab, Cascio said. Earlier this year, researchers in the Netherlands showcased their lab-grown burger, and allowed a taste test. However, right now, the cost is exorbitant (a single burger costs $325,000), and it does not taste exactly like meat (taste-testers said the burger was dry). But with future research, the price is likely to come down, and the product's taste could improve, Cascio said.

And don't forget the insects. Beetles, wasps, grasshoppers and other insects are very efficient at converting the food they eat into body mass, take up very little space and emit fewer greenhouse gases than livestock, according to the U.N.'s FAO report. Although eating insects comes with an "ick factor" for many Westerners, bugs are a part of the diet of about 2 billion people worldwide, according to the report.

Grow differently

Farmers could also focus on growing crops that provide the most calories while using the fewest resources, said Clay, of the WWF. Bananas and plantains are examples of crops that provide a lot of calories compared with the resources it takes to grow them, Clay said.For instance, 1 kilogram of bananas (2.2 lbs.) contains about 1,000 calories, and uses about 500 to 790 liters of water to grow. On the other hand, producing 1,000 calories of beef takes about 5,133 liters of water. (One kilogram of beef contains about 3,000 calories and requires about 15,400 liters of water to produce.)

In addition, crop production in certain parts of the world is not very efficient, Clay said. Efforts should be made to improve crop production in those areas, using the foods that are already grown and eaten by the people there, Clay said. Some native crops — such as pigeon peas and pulses in South Asia, and cowpeas and millet in Africa — have not yet benefited from plant-breeding techniques that could improve productivity, he said.

Innovations from scientists to come up with hardier crops, either through genetic engineering or traditional crop-breeding techniques, may also help protect against crop losses in the future due to extreme weather conditions, said Tim Thomas, an economist at the Washington, D.C.-based International Food Policy Research Institute, an international nonprofit organization that aims to find sustainable solutions for ending world hunger and poverty.

"You could picture developing varieties that are resilient to more than one shock," Thomas said, referring to varying weather and climate conditions, such as rains, flooding and heat.

Such a strategy would be similar to the one employed in the green revolution, in which research and development was used to increase crop production worldwide from the 1940s to 1970s.But this time, humans will have to work with the land they have, rather than bringing new land into production, Thomas said. Improving crop varieties will help use land more efficiently, he said.

"We need a new green revolution," Thomas said.

Follow Rachael Rettner @RachaelRettner. FollowLiveScience @livescience, Facebook & Google+. Original article on LiveScience.

Feeding the world's growing population will not be an easy task. By 2100, it's estimated that there will be 11 billion people on the planet, 3 billion more than there are today. And 870 million people worldwide are already chronically hungry.

The planet can definitely produce enough food for 11 billion people, experts say, but whether humans can do it sustainably, and whether consumers will ultimately be able to afford that food, are separate matters. Humans can't rely on a single solution to feed a population of this size, experts say. A number of different strategies will be required, each of which will move humans a little bit closer toward closing the gap between the amount of food they have, and the amount of food they need.

Here are six possible strategies to help feed 11 billion people:

Eat less meat

As society tries to feed the growing population, it will have to pay close attention to the use of the Earth's resources, or risk making the situation worse.

Beef in particular is not a very sustainable food to eat, said Jamais Cascio, a distinguished fellow at the Institute for the Future, a think tank in Palo Alto, Calif. According to Cascio's calculation, the greenhouse gas emissions generated by the production of cheeseburgers in the United States each year is about equal to the greenhouse gas emissions from 6.5 million to 19.6 million SUVs over a year.

To feed 11 billion people, Americans will need to eat differently than they do today, which may involve eating more vegetables, which take much less energy to produce, and less meat, Cascio said. [What 11 Billion People Mean for Food Security]

Eat fake meat?

Another, perhaps more outlandish, solution may be to eat meat that isn't from an animal at all. Scientists have been working to develop cultured meat, or synthetic meat grown in a lab. Earlier this year, researchers in the Netherlands showcased their lab-grown burger, and allowed a taste test. However, right now, the cost is exorbitant (a single burger costs $325,000), and it does not taste exactly like meat (taste-testers said the burger was dry). But with future research, the price is likely to come down, and the product's taste could improve, Cascio said.

However, some have been skeptical that lab-grown meat would be truly more sustainable than meat from cows. Cultured meat still requires nutrients, and currently, researchers "feed" lab meat, in part, with blood from cow fetuses, according to a 2012 Discover Magazine post by Christina Agapakis, a synthetic biologist at UCLA. Researchers have proposed that they could one day use algae to feed cultured meat, but this has not been proven.

Throw less food away

One big inefficiency in today's food systems is how much food is wasted: One out of every four calories that's produced for human consumption today is not ultimately consumed because it is lost or wasted, according to the World Resource Institute, a nonprofit organization that aims to protect the Earth for current and future generations. 

Some 56 percent of global food loss and waste occurs in the developed world. And the average American household loses $1,600 a year on wasted food, WRI says.

At home, Americans can reduce the amount of food they throw away by eating leftovers, or not preparing more food than they'll need for a given meal, said Craig Hanson, director of the People & Ecosystems Program at WRI.

Aquaponics

One up-and-coming idea for sustainable food production is actually based on an ancient concept called aquaponics, a system that combines fish farming with plant farming in water. The fish fertilize the plants, and the plants clean the water for the fish, according to Johns Hopkins Bloomberg School of Public Health, which has its own aquaponics project.

"You can eat the fish, you can eat the plants, and it keeps going," Cascio said of the system. "It gives a much more efficient use of land," he said.

The idea for aquaponics appears to have arisen hundreds of years ago, when farmers in Southeast Asia found that they could add tilapia to their rice paddy fields to improve production yields, according to Michigan Technological University.

Vertical farming

Given the scarcity of new agricultural land to grow food, some have proposed taking farming into the sky: growing crops in so-called vertical farms. Dickson Despommier, an ecologist and professor at Columbia University, said that food grown in skyscrapers would have many advantages. Food produced in vertical farms would not be in danger of being lost due to extreme weather events, and because the farms would be inside cities themselves, crops would not need to be shipped thousands of miles, Despommier wrote in an essay on his website.

However, the idea of vertical farming has not been proven. And some researchers have argued that the cost of lighting indoor vertical farms would be too expensive, according to a 2010 Economist article.

Improve crop production worldwide

Crop production in certain parts of the world is not very efficient, said Jason Clay, an expert in natural resources management at the World Wildlife Fund (WWF), a conservation organization. Efforts should be made to improve crop production in those areas, using the foods that are already grown and eaten by the people there, Clay said. Some native crops, such as pigeon peas and pulses in South Asia, and cowpeas and millet in Africa, have not yet benefited from plant breeding techniques, which could improve productivity, he said.

"We need to recognize that the best producers in the world of a crop are 100 times better than the worst. Bottom producers [who are the least efficient] have the most to gain," Clay said.

