Global Warming The greenhouse effect, in environmental science, is a popular term for the effect that certain variable constituents of the Earth's lower atmosphere have on surface temperatures. It has been known since 1896 that Earth has been warmed by a blanket of gasses (This is called the "greenhouse effect." ). The gases -- water vapor (H 2 O), carbon dioxide (CO 2), and methane (CH 4) -- keep ground temperatures at a global average of about 15 degrees C (60 degrees 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. Since 1850 there has been a mean rise in global temperature of approximately 1^0 C (approximately 1. 8^0 F). Even a limited rise in average surface temperature might lead to at least partial melting of the polar icecaps and hence a major rise in sea level, along with other severe environmental disturbances.

An example of a runaway greenhouse effect is Earth's near-twin planetary neighbor Venus. Because of Venus's thick CO 2 atmosphere, the planet's cloud-covered surface is hot enough to melt lead. Water vapor is an important 'greenhouse' gas. It is a major reason why humid regions experience less cooling at night than do dry regions, . However, variations in the atmosphere's CO 2 content are what have played a major role in past climatic changes. In recent decades there has been a global increase in atmospheric CO 2, 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 CO 2 increase should raise the average temperature at the Earth's surface. As the atmosphere warmed, the amount of H 2 O would probably also increase, because warm air can contain more H 2 O 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 CO 2 by phytoplankton in the ocean.

These would act as natural feedback's, lowering temperatures. 8 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 CH 4, 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. 2 Despite such uncertainties, numerous scientists have maintained that the rise in global temperatures in the 1980 s and early 1990 s is a result of the greenhouse effect. A report issued in 1990 by the Intergovernmental Panel on Climate Change (IPCC), prepared by 170 scientists worldwide, further warned that the effect could continue to increase markedly.

Most major Western industrial nations have pledged to stabilize or reduce their CO 2 emissions during the 1990 s. The U. S. pledge thus far concerns only chlorofluorocarbons (CFCs).

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