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Greenhouse Effect

 

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Overview
Climate; that is, the weather over a long period; depends on Earth's average temperature. This temperature stays relatively constant because Earth's surface absorbs energy from sunlight, changing it to heat (infrared radiation). Greenhouse gases, particularly water vapor, absorb the resulting heat energy and hold it in the atmosphere instead of allowing it to radiate out into space. (In an actual greenhouse, the glass windows block the heat's exit.) This greenhouse effect keeps us warm, but scientists are concerned that humans may be creating problems by adding certain greenhouse gases to the atmosphere, such as carbon dioxide (from carbon-based fuels), chlorofluorocarbons (from aerosol cans), and methane (from cow digestion). The accumulation of greenhouse gases could result in global warming, an increase in the average temperature that would probably lead to climate change. If the worst predictions come true, we may have to deal with melting polar ice caps; rising sea levels; uninhabitable coastal areas (where half the world's population now lives); and wild, unpredictable storms. Agricultural areas might turn to desert, while barren areas might become fertile. Researchers have analyzed air that was trapped in glacial ice 160,000 years ago. By comparing that air to the air in our current atmosphere, they have discovered an increase in carbon dioxide as the use of fossil fuels has increased. Some scientists aren't convinced that excess greenhouse gases will actually cause global warming. They point out that cooling effects also are taking place. For instance, the oceans absorb much of the carbon dioxide that human activity contributes to the atmosphere. Higher temperatures cause more water to evaporate into clouds, which shade Earth from sunlight, cooling it. Particulate matter from volcanic eruptions and other pollutants deflects sunlight and also contributes to cooling. The greenhouse effect is a very complex issue. Much of the information we have about global warming comes from computer models that estimate climate change. These estimates may be inexact because the atmosphere is so huge. In addition, an observed temperature increase may be caused by something else. Some measurements suggest that variations in the sun's light output cause temperature changes far more significant than those caused by greenhouse gases.

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