Follow Rachael Rettner @RachaelRettner. FollowLiveScience @livescience, Facebook & Google+. Original article on LiveScience.

A population explosion

The human population is exploding. Earlier this year, the United Nations released a new report that said the global population was on pace to reach 11 billion people by the end of the century — a full 800 million more people than were expected by 2100 (with a range between 9 million and 13 million), and a whopping 4 billion more than call Earth home today. The bump in the predicted number was made in part because birthrates in sub-Saharan Africa are not dropping as fast as predicted.

All of those people mean a lot of extra mouths to feed, more strain on water supplies, a lot more trash and human waste to put somewhere and an increased threat of a major deadly global pandemic, among other problems.

As part of a weeklong series, LiveScience is taking a look at what impact a population of 11 billion might have on our Pale Blue Dot, and in what ways humans might need to adapt. The series explores food security, water security, climate change, Earth's animals, disease outbreaks, sanitation and space travel. Here's a brief look at each issue.

Food security

Experts agree the planet can produce enough food for 11 billion people, but whether humans can do it sustainably, and whether consumers will ultimately be able to afford that food, are not guarantees.

The world's food security future is not a simple matter of producing more food. Rather, food security is affected by a number of intertwining factors, including population size, climate change, food production, food use (for things like animal feed and biofuels) and prices, experts say. The world's population will also have to pay close attention to its use of Earth's resources, or risk making the situation worse. [Read how 11 billion people will affect food security here.]

Water security

Today, 2.7 billion people around the world face some water shortage in their daily lives. Clean, fresh water is a source of conflict in the U.S. Southwest, the Middle East and sub-Saharan Africa. Most people expect these conflicts to grow in the coming years. But these water woes are just a foreshadowing of the problems to come if the world's population hits 11 billion people by the end of the century, which will make providing clean water for everyone an increasing challenge.

In order to meet this challenge, scientists will need a better estimate of how much water is available, and people will need to invest in efficient water infrastructure and employ water-sparing farming techniques around the world, experts say. But even that may not be enough to provide for a thirsty planet. Some regions of the world may have to stop producing water-hungry crops and products altogether, leaving that to countries with more ample water supplies.

Climate Change

If greenhouse gas emissions continue to rise, and the global population swells to 11 billion people, could this growth worsen the effects of climate change, and overwhelm humanity's ability to fight global warming? Within the scientific community, this debate is brewing, but there is little consensus about how — or even if — population growth and climate change are directly linked.

Population growth is expected to surge in developing nations, particularly in sub-Saharan Africa. These regions have much smaller carbon footprints than developed countries. But, some climatologists say rapid industrialization of developing nations and changes in their consumption levels could add stress to Earth's fragile ecosystems.

[Read how 11 billion people might affect climate change here]

Earth's animals

Growing human populations spell trouble for animals, and people are in the midst of driving the sixth-largest mass-extinction in the history of Earth, most biologists say. In general, conservationists and scientists are extremely worried about what the world's animal populations may look like if the human population grows to 11 billion.

Population growth is leading to destruction of wildlife habitat, and increasing demand for wildlife products. Some good news is that the richest animal diversity is found in a few spots, which could make conservation of these vital places easier. But it has to be made a priority, which is often not the case, scientists say.

Disease outbreaks

Scientists have documented an increasing rate of infectious diseases caused by new viruses and bacteria over the past few decades, which has happened alongside population rise. They say the rate of infections is likely to keep rising in the future.

Higher numbers of people, clustered in dense areas and traveling all over the globe could make it easier for a virus to be transmitted to more people, spreading any outbreak wider and faster than previously possible. And the fate of outbreaks is also being altered through the interactions of humans with the environment, as people move into areas inhabited by wildlife in search of places to live and grow food. Many agents of infectious diseases originate in animals, and human contact with wildlife species facilitates the transmission of novel viruses.

Some researchers are out in tropical regions hunting for emerging viruses while others emphasize bolstering public health research so we're better able to face the next pandemic.

Sanitation

Nobody likes to mention it, but the world faces a major poop problem with the sharp rise in the human population. Not to mention all the trash humans generate.

Hong Kong and other small countries are already grappling with overflowing landfills. The amount of waste generated per capita is rising as urbanization causes greater consumerism. And worldwide, 2.6 billion people live without access to basic sanitation. The improper disposal of human waste leads to serious health problems. In some regions, the lack of sanitary facilities limits children's access to education.

The population outlook is grim, unless humans find ways to recycle waste in sanitary and energy-efficient ways, experts say.

Space travel

The ongoing population boom could accelerate the pace of climate change, strain the availability of key resources such as fresh water and threaten biodiversity around the globe. But it may have an impact beyond Earth as well, making humanity more able, and perhaps more willing, to leave our home planet and begin settling the solar system. More people overall means more millionaires and billionaires to pay for private spaceflight, which could help the industry develop, some experts said, though they warn that it won't be a Band-Aid for fixing overpopulation of our home planet.

Eleven billion people. That's the number of humans the United Nations estimates could call Earth home by the end of this century — or more than 4 billion people than exist today. That's a staggering amount compared to the mere 2.5 billion people alive in 1950.

Those 11 billion people stand to stamp an enormous footprint onto the Earth: All of them must eat and have enough drinking water; they all generate waste and potentially help to spread disease; they could affect the planet's already changing climate and how Earth's other species fare; they could even affect whether humans become an interplanetary species.

In a weeklong series, LiveScience is taking a look at the impacts a population of 11 billion people might have on our home planet. Our reporting will transport you to America's heartland farms, Hong Kong's rapidly-filling landfills, lemur territory within the jungles of Madagascar, and many other places where the pressures of the world's burgeoning population are increasingly being felt, and finally to Mars, to where we may one day expand.

Each day, we'll publish a new story, along with accompanying features, according to the schedule below. So check back each day to find out how world population affects Earth and humanity's ability to live on it.

Tuesday, Nov. 19

What 11 Billion People Mean for Food Security

What 11 Billion People Mean for the Planet

6 Ways to Feed 11 Billion People

Can the World Feed 11 Billion People? (Infographic)

Wednesday, Nov. 20

What 11 Billion People Mean for Water Scarcity

5 Ways We Waste Water

Thursday, Nov. 21

What 11 Billion People Mean for Climate Change

5 Places Already Feeling the Effects of Climate Change

Friday, Nov. 22

What 11 Billion People Mean for Earth's Animals

7 Iconic Animals Humans Are Driving to Extinction

Monday, Nov. 25

What 11 Billion People Mean for Disease Outbreaks

5 Scariest Outbreaks of the Past Century

Tuesday, Nov. 26

What 11 Billion People Mean for Sanitation

5 Ways Toilets Change the World

Managing the World's Waste (Infographic)

Wednesday, Nov. 27

What 11 Billion People Mean for Space Travel

Red Planet or Bust: 5 Manned Mars Mission Ideas

The world's population could reach 11 billion by the year 2100, according to a new statistical analysis.

