Earth Checkup: 10 Signs of the Planet's Health

With world leaders gathering at a U.N. summit in Copenhagen to brainstorm ways to quash increasing temperatures and hold back rising seas, LiveScience takes a look at the state of Earth's ecosystems and its inhabitants — from polar bears to us. Here are 10 signs of how well (and not-so-well) our planet is doing.

10. Arctic Meltdown

After dramatic meltdowns in recent summers that have left Arctic ice thinner than in the past, some scientists are increasingly worried about the future survival of Arctic sea ice. One recent study estimated that Arctic waters could be ice free during the summer in as few as 30 years, much sooner than previous estimates. Such catastrophic melt could reinforce the global warming trend and further imperil Arctic residents, from humans to narwhals and polar bears, which were first listed as an Endangered Species in May 2008.

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Antarctica has seen its share of melt as well: In April, an ice bridge believed to pin the Wilkins Ice Shelf in place snapped. Wilkins is one of nine Antarctic ice shelves that have receded or collapsed in recent decades — the most dramatic collapses were those of the Larsen A and B shelves, which abruptly crumbled in 1995 and 2002, respectively. Most of the dramatic melting has occurred in the Antarctic Peninsula, the only part of the southernmost continent that juts north of the Antarctic Circle. In contrast, the interior of the frozen continent was thought to be cooling, but earlier this year, new research suggested that these vast ice sheets are also experiencing warming, though the trend has so far been masked by the cooling influence of the ozone hole. The 47 countries that have ratified an agreement called the Antarctic Treaty have agreed to tourism limits to protect the continent's fragile ecosystems.

It's been more than 20 years since scientists discovered the gaping hole in the ozone layer, which normally protects Earth's denizens from the sun's harmful ultraviolet rays. Since then efforts to ban or reduce the chemicals that eat away at the ozone in the stratosphere have initiated the gradual recovery of the hole. This recovery will take decades, though, because these pollutants hang around for a long time. So far the ozone hole over Antarctica has remained about the same size, fluctuating year to year with changes in wind circulation patterns. While it will still take some time for the ozone hole to recover, if countries hadn't acted to ban ozone-destroying substances, the situation could have been much worse.

For years now, so-called oceanic dead zones — pockets of the sea where oxygen is so depleted that many fish, crustaceans and other aquatic species can't survive, such as in the Gulf of Mexico — have been a growing concern. These suffocating spaces are primarily formed when fertilizer runoff pours in from rivers and promotes algae blooms that eat up all the oxygen as they die and decompose. Controlling fertilizer runoff could improve the situation fairly quickly. But some studies have suggested that increased crop growth for producing biofuels could send more fertilizer running downstream.

Coral reefs, sometimes called the "rainforests of the ocean," are critical marine habitats. But reefs from the Caribbean to the Great Barrier Reef have been under pressure in recent decades from overfishing, pollution, disease, warming waters and ocean acidification. Ocean waters become more acidic as they absorb carbon dioxide from the atmosphere. As the water becomes more acidic, it dissolves the minerals used by corals and other animals to build their skeletons. A 2007 study found that this stressor alone could make most current coral habitats too acidic for reef growth by 2050. And so the outcome of the climate summit in Copenhagen, where great minds are hashing out ways to reduce carbon dioxide emissions among other issues, will have implications for the survival of the world's coral reefs.

On land, the actual rainforests aren't fairing much better, thanks in large part to deforestation. Forested areas, particularly rainforests, are key areas of biodiversity. They also absorb carbon dioxide and produce oxygen, and so clearing such trees could boost greenhouse gas emissions. Globally, the rate of deforestation is about 32 million acres a year, or 36 football fields a minute. This amount of forest clearing generates nearly 20 percent of greenhouse gas emissions, according to the WWF (formerly the World Wildlife Fund). One verdant sign: Fewer trees were cut down in the Brazilian Amazon this year than since record-keeping began in 2000, according to the WWF. Even so, the Amazon did lose swaths of forest, just not as much. Asia and Africa have seen rising rates of deforestation. Forests in the United States and Europe are fairing better, as reforestation has occurred in the last decade.

It's essential to life as we know it, and though the planet's surface is two-thirds water, pollution is making it unsuitable for the humans who drink it and the animals that live in it. The effects of global warming are also altering the patterns of water availability for drinking and agriculture: Already arid regions will likely get drier, and rising sea levels could force salty sea water into normally freshwater aquifers. Some scientists say western U.S. water supplies are already being impacted by climate change and that policy advisors need to set better management practices. Depending on where they are grown, the crops used to make biofuels could stress local water supplies.

This year, the U.S. Environmental Protection Agency formally declared that carbon dioxide and five other heat-trapping gases are pollutants under the Clean Air Act, paving the way for regulations of emissions. Some companies and nations have already pledged to reduce greenhouse gas emissions, but many of these goals have not been met. That and the rapid pace of development in countries like China and India have kept levels of carbon dioxide and other greenhouse gases rising globally, and at a faster rate than in previous years. China leads all nations in total emissions, but the United States is still number one in emissions per capita. Many proposals for cap-and-trade systems, methods for trapping carbon dioxide emissions underground and alternative forms of energy have been put forward, but it's up to governments and other groups to put them into action.

Tigers, elephants, rhinos, and several species of primates are known victims of habitat change — and poaching — in Africa and Asia. Frog populations across the globe have been decimated by the spread of a deadly fungus. In the oceans, sharks, whales, dolphins and some species of fish are also hurting. The news isn't all bad, as many bird populations are recovering thanks to the ban on DDT. Polar bears were placed on the Endangered Species List last year, which means they will have protection under the Endangered Species Act. On the other end of the Earth, however, new studies have found that penguins are also in peril due to a combination of changes in climate, overfishing and pollution.

While we are the significant force behind much of the change to Earth's systems, those effects can come back and impact us through our health and changing environmental conditions that we must adapt to. This feedback will be magnified as human populations continue to grow. In 2007, the world population passed the 6 billion mark. That year also marked the first time in human history that more people were living in urban settings than rural areas. All 6 billion of us must compete for limited resources, including water, food and fuel. Some scientists say that we have already reached the limits of what our planet can support and that we need to curb population growth for the health of our species and the planet.

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