What Is Global Warming?
This image shows the five-year average variation of global surface temperatures in 2012.
Credit: NASA/Goddard Scientific Visualization Studio

Global warming is the gradual heating of Earth's surface, oceans and atmosphere. Scientists have documented the rise in average temperatures worldwide since the late 1800s. Earth's average temperature has risen by 1.4 degrees Fahrenheit (0.8 degrees Celsius) over the past century, according to the Environmental Protection Agency (EPA). Temperatures are projected to rise another 2 to 11.5 degrees F (1.133 to 6.42 degrees C) over the next 100 years.

Most of the leading scientific organizations in the world acknowledge the existence of global warming as fact, according to a NASA report. Furthermore, 97 percent of climate scientists agree that the rate of global warming trends the planet is now experiencing is not a natural occurrence, but is primarily the result of human activity. That consensus was made clear in a major climate report released Sept. 27, 2013, by the Intergovernmental Panel on Climate Change (IPCC). In that report, climate scientists indicated they are more certain than ever of the link between human activities and global warming.

Global warming begins with 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 (Svante Arrhenius in 1896)," Josef Werne, an associate professor in the department of geology and planetary science at the University of Pittsburgh, told Live Science.

Solar radiation passes through the atmosphere to the surface of Earth, where it is absorbed and then radiated upward as heat. Gases in Earth's atmosphere absorb about 90 percent of this heat and radiate it back to the surface, which is warmed to a life-supporting average of 59 F (15 C). This very helpful process is called the greenhouse effect.

Human-caused global warming occurs when human activity introduces too much of certain types of gas into the atmosphere. More of this gas equals more warming. The atmospheric gases primarily responsible for the greenhouse effect are known as "greenhouse gases" and include water vapor, carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). The most prevalent greenhouse gas is CO2.

Some atmospheric CO2 is natural. For example, before the Industrial Revolution, there were about 280 parts per million (ppm) of CO2 in the atmosphere, and during most of the past 800,000 years, CO2 fluctuated between about 180 ppm during ice ages and 280 ppm during interglacial warm periods. Since the Industrial Revolution, though, the amount of CO2 has dramatically increased. Currently, the increase is 100 times faster than that when the last ice age ended, according to the National Oceanic and Atmospheric Administration (NOAA).

In May 2013, scientists reported measuring atmospheric carbon dioxide levels as high as 400 ppm. Levels of CO2 haven't been that high since the Pliocene Epoch, which was between 3 million and 5 million years ago, according to the Scripps Institution of Oceanography.

In 2012, CO2 accounted for about 82 percent of all U.S. greenhouse gas emissions, according to the EPA. "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, for example, releases CO2. 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 for photosynthesis. According to the 2010 Global Forest Resources Assessment, deforestation releases nearly a billion tons of carbon into the atmosphere per year.  

But fossil fuel combustion is the number one anthropogenic source of carbon dioxide. The EPA lists this source as the cause of 32 percent of total U.S. CO2 emissions and 27 percent of total U.S. greenhouse gas emissions in 2012.

Methane is the second most common greenhouse gas, but it is much more destructive. In 2012, the gas accounted for about 9 percent of all U.S. greenhouse gas emissions, according to the EPA. There may be less methane in the atmosphere, but this gas is much more efficient at trapping radiation. The EPA reports that methane has 20 times more impact 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.

The effects of global warming are already visible in many areas of the world. For example, in Montana's Glacier National Park, where about 150 glaciers were once located, only 25 glaciers larger than 25 acres remain, according to the U.S. Geological Survey (USGS). A commonly accepted guideline for identifying a glacier — a body of snow and ice that moves — is that the object must be about 101,000 square meters or about 25 acres in size. Below this size, the ice is generally stagnant and does not move, unless it is on a steep slope.