That represents 800 million more people than was forecast in 2011. Most of that increase comes because birth rates in Africa haven't dropped as fast as projected.

"The fertility decline in Africa has slowed down or stalled to a larger extent than we previously predicted, and as a result the African population will go up," said study co-author Adrian Raftery, a statistician at the University of Washington, in a statement.

Ever increasing

The United Nations reported that the population hit 7 billion in October 2011. That's an amazing increase from the mere 5 million people who lived on the planet in 8000 B.C. or the 1 billion who were alive in 1805.

The huge surge in population is expected to cause mega-city populations to swell, which could worsen environmental problems and overcrowding.

Right now, Africa's population stands at 1.1 billion, but that is expected to increase four-fold, to 4.2 billion, by 2100.

Rest unchanged

The rest of the world is unlikely to see big changes from the past estimate. Europe may see a slight dip in population, because it continues to have a below-replacement birth rate, meaning more people are dying than being born.

The new analysis used a more sophisticated method for estimating life expectancy, updated fertility forecasting methods and new population data.

The model predicts that the population will likely reach between 9 billion and 13 billion by 2100. By contrast, the U.N.'s population estimates assume the average birth rate may vary by up to 0.5 children per woman, which results in a large range for the world's population at the end of the century, between 7 billion and 17 billion.

The findings suggest that experts should redouble their efforts to curb population growth in Africa, Raftery said.

"These new findings show that we need to renew policies, such as increasing access to family planning and expanding education for girls, to address rapid population growth in Africa," Raftery said in a statement.

Follow Tia Ghose on Twitter and Google+. Follow LiveScience @livescience, Facebook & Google+. Original article on LiveScience.com.

Sir David Attenborough, the famed British naturalist and television presenter, has some harsh words for humanity.

"We are a plague on the Earth," Attenborough told the Radio Times, as reported by the Telegraph. "It's coming home to roost over the next 50 years or so."

Attenborough went on to say that both climate change and "sheer space" were looming problems for humanity.

"Either we limit our population growth or the natural world will do it for us, and the natural world is doing it for us right now," he said.

Sir David is not the only naturalist who has warned of population growth outstripping resources.  Paul Ehrlich, the president of the Center for Conservation Biology at Stanford University and author of "The Population Bomb" (Sierra Club-Ballantine, 1968) has long used language similar to Attenborough's. And in 2011, an analysis of species loss suggested that humans are beginning to cause a mass extinction on the order of the one that killed the dinosaurs.

When asked about Attenborough's comments on humanity as its own scourge, Ehrlich told LiveScience he "completely agree[d], as does every other scientist who understands the situation." [Top 10 Ways to Destroy Earth]

Even so, that doesn't mean forceful measures must be taken. "Government propaganda, taxes, giving every sexually active human being access to modern contraception and backup abortion, and, especially, giving women absolutely equal rights and opportunities with men might very well get the global population shrinkage required if a collapse is to be avoided," Ehrlich said.

In fact, providing free, reliable birth control to women could prevent between 41 percent and 71 percent of abortions in the United States, according to a study detailed in the Oct. 4, 2012, issue of the journal Obstetrics and Gynecology.

Other scientists also agreed to some extent with the heart of Attenborough's message.

"It's clear that increasing population growth makes some of our biggest environmental challenges harder to solve, not easier," said from Jerry Karnas, population campaign director for the Center for Biological Diversity in Tucson, Ariz.

Karnas added, however, "What's needed is not population control but a real emphasis on reproductive rights, women's empowerment, universal access to birth control and education, so more freedom for folks to make better, more informed family planning choices."

And population numbers would matter less for the planet's health if clean renewable energy were widely adopted as well as planning laws, he told LiveScience during an interview.

Attenborough is famous for his "Life on Earth" series of wildlife documentaries, among other nature programming. In 2009, he became a patron of the Optimum Population Trust, a group that advocates voluntary population limitation. At the time, he released a statement saying, "I've seen wildlife under mounting human pressure all over the world and it's not just from human economy or technology — behind every threat is the frightening explosion in human numbers."

Earth's population reached 7 billion people on or around Oct. 31, 2011, according to United Nations estimates.

LiveScience Staff Writer Tia Ghose contributed reporting to this article.

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Humanity is 17 million tons (15 million metric tons) overweight, according to a study that calculates the adult portion of the human race's collective weight at 316 million tons (287 million metric tons).

That's the equivalent of about 170 military aircraft carriers of extra weight. Or in people weight, it's like having an extra 242 million people of average body mass on the planet.

This is more than just an attempt to make the human race feel uncomfortable about its waistline; looking at the collective mass of humanity can improve understanding of the effects of population growth, contends a team of European researchers.

"[United Nations] world population projections suggest that by 2050 there could be an additional 2.3 billion people," they write in research published online Sunday (June 17) in the journal BMC Public Health. "The ecological implications of rising population numbers will be exacerbated by increases in average body mass." [7 (Billion) Population Milestones]

The argument is simple. More body mass takes more energy to maintain and move; therefore as someone's weight goes up, so do the calories they need to exist. This means increases in population counts don't tell the whole story when it comes to demand for resources, according to the authors.

"Although the largest increase in population numbers is expected in Asia and sub-Saharan Africa, our results suggest that population increases in the USA will carry more weight than would be implied by numbers alone," they write.

The United States ranked at the top of the "Heaviest 10" category, while the "Lightest 10" list is composed entirely of African and Asian nations. For example, North America has 6 percent of the world population but 34 percent of biomass due to obesity. Meanwhile, Asia has 61 percent of the world population but just 13 percent of biomass due to obesity. [List of heaviest and lightest nations]

Using data from around the world for 2005, researchers used body mass indexes (BMI, or a measure of body fatness) and height distributions to estimate average adult body mass. They then multiplied these results by population size to get a total mass, referred to as biomass.  They evaluated body mass using BMI thresholds of greater than 25 for overweight and greater than 30 for obese. The collective mass of the adult population in 2005 due to obesity was 3.9 million tons (3.5 million metric tons), they calculated.

Globally, average body mass globally for an individual was calculated at 137 pounds (62 kilograms). 

"Our scenarios suggest that global trends of increasing body mass will have important resource implications and that unchecked, increasing BMI could have the same implications for world energy requirements as an extra 473 million people," they write. "Tackling population fatness may be critical to world food security and ecological sustainability."

Follow LiveScience writer Wynne Parry on Twitter @Wynne_Parry or LiveScience @livescience. We're also on Facebook & Google+.

The 7 billionth person on Earth will draw his or her first breath on Oct. 31, at least according to estimates by the United Nations. Assuming all systems are in working order, that baby will also create its first output that same day, in the form — to put it delicately — of a dirty diaper.

