Global warming — the gradual heating of Earth's surface, oceans and atmosphere — has emerged as one of the most vexing environmental issues of our time.
The rise in average temperatures worldwide has been documented by scientists since the late 1800s. The Earth's average temperature has risen by 1.4 degrees Fahrenheit (0.8 degrees Celsius) over the past century, the Environmental Protection Agency (EPA) reports.
Though the existence of global warming was once considered controversial, it is now acknowledged as real by an overwhelming majority of researchers throughout the international scientific community, according to the National Aeronautics and Space Administration (NASA).
Furthermore, most scientists agree that the rate of global warming we're 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 it, climate scientists indicated they are more certain than ever of the link between human activities and global warming.
The greenhouse effect
Global warming begins with the greenhouse effect, which is caused by the interaction between Earth's atmosphere and incoming radiation from the sun.
Solar radiation passes through the atmosphere and is partially absorbed on the surface of Earth. Some of the incoming radiation, however, is reflected back out toward space.
Gases in Earth's atmosphere absorb some of that reflected radiation; as a result, the atmosphere heats up.
This atmospheric warming is known as the "greenhouse effect" because the same process keeps a greenhouse warm during cold weather: Solar radiation is trapped by the glass walls of a greenhouse, heating the greenhouse and keeping its plants warm throughout the winter.
The atmospheric gases primarily responsible for the greenhouse effect are known as "greenhouse gases" and include water vapor, carbon dioxide (CO2), methane and nitrous oxide. Not all greenhouse gases are the same: methane, for example, has roughly 21 times the heat-trapping ability of carbon dioxide, according to the EPA.
Nonetheless, CO2 is frequently cited as the principal driver of global warming because human activity — primarily, the burning of fossil fuels like coal and oil — has released unprecedented amounts of CO2 into the atmosphere since the Industrial Revolution began in the late 1700s.
Global warming statistics
Before the Industrial Revolution, the amount of atmospheric CO2 was about 280 parts per million (ppm), according to the National Oceanic and Atmospheric Administration (NOAA).
But since the late 1700s, CO2 levels have been increasing steadily; beginning in the year 2000, the rate of increase has been about 1.9 ppm per year, according to NOAA.
In May 2013, scientists reported measuring carbon dioxide levels as high as 400 ppm, a symbolic benchmark that nonetheless has climate scientists concerned: Levels of CO2 haven't been that high since the Pliocene Epoch, between 3 million and 5 million years ago, according to the Scripps Institution of Oceanography.
During that era, global average temperatures were between 5.4 and 7.2 degrees F (3 to 4 degrees C) warmer than today, and sea level was up to 131 feet (40 meters) higher in some areas.
The effects of global warming are already visible in many areas of the world: In Montana's Glacier National Park, where about 150 glaciers were once found, only 25 glaciers larger than 25 acres remain, according to the U.S. Geological Survey.
And temperatures in North America reached record highs in 2012, making it the hottest year since recordkeeping began in 1895. Scientists also recorded the second-greatest number of temperature extremes (extreme highs and lows) in 2012.
Global warming and climate change
The rise in average global temperatures, serious as it is, is just one aspect of global warming. Scientists are also concerned that global warming will cause climate patterns to change worldwide.
Climate change resulting from warming temperatures will likely include major changes in wind patterns, annual precipitation and seasonal temperatures variations. These changes are expected to last for several decades or longer, according to the EPA.
In the northeastern United States, for example, climate change is likely to bring increased annual rainfall.
In the Pacific Northwest, however, summer rainfall is expected to decrease, while winter precipitation is more likely to fall as rain instead of snow. This will reduce the amount of water available as snowmelt during the summer months.
How to address global warming
A growing number of business leaders, government officials and private citizens are increasingly concerned about global warming and its implications, and are proposing steps to reverse the trend.
Reducing greenhouse gas emissions is perhaps the most fundamental way to address global warming, and 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 to power electrical generating plants.
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, will 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.
This reference article was first published on May 30, 2013.
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