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atmospheric concentrations
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1
Introduction
Greenhouse gases in the atmosphere have an important influence
on the climate of our planet. Simply stated, greenhouse gases
impede the outward flow of infrared radiation more effectively than
they impede incoming solar radiation. Because of this asymmetry,
the earth, its atmosphere, and its oceans are warmer than they
would be in the absence of such gases.
The major greenhouse gases are water vapor, carbon dioxide
(CO2), methane (CH4), chlorofluorocarbons (CFCs) and
hydrogenated chlorofluorocarbons (HCFCs), ozone (O3), and nitrous oxide (N2O). Without the naturally occurring
greenhouse gases (principally water vapor and CO2), the earth's average temperature would
be about 33°C (59°F) colder than it is, and the planet
would be much less suitable for human habitation.
Human activity has contributed to increased atmospheric
concentrations of CO2, and CH4, and CFCs. The increased atmospheric
concentrations of greenhouse gases may increase average global
temperatures. The possible warming due to increased concentrations
of these gases is called ''greenhouse warming." The atmospheric
concentration of CO2 in 1990 was 353
parts per million by volume (ppmv), about 25 percent greater than
it was before the Industrial Revolution (about 280 ± 10 ppmv
prior to 1750). Atmospheric CO2 is
increasing at about 0.5 percent per year. The concentration of
CH4 was 1.72 ppmv in 1990, or
slightly more than twice that before 1750. It is rising at a rate
of 0.9 percent per year. CFCs do not occur naturally and were not
found in the atmosphere until production began a few decades ago.
Continued increases in atmospheric concentrations of greenhouse
gases would affect the earth's radiative balance and might cause a
significant amount of additional greenhouse warming.
General circulation models (GCMs) are the principal tools used
to project climatic changes. At their present level of development,
GCMs project that
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an increase in greenhouse gas concentrations equivalent to a
doubling of the preindustrial level of atmospheric CO2 would produce global average temperature
increases between 1.9° and 5.2°C (3.4° and 9.4°F).
The larger of these temperature increases would mean a climate
warmer than any in human history. The consequences of this amount
of warming are unknown and could include extremely unpleasant
surprises.
During the last 100 years the average global temperature has
increased between 0.3° and 0.6°C (0.5° and 1.1°F).
This temperature rise could be attributable to greenhouse warming
or to natural climate variability; with today's limited
understanding of the underlying phenomena, neither can be ruled
out.
Increases in atmospheric greenhouse gas concentrations probably
will be followed by increases in average atmospheric temperature.
We cannot predict how rapidly these changes will occur, how intense
they will be for any given atmospheric concentration, or, in
particular, what regional changes in temperature, precipitation,
wind speed, and frost occurrence can be expected. So far, no large
or rapid increases in the global average temperature have occurred,
and there is no evidence yet of imminent rapid change. But if the
higher GCM projections prove to be accurate, substantial responses
would be needed, and the stresses on this planet and its
inhabitants would be serious.
It is against this backdrop that prudent, necessarily
international, plans should be made and actions undertaken. These
plans and actions should start with responses justified by the
current credibility of the threat. They also should include
preparatory measures that can set the stage for more far-reaching
responses if the evidence of need becomes persuasive. It is in this
setting that the Synthesis Panel performed its analyses and
developed recommendations for action by the United States.
The principal findings and conclusions of the panel are
summarized in Chapter 8, and its recommendations are in Chapter 9.
Appendix A, "Questions and Answers About Greenhouse Warming,"
discusses relevant issues in a format the panel believes may be
especially useful to the reader.
