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 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 CH4,
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.

Despite such uncertainties, numerous scientists have maintained that the
rise in global temperatures in the 1980s and early 1990s 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 CO2 emissions during the 1990s.
The U.S. pledge thus far concerns only chlorofluorocarbons (CFCs). CFCs attack
the ozone layer and contribute thereby to the greenhouse effect, because the
ozone layer protects the growth of ocean phytoplankton. 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.

Category: Science