(From Grolier Multimedia Encyclopedia)

Greenhouse Effect

The greenhouse effect is a popular term in environmental science for the effect that certain variable constituents of the Earth's lower atmosphere have on surface temperatures. These gases—water vapor, carbon dioxide, and methane—keep ground temperatures at a global average of about 15° C (60° F). Without them the average would be below the freezing point of water. The gases have this effect because as incoming solar radiation strikes the surface, the surface gives off infrared radiation, or heat, that the gases trap and keep near ground level. The effect is comparable to the way in which a greenhouse traps heat, hence the term. Environmental scientists are concerned that changes in the variable contents of the atmosphere—particularly changes caused by human activities—could cause the Earth's surface to warm up to a dangerous degree. Even a small rise in average surface temperature might lead to at least partial melting of the polar ice caps and hence a major rise in sea level, along with other severe environmental disturbances.

Water vapor is an important "greenhouse" gas. It is a major reason why humid regions experience less cooling at night than do dry regions. CO2 is a more important factor than water vapor, however, in the greenhouse effect. Variations in the atmosphere's CO2 content played a major role in past climatic changes, and in recent decades there has been a global increase in atmospheric CO2, largely as a result of the burning of fossil fuels. If the many other determinants of the Earth's present global climate remain more or less constant, the CO2 increase should raise the average temperature at the Earth's surface. With the warming of the atmosphere, the amount of water vapor would probably also increase, because warm air can contain more water than can cooler air. This process might go on indefinitely. On the other hand, reverse processes could develop such as increased cloud cover and increased absorption of CO2 by phytoplankton in the ocean. These would act as natural feedbacks, lowering temperatures.

In fact, a great deal remains unknown about the cycling of carbon through the environment and in particular about the role of oceans in this atmospheric carbon cycle. Many further uncertainties exist in greenhouse-effect studies because the temperature records being used tend to represent the warmer urban areas rather than the global environment. Beyond that, the effects of methane, natural trace gases, and industrial pollutants—indeed, the complex interactions of all of these climate controls working together—are only beginning to be understood by workers in the environmental sciences.

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