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 (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 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°
C (approximately 1.8° 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 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.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
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.2 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.


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Earth (1990)

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