What Is Global Warming?
Earth as seen by the EPIC imager on the DSCOVR satellite on March 29, 2017.
Credit: NASA

The globe is heating up. Both land and oceans are warmer now than record-keeping began in 1880, and temperatures are still ticking upward. This temperature rise, in a nutshell, is global warming.

Here are the bare numbers, according to the National Oceanic and Atmospheric Administration (NOAA): Average surface temperatures rose a total of 1.71 degrees Fahrenheit (0.95 degrees Celsius) between 1880 and 2016. The pace of change has been an additional 0.13 degrees F (0.07 degrees C) per decade, with the land surface warming faster than the ocean surface — 0.18 degrees F (0.10 degrees C) versus 0.11 degrees F (0.06 degrees C) per decade, respectively.

The Paris Agreement, ratified by 159 nations as of the summer 2017, aims to halt that warming at 2.7 degrees F (1.5 degrees C) above Earth's average temperature during preindustrial times — a goal most scientists and policy makers agree will be a challenge to meet. (The United States participated in the crafting of that nonbinding treaty under President Barack Obama, but President Donald Trump has said that his administration will not participate.) Here's how humanity has managed to heat up the planet.  

The main driver of today's warming is the combustion of fossil fuels. These hydrocarbons heat up the planet via the greenhouse effect, which is caused by the interaction between Earth's atmosphere and incoming radiation from the sun. "The basic physics of the greenhouse effect were figured out more than a hundred years ago by a smart guy using only pencil and paper," Josef Werne, a professor of geology and environmental science at the University of Pittsburgh, told Live Science.

That "smart guy" was Svante Arrhenius, a Swedish scientist and eventual Nobel Prize winner. Simply put, solar radiation hits Earth's surface and then bounces back toward the atmosphere as heat. Gases in the atmosphere trap this heat, preventing it from escaping into the void of space (good news for life on the planet). In a paper presented in 1895, Arrhenius figured out that greenhouse gases such as carbon dioxide could trap heat close to the Earth's surface — and that small changes in the amount of those gases could make a big difference in how much heat was trapped.

Since the beginning of the industrial revolution, humans have been rapidly changing the balance of gases in the atmosphere. Burning fossil fuels like coal and oil releases water vapor, carbon dioxide (CO2), methane (CH4), ozone and nitrous oxide (N2O) — the primary greenhouse gases. Carbon dioxide is the most common greenhouse gas. Between about 800,000 years ago and the beginning of the Industrial Revolution, its presence in the atmosphere amounted to about 280 parts per million (ppm). Today, it's about 400 ppm. (This number means there are 400 molecules of carbon dioxide in the air per every million air molecules.)

Levels of CO2 haven't been that high since the Pliocene epoch, which occurred between 3 million and 5 million years ago, according to the Scripps Institution of Oceanography.

In 2015, CO2 accounted for about 82 percent of all U.S. greenhouse gas emissions, according to an EPA inventory.

"We know through high-accuracy instrumental measurements that there is an unprecedented increase in CO2 in the atmosphere. We know that CO2 absorbs infrared radiation [heat] and the global mean temperature is increasing," Keith Peterman, a professor of chemistry at York College of Pennsylvania, and his research partner, Gregory Foy, an associate professor of chemistry at York College of Pennsylvania, told Live Science in a joint email message.

CO2 makes its way into the atmosphere through a variety of routes. Burning fossil fuels releases CO2 and is by far the primary way that U.S. emissions warm the globe. According to the EPA's 2015 report, U.S. fossil fuel combustion, including electricity generation, releases just over 5.5 billion tons (5 billion metric tons) of CO2into the atmosphere annually. Other processes — such as non-energy use of fuels, iron and steel production, cement production and waste incineration — boost the total annual CO2 release in the U.S. to almost 6 billion tons (5.5 billion metric tons).

Deforestation is also a large contributor to excessive CO2 in the atmosphere. In fact, deforestation is the second largest anthropogenic (human-made) source of carbon dioxide, according to research published by Duke University. When trees are killed, they release the carbon they have stored during photosynthesis. According to the 2010 Global Forest Resources Assessment, deforestation releases nearly a billion tons of carbon into the atmosphere per year.  

Methane is the second most common greenhouse gas, but it is much more efficient at trapping heat. In 2012, the gas accounted for about 9 percent of all U.S. greenhouse gas emissions, according to the EPA. The EPA reports that methane has 20 times more impact than carbon dioxide on climate change over a 100-year period.