That dirty diaper is only the tip of an iceberg of human manure produced around the globe every day. It might seem a reasonable question to ask how humanity will deal with this output of feces as the world's population creeps toward 10 billion by 2100. But that question presumes we have the poop problem under control now. Here's the bad news: We don't.

Approximately 2.6 billion people around the world lack any sanitation whatsoever. More than 200 million tons of human waste goes untreated every year. In the developing world, 90 percent of sewage is discharged directly into lakes, rivers and oceans. And even in developed countries, cities depend on old, rickety sewage systems that are easily overwhelmed by a heavy rain.

All this untreated sewage adds up to a major public health crisis that kills an estimated 1.4 million children each year, according to the World Health Organization. That's one child every 20 seconds, or more than AIDS, malaria and tuberculosis combined. Despite this massive death toll, sanitation hasn't gotten the same attention as other world development goals. The United Nations, which set a goal to halve the number of people without basic sanitation by 2015, now calls that target "out of reach."

"Sanitation is not a sexy issue," said Dan Yeo, a senior policy analyst at WaterAid, a nongovernmental organization dedicated to hygiene, water and sanitation issues. "It's about s---, and that's not particularly attractive. It's a taboo to talk about in a lot of contexts." [See images of the world's toilets]

Learning to talk about toilets

That taboo is one reason that sanitation hasn't taken off as a major issue in the public's mind, Yeo said. But providing sanitation is also more complex than "if you build it, they will come," according to Rose George, the author of "The Big Necessity: The Unmentionable World of Human Waste and Why It Matters" (Metropolitan Books, 2008).

"The assumption was that latrines would be used and that everyone needs a flush toilet," George told LiveScience of early sanitation efforts. "People not necessarily wanting latrines was provedin India when the government provided millions almost free in the 1980s, and then millions of these kind-of-adequate latrines got turned into goat sheds or storage areas, because people are used to just going and crapping in the bush."

"Crapping in the bush," also known as "open defecation," is a major problem, George said, because the pathogens from the feces invariably end up tracked back into the village, often contaminating the community water supply. 

Open defecation also puts people in rural areas such as sub-Saharan Africa at risk for snakebites as they go tramping into the bushes in the dark, George said. Women looking for a private place to go are at risk of being followed and sexually assaulted, she said. According to WaterAid, many women in Africa wait until nightfall to relieve themselves, putting themselves at risk of urinary tract infections, because propriety dictates that women don't go where someone might see them. [Sidebar: Top 16 Countries Without Sanitation]

To tackle the open-defecation problem, aid organizations had to learn to work with the people on the ground to explain why sanitation matters, Yeo told LiveScience. In Bangladesh, for example, WaterAid works with a local music-theater performance troupe that puts on sanitation-related skits for schoolchildren. 

In her travels, George uncovered enormous cultural differences in the way people think about using the bathroom. In China, for example, plenty of public bathrooms lack doors on the stalls — or even stalls. Meanwhile, Americans happily use toilets in stalls with large gaps below, above and on either side of the door, a fact that seems bizarre in George's native Britain. In the U.K., she said, public toilet stalls are completely closed off.

"You have to understand that it's about software — psychology — as well as just the hardware of putting in pipes and toilets," George said.

Providing the plumbing

But the hardware matters, too. For one thing, the latrines can't be more disgusting than the alternative they're replacing, George said. Who wants to hang out in a dank, fly-infested latrine when you could just move your bowels down by the river?

Urbanization is another challenge, Yeo said. According to the U.N., the proportion of people living in urban slums around the world has declined from 39 percent in 2000 to 33 percent in 2010. But the absolute number of people living in slums is actually growing, with about 828 million slum-dwellers worldwide in 2010.

In many cases, these slums are informal communities that local governments would rather not recognize officially, Yeo said.

"They're often on land they don't own, and they aren't recognized as having the rights to that land," he said.

That makes officials reluctant to solve the sewage problems in these slums, since adding them to the grid would amount to tacit approval of their existence, Yeo said.

Meanwhile, just the physical layout of urban slums makes adding latrines difficult. A high density of human beings means a high density of human waste. Narrow streets make it tough to get latrine-emptying trucks into the area. In urban settlements, Yeo said, it's often important to encourage planning by local governments so these engineering problems don't blindside cities later on. [Read: How Many People Can Earth Support?]

Sewers and 'fatbergs'

Investing in sanitation is by any measure a winning bet: According to the U.N., for every dollar invested in sanitation, $8 are returned in reduced public health costs and lost productivity due to disease. According to WaterAid, a $30 donation buys one person access to both clean water and sanitation.

The availability of a toilet can have wide-ranging effects, George said. In developing areas, she said, up to 20 percent of girls drop out of school, because they have no place to relieve themselves. Providing a latrine can mean the difference between illiteracy and education.

But while the developing world undoubtedly bears the burden of poor sanitation, it would be a mistake to think that developed countries have it all figured out, George said. Urbanization and population growth have taken their toll on the crumbling sewer systems beneath many municipalities, she said, and many sewer systems are forced to release untreated sewage when a sudden downpour swamps the system.

"In the U.S., there's a massive, multimillion-dollar gap between the funding that is needed to maintain the sewer system and what is being given," George said. "Even a five-minute rainstorm can overwhelm the sewer system."

Even worse, she added, people's "out of sight, out of mind" attitude means they abuse the sewer system we do have.

"I've been down the London sewers, and all the 'flushers' who work down there say, 'We don't mind the s---, but we do mind the fat,'" she said.

The fat, George said, is household and restaurant grease that gets poured down drains and congeals into enormous "fatbergs," floating chunks of grease and oil. These grease bombs wreak havoc on an already strained system.

"We think we have it all sorted in the West," George said. "But we absolutely don't."

You can follow LiveScience senior writer Stephanie Pappas on Twitter @sipappas. Follow LiveScience for the latest in science news and discoveries on Twitter @livescience and on Facebook.

According to the United Nations, about 2.6 billion people on Earth go without access to sanitary toilet and sewage facilities. In many cases, people still practice "open defecation," or going in the bush near villages. This practice can be deadly, as bacteria from excrement often get tracked back into the community, contaminating water supplies and spreading disease. [Read full article]

Here are the top countries with sanitation problems, by raw number of people in each country without access to sanitation. All numbers are from the World Health Organization/UNICEF joint monitoring program. 

1. India (818 million)

2. China (607 million)

3. Indonesia (109 million)

4. Nigeria (103 million)

5. Pakistan (98 million)

6. Bangladesh (75 million)

7. Ethiopia (71 million)

8. Congo, DR (50 million)

9. Brazil (39 million)

10. Tanzania (32 million)

11. Sudan (27 million) 

12. Kenya (27 million) 

13. Philippines (22 million)

14. Vietnam (22 million)

15. Ghana (20 million) 

16. Nepal (20 million) 

This year marks the seventh "billion-person" milestone in the planet's history. On or around Oct. 31, 2011, the world's 7 billionth person will be born, the United Nation estimates.

