What Is Global Warming & What Causes It?

Global warming is a form of climate change that has been much talked about--and argued over--since scientists perceived an increase in the Earth's global temperatures from the middle of the 20th century through the present. While the cause of global warming has been the subject of heated debate, more and more evidence suggests it is well underway.

Global Warming

Global warming refers to increasing worldwide temperatures, and in popular use most often references a general warming trend beginning in the 20th century. Periods of warming--and of cooling--have occurred cyclically throughout the Earth's history, such as following the end of the last major Ice Age 10,000 years ago. But many scientists believe the modern pattern is mostly due to the activities of humankind, not fluctuations in the planet's orbit or other "natural" processes.

The Greenhouse Effect

The Earth's surrounding atmosphere ensures that radiation from the Earth, gleaned from daylight insolation from the sun, is not simply lost to space. Gases in the atmosphere absorb the long-wave terrestrial radiation and re-radiate it within the troposphere, or lower atmosphere, and back to Earth, keeping it substantially warmer.

Greenhouse Gases

The primary gases native to Earth's atmosphere that are responsible for the greenhouse effect are carbon dioxide, water vapor, methane and nitrous oxide. Levels of these and other synthetic greenhouse compounds like chlorofluorocarbons have increased as a result of agricultural and industrial byproducts and emissions as well as deforestation and other human-induced habitat changes. Boosted atmospheric carbon dioxide, in particular, released from sources like combustion and destroyed forests, appears to be responsible for the strengthening greenhouse effect. The National Oceanic and Atmospheric Administration reports that present levels of carbon dioxide exceed 380 parts per million by volume, whereas before the Industrial Revolution, which began in the 1700s, the statistic was closer to 280 parts per million by volume--and the gas's concentration has risen by 1.9 parts per million per year since the turn of the 21st century.

Changing Temperatures

Researchers have identified warming at both the Earth's surface and in the lower ranks of its troposphere some 2,500 to 26,000 feet above sea level, according to the NOAA. The administration notes that global surface temperatures are roughly 33 degrees Fahrenheit warmer than they were in the late 1800s, although the trend is not uniformly experienced.

Impacts of Warming

Even slight changes in average surface temperatures can greatly transform terrestrial and oceanic environments. For example, a hotter globe could melt off terrestrial glaciers and ice masses, raising sea levels--a trend that is already occurring. This would transform the coasts of continents and islands alike. Precipitation and local temperature regimes would change--possibly upping the frequency of droughts in the middle latitudes, for example. A warming climate will cast ripple effects across the Earth's ecological systems on both large and small scales. For example, increased incidences of insect infestations in the pine forests of western North America may be due to milder winters promoting the pest populations.