Methane can come from many natural sources, but humans cause a large portion of methane emissions through mining, the use of natural gas, the mass raising of livestock and the use of landfills, according to the Inventory of U.S. Greenhouse Gas Emissions and Sinks report from 1990 to 2012. In fact, according to the EPA, humans are responsible for more than 60 percent of methane emissions.

There are some hopeful trends in greenhouse gas emissions. Though U.S. emissions increased by a total of 7.7 percent between 1990 and 2014, according to EPA data, they have declined 8 percent in the timeframe between 2005 and 2014. Much of the reason for this recent decline is the replacement of coal with natural gas, according to the Center for Climate and Energy Solutions. The U.S. economy is also transitioning from manufacturing-based to a less carbon-intense service economy. Fuel-efficient vehicles and energy-efficiency standards for buildings have also improved emissions, according to the EPA.

Global warming doesn't just mean warming — which is why "climate change" has become the trendier term among researchers and policy makers. While the globe is becoming hotter on average, this temperature increase can have paradoxical effects, such as more serious snowstorms. There are several big ways climate change can and will affect the globe: By melting ice, by drying out already-arid areas, by causing weather extremes and by disrupting the delicate balance of the oceans.

Perhaps the most visible effect of climate change so far is the melting of glaciers and sea ice. The ice sheets have been retreating since the end of the last Ice Age about 11,700 years ago, but the last century's warming has hastened their demise. A 2016 study found that there is a 99 percent chance that global warming has caused the recent retreat of glaciers; in fact, the research showed, these rivers of ice retreated 10 to 15 times the distance they would have if the climate had stayed stable. Glacier National Park in Montana had 150 glaciers in the late 1800s. Today it has 26. The loss of glaciers can cause the loss of human life when icy dams holding back glacier lakes destabilize and burst, or when avalanches caused by unstable ice bury villages.

At the North Pole, warming is proceeding twice as quickly as it is at middle latitudes, and the sea ice is showing the strain. Fall and winter ice in the Arctic hit record lows in both 2015 and 2016, meaning the ice expanse did not cover as much of the open sea as previously observed. According to NASA, the 13 smallest maximum winter extents of sea ice in the Arctic have all happened in the last 13 years. The ice also forms later in the season and melts more readily in spring. Some scientists think the Arctic Ocean will see ice-free summers within 20 or 30 years.

In the Antarctic, the picture has been a little less clear. The Western Antarctic Peninsula is warming faster than anywhere else besides some parts of the Arctic, according to the Antarctic and Southern Ocean Coalition. The peninsula is where the Larsen C ice shelf just rifted in July 2017, spawning an iceberg the size of Delaware. The sea ice off Antarctica is very variable, though, and some areas have actually hit record highs in recent years — though those record highs could bear the fingerprints of climate change, as they might result from land-based ice moving out to sea as the glaciers melt, or in warming-related changes to wind. In 2017, though, this pattern of record-high ice abruptly reversed, with a record low. On March 3, 2017, Antarctic sea ice was measured at an extent of 71,000 square miles (184,000 square kilometers) less than the previous low from 1997.

Global warming will change things between the poles, too. Many already-dry areas are expected to become even drier as the world warms. The Southwest and Central Plains of the United States, for example, are expected to experience decades-long "megadroughts" harsher than anything else in human memory.

"The future of drought in western North America is likely to be worse than anybody has experienced in the history of the United States," Benjamin Cook, a climate scientist at NASA's Goddard Institute for Space Studies in New York City who published research projecting these droughts in 2015, told Live Science. "These are droughts that are so far beyond our contemporary experience that they are almost impossible to even think about."

The study predicted an 85 percent chance of droughts lasting at least 35 years in the region by 2100. The main driver, the researchers found, is the increasing evaporation of water from hotter and hotter soil. Much of the precipitation that does fall in these arid regions will be lost.

Meanwhile, 2014 research finds that many areas will likely see less rainfall as the climate warms. Subtropical regions, including the Mediterranean, the Amazon, Central America and Indonesia will likely be hardest hit, that study found, while South Africa, Mexico, western Australia and California will also dry out.

Another impact of global warming: extreme weather. Hurricanes and typhoons are expected to become more intense as the planet warms. Hotter oceans evaporate more moisture, which is the engine that fuels these storms. The International Panel on Climate Change (IPCC) predicts that even if the planet diversifies its energy sources and transitions to a less fossil-fuel intense economy (known as the A1B scenario), tropical cyclones are likely to be up to 11 percent more intense on average. That means more wind and water damage on vulnerable coastlines. (The IPCC is an international organization created by the United Nations to report on the state of climate change science and to provide the best projections of climate impacts and strategies for adapting to the projections.)