Scientists have expressed confidence that climate change will make hurricanes more intense, as well; the unusually strong hurricanes that have formed over the past few years give evidence for this. "We are confident not just because models predict hurricane intensification, but because we understand the reasons why they do and can explain those reasons in terms of what we know about how hurricanes work today," said atmospheric scientist Adam Sobel, author of "Storm Surge: Hurricane Sandy, Our Changing Climate, and Extreme Weather of the Past and Future" (HarperWave, 2014).

Sobel, a Columbia University professor in the departments of Earth and Environmental Sciences, and Applied Physics and Applied Mathematics, explained that hurricanes get their energy from the temperature difference between the warm tropical ocean and the cold upper atmosphere. Global warming increases that temperature difference.

Temperatures are getting more intense, as well. North America reached record highs in 2012, making it the hottest year since record keeping began in 1895. Scientists also recorded the second greatest number of temperature extremes (unusual highs and lows) in 2012. According to NOAA, 2013 tied with 2003 as the fourth warmest year globally since 1880, which is when the first year of global-temperature recording began.

These images show the five-year average variation of global surface temperatures in 1884, 1927, 1969 and 2012. Dark blue indicates areas cooler than average. Dark red indicates areas warmer than average.
These images show the five-year average variation of global surface temperatures in 1884, 1927, 1969 and 2012. Dark blue indicates areas cooler than average. Dark red indicates areas warmer than average.
Credit: NASA/Goddard Scientific Visualization Studio

In recent years, record-breaking cold weather has made some wonder if global warming is actually happening. "First, we need to understand the difference between 'weather' and 'climate,'" said Peterman and Foy. Weather is atmospheric activity over a short time period, like a cold winter snap or a hot spell. Climate is the "average weather," meaning the sum of weather events averaged over decades, centuries or even thousands of years.

Global warming is related to climate and is a global phenomenon. Even though there are regional cold snaps (weather), the average global temperature (climate) continues to increase even during different regions' winter months, the two scientists went on to explain.

Surprisingly, global warming can actually cause unusually cold weather. One of the key atmospheric features of climate is that it is affected by warming in air circulation patterns, including the jet stream, which is like a river of wind high above in the atmosphere. "If you perturb the jet stream in the right way, it migrates south, bringing with it cold, Arctic air," Werne said. "This is precisely why you can get a cold snap in the short term, and also why a given winter in North America might be colder than average, even during a long-term trend of global warming."

According to NASA:

  • Carbon dioxide levels are at 399.2 ppm as of November 2014
  • The global temperature has risen 14 F (7.8 C) since 1880
  • The global Arctic ice minimum (the extent of sea ice in warm months) is decreasing by 13.3 percent each decade
  • Land ice is decreasing by 258 billion tons (234 million kilotons) each year
  • Due to melting ice, the sea level has risen by 0.12 inches (3.17 millimeters) per year

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.

Many scientists say that reversal is not possible and that certain types of destruction, such as the melting of the polar ice caps, have already gone past the point of no return. Others say that the planet Earth has the ability to heal itself. This takes time, though.

"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," 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."

There are many steps humans can take to lessen the effects of global warming. NASA is committed to a two-pronged approach:

  • Reducing emissions of greenhouse gases and stabilizing the levels of these gases in the atmosphere
  • Adapting to the climate change already happening

Decreasing the rate at which fossil fuels are burned is critical to that effort. Development of clean energy, including solar, wind and geothermal energy, has immense potential to reduce the amount of coal and oil burned in powering electrical generating plants, NASA said.

More-sustainable transportation options, such as mass transit and alt-fuel vehicles, will also reduce the amount of greenhouse gases in the atmosphere. (The Union of Concerned Scientists estimates that about 25 percent of energy-related CO2 emissions in the United States come from passenger vehicles.)

Even individual efforts, such as lowering thermostats in winter and using energy-efficient light bulbs, can help to address global warming. But most climate researchers also stress the immediate need for large-scale, international policies to address the complex causes and effects of global warming.

Marc Lallanilla contributed to this article.

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