Even more staggering is that of the 7 billion people on Earth, about 1.4 billion of them will be old enough to have observed the arrivals of the 6 billionth, 5 billionth, 4 billionth and 3 billionth people in the world. About 42.5 million people could have blown the party horn for the birth of the 2 billionth baby.

Yes, population has risen very quickly over the last century. Demographers do expect a decline in the population growth rate, but absolute numbers will continue to rise, likely hitting 9 billion by 2050. Meanwhile, we look back at history's past population milestones, asking: "How has the world changed?"

1805  –  The 1 billionth baby

The world's first billion-person milestone was a long time coming. Estimates of historical populations can be rough, but the U.S. Census Bureau pegs the global population at a paltry 5 million people in 8000 B.C. Certainly, humans remained scarce until the development of agriculture some 10,000 years ago. Even after our kind began farming, it was a slow climb to the 100-million-person mark around 500 B.C.

From there, population growth gets a bit more exciting. Somewhere around A.D. 500 to 600, humans hit the 200-million mark. By about 1250, the population had doubled to between 400 million and 416 million. Plagues and wars took a toll on the global human population before the 1400s, but the numbers then started a steady tick upward. [Infographic: Urban Population Explosion]

The birth year of the world's billionth baby will never be certain, but it's likely he or she came into the world around 1805. Beethoven was big that year, and already going deaf. Lewis and Clark made it to the Pacific Ocean. Napoleon was on a roll in Europe, 10 years from his defeat in the Battle of Waterloo. With the exception of a few coastal outposts, most of Africa was a complete mystery to Europeans. In China, the Qing Dynasty had just put down the White Lotus Rebellion, a tax protest that ultimately killed about 16 million people — a reminder that mass death and population growth don't necessarily cancel one another out.

1927  –  Race to 2 Billion

It had taken thousands of years for the population to reach 1 billion, but 2 billion was barely more than a century away. The 2 billionth baby was likely born in the late 1920s, perhaps 1927; the U.N. estimates that by 1930 there were 2.07 billion people on the planet.

This sudden takeoff of population makes sense in the context of a phenomenon called the demographic transition. In the transition, a population goes from one with high birth rates and high death rates (imagine farming families having seven or eight kids in hopes of a few reaching adulthood) to one with low birth rates and low death rates (where parents usually have one or two kids and expect them to grow up).

In the midst of this transition is a period when death rates are declining but people have yet to alter their behavior: They still have lots of kids. Even if those kids decide to have only a few children of their own, population will remain high, because there are so many potential parents.

The agricultural and industrial revolutions led to gradually declining death rates in the Western world starting in the 1700s, while birth rates remained fairly high for generations. As developed countries began to see falling birth rates, developing countries entered the transition period of declining death rates and high birth rates. Some countries in sub-Saharan Africa are just entering the demographic transition today.

But back to 1927. If the 2 billionth baby was indeed born in this year, he or she came into being the same year that the first trans-Atlantic telephone call was made (from New York City to London). The world was on the cusp of the Great Depression. Mao Zedong battled the Kuomintang in Hunan, China, and lost — for a time. An enormous flood along the Mississippi River inundated 27,000 square miles (70,000 square kilometers), the greatest natural disaster in U.S. history at that point.

1959  –  3 Billion People

The 3 billionth baby was a Cold War baby, born in approximately 1959. If he or she was a westerner, baby No. 3 billion would have been a baby boomer, part of the generation born after World War II. During this period, births in the U.S. rose from around 2.8 million a year in the 1930s and early 1940s to a peak of about 4.3 million births per year in the late 1950s.

Worldwide, the population growth rate hit a peak of more than 2 percent per year in the early 1960s, just after our 3 billionth baby's birth. The growth rate had been increasing since 1950, according to the U.S. Census Bureau, with one rapid downward plunge between 1959 and 1960. This hiccup was because of China's Great Leap Forward, a disastrous industrialization and collectivization push that killed millions and caused China's fertility rates to plummet.

The year the 3 billionth person was born was also the first year that rocket scientists were able to send monkeys to space and bring them back alive. The Dalai Lama fled Tibet during an uprising and set up a government-in-exile in India. And in a fitting tie to humanity's evolutionary roots, Louis and Mary Leakey discovered the first skull of the ancient hominid Australopithecus.  

1974  –  A Billion More

Only 15 years after humanity reached the 3 billion mark, it was time to ring in the 4 billionth. It all seemed rather sudden, and people were noticing. In 1968, biologist Paul Ehrlich and his wife, Anne, published "The Population Bomb" (Sierra Club/Ballentine Books), sounding a doleful alarm about overpopulation and predicting mass famine in the 1970s.

Those predictions didn't come to pass, though the book did catapult serious issues of overconsumption and resource scarcity into the limelight. In 2009, the Ehrlichs wrote in the Electronic Journal of Sustainable Development that many of their dire concerns persisted, including fishery collapse, major extinctions and fears of an epidemic. The researchers admitted they had underestimated agricultural advances in feeding a hungry world, but argued that they were partially right: Between 1968 and 2009, they wrote, "perhaps 300 million people overall have died of hunger and hunger-related diseases." [7 Devastating Infectious Diseases]

So baby No. 4 billion was not entering the world at an optimistic time, population-wise. But the world had changed in many ways in the 15 years since baby No. 3 billion took his or her first breath. Forget space monkeys: The human crew of Skylab 4 returned to Earth alive and well after 84 days in orbit. Second-wave feminism had hit a stride in the U.S., ushering in a number of legal victories aimed at workplace and educational equality for women. And just as the first Australopithecus skull was found the year the world's population rose to 3 billion, "Lucy," the famed 40-percent-complete Australopithecus afarensis skeleton, was found in 1974, the year the population hit 4 billion.

1987  –  The Day of 5 Billion

On July 11, 1987, the United Nations marked "The Day of 5 Billion," the organization's best estimate of when the 5 billionth human might be born. The event was such a hit that July 11 is now "World Population Day" every year.

Which isn't to say the Day of 5 Billion was a celebration, exactly. The then-president of the Population Institute in Washington, D.C., told the New York Times in 1986 that the 5 billionth baby was "a sobering symbol of the shocking rapidity at which the world's population is multiplying." (In a sign of how difficult exact population counts are, the Population Institute pegged the 5-billion milestone to July 1986 instead of 1987.)

But people in the 1980s were no longer as panicked over population as they had been in the 1970s. A 1986 National Academy of Sciences report argued that the time of peak population growth was past; birth rates in developing countries were down sharply, as was fertility in the developing world, though population was still expected to climb. The Carter administration had been concerned about overpopulation, the New York Times reported in 1986, while President Ronald Reagan's advisers argued that market forces and scientific progress would solve any problems caused by population growth.