Paradoxically, climate change may also cause more extreme snowstorms. According to the National Centers for Environmental Information, extreme snowstorms in the eastern United States have become twice as common since the early 1900s. Again, warming ocean temperatures lead to increased evaporation of moisture into the atmosphere. This moisture powers storms that hit the continental United States.

Some of the most immediate impacts of global warming are beneath the waves. Oceans act as a carbon sink — they absorb dissolved carbon dioxide. That's not a bad thing for the atmosphere, but it isn't great for the marine ecosystem. When carbon dioxide reacts with seawater, it leads to a decline in pH, a process known as ocean acidification. Increased acidity eats away at the calcium carbonate shells and skeletons that many ocean organisms depend on for survival. These include shellfish, pteropods and corals, according to NOAA.

Corals, in particular, are the canary in a coal mine for climate change in the oceans. Marine scientists have observed alarming levels of coral bleaching, events in which coral expel the symbiotic algae that provide them with nutrients and give them their vivid colors. Bleaching occurs when coral are stressed, and stressors can include high temperatures. In 2016 and 2017, Australia's Great Barrier Reef experienced back-to-back bleaching events. Coral can survive bleaching, but repeated bleaching events make survival less and less likely.

Despite overwhelming scientific consensus about the causes and reality of global warming, the issue is contentious politically. For instance, deniers of climate change have argued that warming slowed between 1998 and 2012, a phenomenon known as the "climate change hiatus."

Unfortunately for the planet, the hiatus never existed. Two studies, one published in the journal Science in 2015 and one published in 2017 in the journal Science Advances, reanalyzed the ocean temperature data that showed the warming slowdown and found that, in fact, it was a mere measurement era. Between the 1950s and 1990s, most measurements of ocean temperature were taken aboard research boats. Water would be pumped into pipes through the engine room, which ended up heating the water slightly. After the 1990s, scientists began using ocean buoy-based systems, which were more accurate, to measure ocean temperatures. The problem came because no one corrected for the change in measurements between boats and buoys. Making those corrections showed that the oceans warmed 0.22 degrees Fahrenheit (0.12 degrees Celsius) on average per decade since 2000, almost twice as fast as earlier estimates of 0.12 degrees F (0.07 degrees C) per decade.

A growing number of business leaders, government officials and private citizens are concerned about global warming and its implications, and are proposing steps to reverse the trend.

"While some argue that 'the Earth will heal itself,' the natural processes for removing this human-caused CO2 from the atmosphere work on the timescale of hundreds of thousands to millions of years," the University of Pittsburgh's Werne said. "So, yes, the Earth will heal itself, but not in time for our cultural institutions to be preserved as they are. Therefore, in our own self-interests, we must act in one way or another to deal with the changes in climate we are causing."

The most ambitious effort to forestall warming is the Paris Agreement. This nonbinding international treaty entered into force in November 2016. The aim is to keep warming "well below 2 degrees Celsius above pre-industrial levels and to pursue efforts to limit the temperature increase even further to 1.5 degrees Celsius," according to the United Nations. Each signatory to the treaty agreed to set their own voluntary emissions limits and to make them stricter over time. For the United States under President Obama, that meant limiting greenhouse emissions to less than 28 percent of 2005 levels by 2025. Climate scientists said that the emissions limits suggested so far wouldn't keep warming as low as 1.5 or even 2 degrees C, but that it would be an improvement over the "business-as-usual" scenario.

However, President Trump said in June that his administration will not honor the Paris Agreement. Shortly thereafter, more than 1,000 mayors, governors and business executives said they would continue to abide by the promised emissions cuts, Inside Climate News Reported.

Solving climate change will require big shifts in energy production, from fossil fuels to less carbon-intensive sources. Some scientists even think geoengineering will be needed to cool the planet.

According to NASA:

  • Carbon dioxide levels in the atmosphere are at 406.5 ppm as of 2017, their highest levels in 650,000 years.
  • Average global temperature is up 1.7 degrees F (0.94 degrees C) since 1880.
  • The minimum expanse of Arctic summer sea ice has declined 13.3 percent per decade since the 1980s.
  • Land ice has declined at the poles by 286 gigatons a year since 2002.
  • Global sea level has risen 7 inches (176 millimeters) in the past century.

Marc Lallanilla and Alina Bradford contributed to this article.

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