The Greenhouse Effect
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. These gases--water vapor (H2O), carbon dioxide (CO2), and methane (CH4)--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 H20. 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 limited 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 agitation. An example of a runaway greenhouse effect is Earth's near-twin planetary neighbor Venus. Because of Venus's thick CO2 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 CO2 content are what have played a major role in past climatic changes. 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. As the atmosphere warmed, the amount of H2O would probably also increase, because warm air can contain more H2O than can...
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