In fact, it had taken about 13 years to add another billion people to the planet, and it would take 12 the next time around. In the meantime, Baby 5 Billion actually got a name: Matej Gaspar, a 7-pound, 9-ounce Croatian boy, was declared by the United Nations to be the official 5 billionth person on the planet. Matej was born at 8:35 a.m. local time in the city of Zagreb, according to Associated Press reports from the time. The secret of being the 5 billionth baby was location, location, location: Officials looked to Zagreb for the birth because the 14th World University Games were being played there at the time.

1999  – Milestone 6 Billion

Baby No. 6 billion got an official designation, too: Adnan Nevic, born in Sarajevo, Bosnia, on Oct. 12, 1999. Adnan got his label seemingly because then-U.N. Secretary-General Kofi Annan happened to be in town for a photo shoot.

In less than 75 years, the global population had tripled. The global death rate in 1999 was half what it was in 1950, the U.N. reported, while the average global life expectancy had increased by 20 years over the same period, from 46 to 66.

Fertility was down worldwide by 1999, though the number of children per mother varied widely by country. In Europe, 1.4 babies were born per woman on average, below replacement level. Meanwhile, in sub-Saharan Africa, the average figure per woman was 5.5 babies. There, the fertility rate had dropped by only one child since 1950.

Notably, Adnan Nevic (or whoever Baby 6 Billion really was) was born into a world with more young people than ever before. Over 1 billion people were between ages 15 and 24 in 1999, all entering peak childbearing years themselves.

2011  – A World of 7 Billion

Who will Baby 7 Billion be? While the United Nations places his or her birthday at around Oct. 31, 2011, the U.S. Census Bureau, going with slightly more conservative numbers, estimates a February 2012 due date.

Where the 7 billionth baby will be born is impossible to know. But Plan International, an international child advocacy agency, is taking a bet on Uttar Pradesh, India's most populous state. The group plans to present a birth certificate to a baby girl in the region on Oct. 31 to draw attention to the problem of sex-selective abortions in India.

Birth rates have been declining in Utter Pradesh since the 1970s, but not as much as the rest of India, making the area a likely spot for the 7 billionth baby. According to the Indian Ministry of Health and Family Welfare, Uttar Pradesh had a fertility rate of 3.8 births per woman in 2008, compared with 2.6 births per woman in India as a whole.

What's next for a growing world? The U.N. estimates that baby No. 8 Billion will take its first breath around 2025. According to the organization's 2010 population report, Baby 9 Billion will show up shortly before 2050, and the world will welcome its 10 billionth person by 2100. Most of the growth will be in high-fertility countries such as those in sub-Saharan Africa: The population in these areas is likely to triple. Meanwhile, intermediate-fertility countries such as the U.S., Mexico and India will grow by 26 percent, and low-fertility regions such as Europe will actually shrink by 20 percent.

You can follow LiveScience senior writer Stephanie Pappas on Twitter @sipappas. Follow LiveScience for the latest in science news and discoveries on Twitter @livescience and on Facebook.

If humans continue having babies at the rate that they do now for the rest of the century, and life expectancy rates hold as well, there will be 27 billion people on planet Earth by 2100.

"This is the 'constant fertility scenario,'" said Gerhard Heilig, chief of population estimates and projections at the United Nations, "but no one thinks this will be the case." Instead, the U.N. predicts that the population will hit 10 billion at the end of the century, and then stabilize.

This may sound like false optimism, but actually, it isn't. To arrive at their predictions for the future population, the U.N. uses complex models to project past and current population trends into the future.

The data compiled by the U.N. shows that the fertility rate — the average number of children born per woman — has been declining almost everywhere in the world for the past 50 years, and at a faster and faster pace. In 1950, women averaged five babies apiece, but today, they have just 2.5 babies. There is wide variation between countries, but in general, the rate seems to be converging to the "replacement level" — 2.1 children per woman, the rate at which children exactly replace their parents (with some extra births to make up for children who die young).

This rapid drop comes down to the growing use of contraceptives, more widespread gender equality and improvements in education, health care and standard of living.

The population is still increasing, as the fertility rate in most countries remains above the replacement level, but humanity's numbers will stabilize if current trends continue, and more and more countries modernize. "Fertility begins to decline slowly in most developing countries, and then it declines fast around three to four children, and then it slows down again," Heilig told Life's Little Mysteries.

In developed countries, the fertility rate typically dips below the replacement level and then rises back up. "We predict that in countries with increasing fertility, such as Spain, France and China, the rate is gradually increasing toward 2.1 [children per woman]," he said.

With fertility rates in developing countries approaching the replacement level from above, and modernized countries approaching it from below, U.N. projections see the rate leveling off.

However, there is great uncertainty in how things will turn out. "The uncertainty for 2100 is very, very large," Heilig said. "We cannot say 'This is the end of world population growth.'" So many variables are factored into his team's statistical models that the 10-billion-person projection is just the middle guess among a huge range of others, he explained. Because of the great many unknowns, "I strongly recommend using our 2100 projection just as an illustration of what would be the long-term consequences of the current trend," he said.

In fact, keeping the fertility rate declining along its current trajectory will take a great deal of effort. "What will happen to the world population is not carved in stone," said Joel Cohen, a population biologist at Columbia University, "but is subject to influence by how much we invest today in family planning programs, education and the status of women, and alleviation of poverty. Nobody knows when or at what number the human population will peak because it depends on what we and future people decide to do to improve human well-being."

But this much seems clear: If we decide to do nothing, then instead of stabilizing at 10 billion, the human population could multiply exponentially to 27 billion.

This story was provided by Life's Little Mysteries, a sister site to LiveScience. Follow Natalie Wolchover on Twitter @nattyover. Follow Life's Little Mysteries on Twitter @llmysteries, then join us on Facebook.

It's only September, but humans have used up the Earth's natural resources for the year, according to a sustainability nonprofit group.

The Global Footprint Network (GFN) has declared today (Sept. 27) "Earth Overshoot Day." That's the day when humankind's demand on nature exceeds the planet's ability to regenerate resources and absorb the waste.

"Our research shows that in approximately nine months, we have demanded a level of services from nature equivalent to what the planet can provide for all of 2012," according to a GFN statement. "We maintain this deficit by depleting stocks of things like fish and trees, and by accumulating waste such as carbon dioxide in the atmosphere and the ocean." [Read: Top 10 Alternative Energy Bets]

Running an ecological debt

Earth Overshoot Day is a rough estimate, but other conservation groups raise similar alarms about consumption of resources on planet Earth. The world's 7 billionth person is likely to be born next month, according to estimates by the United Nations. Meanwhile, the World Wildlife Fund (WWF) estimates that humans currently consume the equivalent of 1.5 planets' worth of resources to maintain our activities.

"If current trends continue, by 2030 we will need the capacity of two planets to meet natural resource consumption needs and absorb CO2 [carbon dioxide, a major greenhouse gas] waste," according to the WWF's Living Planet Report. 

According to the GFN, humans have been outliving our means since the mid-1970s, demanding more than the planet can renewably produce. (This is called "ecological overshoot.") In 2011, the GFN estimated that humans will use 135 percent of the resources the Earth can actually generate in one year. [Infographic: Human Demands Tax Earth's Resources]

The results of overshooting

The result of this ecological overshoot, according to the GFN, is similar to being in debt in a household: The bills pile up. Climate change, for example, occurs because excess greenhouse gases get trapped in the atmosphere. Forests shrink because the trees can't grow back faster than humans cut them down. Overfishing causes fisheries to collapse. The WWF estimates that global biodiversity is down 30 percent since 1970.

"The environmental crises we are experiencing are all symptoms of an overall trend — humanity is simply using more than the planet can provide," the GFN wrote.

Last year's Earth Overshoot Day was a few weeks earlier than this year's, but that's due to revised calculation methods rather than to any environmental gains on the part of humanity, according to the GFN. The organization calculates Overshoot Day by estimating the world's biocapacity — including factors like the regrowth of forests and regeneration of fisheries — and comparing it with humanity's ecological footprint, the amount of land and sea it takes to produce everything we consume and dispose of all the waste. The concept of Earth Overshoot Day was originally devised by the U.K.-based New Economics Foundation.

Exact trends are difficult to calculate, but the GFN's best calculations suggest that the real Earth Overshoot Day has been moving three days earlier each year since 2001.

No matter how you do the calculations, the group finds the trends worrying.

"We are in significant overshoot, and overshoot is growing," the GFN said. "By any analysis, we are well over budget."

You can follow LiveScience senior writer Stephanie Pappas on Twitter @sipappas. Follow LiveScience for the latest in science news and discoveries on Twitter @livescience and on Facebook.

The United Nations Population Division (UNPD), which keeps track of the world population, projects that the world's human population will hit 7 billion on Halloween Day 2011. Admittedly, that is just an estimate: There's no way to know exactly how many people are alive at any given moment, and the true date that humanity's ranks will surpass 7 billion could come in the weeks or months before or after Oct. 31. Nonetheless, the UN's guess is the best there is.

How do they make it? By synthesizing a mind-boggling array of data.

According to a chief analyst in the UNPD, its population estimate relies on fertility, mortality and migration information gathered by government censuses, independent demographic and health surveys, vital registers (official birth and death records), the World Health Organization, the UN High Commission on Refugees, and academic studies. UN analysts revise their country and world population curves every five years to account for any new data gathered by those entities since their last revision; they completed the current population projections earlier this year. [Center of U.S. Population Moves West]

Censuses — population statistics gathered periodically by governments — only go so far. "The uncertainty in census data is very high: in the range of 2 to 3 percent in most countries," the UN analyst, who asked not be named, told Life's Little Mysteries. That range might not sound very large, but for a country like China, which has a population of 1 billion, that means a 40-to-60-million-person error.

"In other countries, census data is very limited. Either there hasn't been a census for decades, or the census that has been taken is disputed," the analyst said. In many war-torn areas of the world, for example, it's logistically impossible to accurately count people. Some gaps in census data pertaining to developing countries get filled in by independent organizations such as Demographic and Health Surveys (DHS), a nonprofit based in the United States, but in many countries, uncertainty remains.

Furthermore, if you're trying to predict the population of a given country at some future time — even if that future time is just the publication date of your report — you obviously need to know the fertility, mortality and migration rates in the country. Are people having lots of kids or not? Are they dying young? Are they going elsewhere? [Why Do We Have Sex?]

The DHS, as well as UNICEF, the World Health Organization (WHO), and the UN Commission on Refugees all contribute to gathering information about fertility, infant and adult mortality rates and migration rates around the world.

"We also use data collected by WHO concerning HIV/AIDS prevalence. This is because, in a number of countries, you cannot estimate mortality independent of the AIDS epidemic," the UN Population Division analyst said.

These various data sources enable the UN Population Division to establish the growth rates of each age bracket in each country, and determine the overall upward trend in the world population over time. (It extends the curves all the way to the year 2100.) "You typically get a data cloud — a range of data points — rather than a simple line, and it's very complicated to come up with estimates and put a line through this data cloud." In short, who knows what the future holds? The UN picks a single projection from among a huge range of possibilities.

UN statistics show that the population will reach 10 billion by 2100, if worldwide fertility converges to "replacement level" by that time — if people have only enough children to replace themselves.

"Many people are not aware that, with our projections, we assume that people change their reproductive behavior. We assume that people in those countries where fertility is still very high will change and have significantly less children," the analyst explained. "But this is an assumption based on experience in other countries; this is by no means guaranteed. It could be that people don't want to reduce their fertility for some reason. If the fertility would not decline in the world, if it would stay at the same level as 2010, we would have 27 billion people in 2100."

Africa, not Asia, is the biggest concern for population analysts, he said. "Essentially all population growth between now and the end of the century is happening in Africa. It's not Asia; in fact, fertility in China is below the replacement level. It's Africa, because there are many countries that have very high fertility. There are many young people there, so it's a very 'high momentum' population."

This story was provided by Life's Little Mysteries, a sister site to LiveScience. Follow Natalie Wolchover on Twitter @nattyover. Follow Life's Little Mysteries on Twitter @llmysteries, then join us on Facebook.

Our planet and its inhabitants - including we humans - are in a precarious position as we approach Earth Day, April 22.

While global warming is widely accepted as a reality by scientists and many governments and industrial leaders, progress to curb greenhouse gases and other forms of pollution remains limited. The current economic climate will likely make pollution control efforts more difficult, analysts say.

Recent studies, as well as the 2007 report by the Intergovernmental Panel on Climate Change (IPCC) have pointed to some of the likely effects of uncurbed greenhouse gas emissions: rising global temperatures, rising sea levels, Arctic sea ice melt, the disappear of glaciers, epic floods in some areas and intense drought in others.

These effects are intensified when combined with other forms of pollution the world's rising population.

Humans will face widespread water shortages. Famine and disease will increase. Earth’s landscape will transform radically, with a quarter of plants and animals at risk of extinction.

While putting specific dates on these traumatic potential events is challenging, this timeline paints the big picture and details Earth's future based on several recent studies and the longer scientific version of the IPCC report. This timeline is an updated version of one first published by LiveScience in 2007.

2008

Arctic sea ice hits its second lowest summer ice extent on record (the lowest extent was in 2007). A massive chunk of ice breaks away from Greenland's Petermann Glacier. Several breakups of ice shelves in Antarctica are observed. (NSIDC; Jason Box, Ohio State University; ESA, NSIDC)

Global oil production peaks sometime between 2008 and 2018, according to a model by a Swedish physicist. Others say this turning point, known as "Hubbert’s Peak," won’t occur until after 2020. Once Hubbert’s Peak is reached, global oil production will begin an irreversible decline, possibly triggering a global recession, food shortages and conflict between nations over dwindling oil supplies. (Doctoral dissertation of Frederik Robelius, University of Uppsala, Sweden; report by Robert Hirsch of the Science Applications International Corporation)

The Bush Administration enacts changes to the Endangered Species Act that affect reviews of government projects.

Polar bears and beluga whales are placed on the Endangered Species List. Gray wolves are delisted in certain areas.

2009

The U.S. Environmental Protection Agency declares carbon dioxide and other heat-trapping gases to be pollutants under the Clean Air Act.

An ice bridge connected to the Wilkins Ice Sheet of Antarctica breaks apart.

Many of the world's major rivers are found to be losing water. (Aiguo Dai, NCAR, Journal of Climate)

2012

The first phase of the Kyoto Protocol, an international environmental treaty created to limit the production of greenhouse gases, expires. Nations will have to draw up and enact a succesor treaty to further limit emissions, should they choose to do so.

2020

Flash floods will very likely increase across all parts of Europe. (IPCC)

Less rainfall could reduce agriculture yields by up to 50 percent in some parts of the world. (IPCC)

World population will reach 7.6 billion people. (U.S. Census Bureau)

2030

Diarrhea-related diseases will likely increase by up to 5 percent in low-income parts of the world. (IPCC)

Up to 18 percent of the world’s coral reefs will likely be lost as a result of climate change and other environmental stresses. In Asian coastal waters, the coral loss could reach 30 percent. (IPCC)

World population will reach 8.3 billion people. (U.S. Census Bureau)

Warming temperatures will cause temperate glaciers on equatorial mountains in Africa to disappear. (Richard Taylor, University College London, Geophysical Research Letters:)

In developing countries, the urban population will more than double to about 4 billion people, packing more people onto a given city's land area. The urban populations of developed countries may also increase by as much as 20 percent. (World Bank: The Dynamics of Global Urban Expansion)

The Arctic Sea could be ice-free in the summer. (James Overland, NOAA, Muyin Wang, University of Washington, Geophysical Research Letters)

2050

The Group of Eight top industrial nations has endorsed cutting greenhouse gas emission in half by this year.

Small alpine glaciers will very likely disappear completely, and large glaciers will shrink by 30 to 70 percent. Austrian scientist Roland Psenner of the University of Innsbruck says this is a conservative estimate, and the small alpine glaciers could be gone as soon as 2037. (IPCC)

Ocean acidification could kill off most coral reefs. (Ken Caldeira, Carnegie Institution of Washington, Science)

At least 400 bird species could become endangered or extinct due to deforestation and climate change. (Walter Jetz, University of California, Davis, PLoS Biology)

In Australia, there will likely be an additional 3,200 to 5,200 heat-related deaths per year. The hardest hit will be people over the age of 65. An extra 500 to 1,000 people will die of heat-related deaths in New York City per year. In the United Kingdom, the opposite will occur, and cold-related deaths will outpace heat-related ones. (IPCC)

World population reaches 9.4 billion people. (U.S. Census Bureau)

Crop yields could increase by up to 20 percent in East and Southeast Asia, while decreasing by up to 30 percent in Central and South Asia. Similar shifts in crop yields could occur on other continents. (IPCC)

As biodiversity hotspots are more threatened, a quarter of the world’s plant and vertebrate animal species could face extinction. (Jay Malcolm, University of Toronto, Conservation Biology)

2070

As glaciers disappear and areas affected by drought increase, electricity production for the world’s existing hydropower stations will decrease. Hardest hit will be Europe, where hydropower potential is expected to decline on average by 6 percent; around the Mediterranean, the decrease could be up to 50 percent. (IPCC)

Warmer, drier conditions will lead to more frequent and longer droughts, as well as longer fire-seasons, increased fire risks, and more frequent heat waves, especially in Mediterranean regions. (IPCC)

2080

While some parts of the world dry out, others will be inundated. Scientists predict up to 20 percent of the world’s populations live in river basins likely to be affected by increased flood hazards. Up to 100 million people could experience coastal flooding each year. Most at risk are densely populated and low-lying areas that are less able to adapt to rising sea levels and areas which already face other challenges such as tropical storms. (IPCC)

Coastal population could balloon to 5 billion people, up from 1.2 billion in 1990. (IPCC)

Between 1.1 and 3.2 billion people will experience water shortages and up to 600 million will go hungry. (IPCC)

Sea levels could rise around New York City by more than three feet, potentially flooding the Rockaways, Coney Island, much of southern Brooklyn and Queens, portions of Long Island City, Astoria, Flushing Meadows-Corona Park, Queens, lower Manhattan and eastern Staten Island from Great Kills Harbor north to the Verrazano-Narrows Bridge. (NASA GISS)

2085

The risk of dengue fever from climate change is estimated to increase to 3.5 billion people. (IPCC)

2100

A combination of global warming and other factors will push many ecosystems to the limit, forcing them to exceed their natural ability to adapt to climate change. (IPCC)

Atmospheric carbon dioxide levels will be much higher than anytime during the past 650,000 years. (IPCC)

Ocean pH levels will very likely decrease by as much as 0.5 pH units, the lowest it’s been in the last 20 million years. The ability of marine organisms such as corals, crabs and oysters to form shells or exoskeletons could be impaired. (IPCC)

Thawing permafrost and other factors will make Earth’s land a net source of carbon emissions, meaning it will emit more carbon dioxide into the atmosphere than it absorbs. (IPCC)

Roughly 20 to 30 percent of species assessed as of 2007 could be extinct by 2100 if global mean temperatures exceed 2 to 3 degrees of pre-industrial levels. (IPCC)

New climate zones appear on up to 39 percent of the world’s land surface, radically transforming the planet. (Jack Williams, University of Wisconsin-Madison, Proceedings of the National Academy of Sciences)

A quarter of all species of plants and land animals—more than a million total—could be driven to extinction. The IPCC reports warn that current “conservation practices are generally ill-prepared for climate change and effective adaptation responses are likely to be costly to implement.” (IPCC)

Increased droughts could significantly reduce moisture levels in the American Southwest, northern Mexico and possibly parts of Europe, Africa and the Middle East, effectively recreating the “Dust Bowl” environments of the 1930s in the United States. (Richard Seager, Lamont Doherty Earth Observatory, Science)

2200

An Earth day will be 0.12 milliseconds shorter, as rising temperatures cause oceans to expand away from the equator and toward the poles, one model predicts. One reason water will be shifted toward the poles is most of the expansion will take place in the North Atlantic Ocean, near the North Pole. The poles are closer to the Earth’s axis of rotation, so having more mass there should speed up the planet’s rotation. (Felix Landerer, Max Planck Institute for Meteorology, Geophysical Research Letters)

{{ video="LS_090415-TakeAIM" title="Climate Change Gets a Music Video" caption="AIM = Adapt Innovate Mitigate. What happens at Earth's poles will rock your world." }}

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