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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 chlorofluoro-
carbons (HCFCs), ozone (O3), and nitrous oxide (N2O). Without the natu-
rally 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, 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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2
POLICY IMPLICATIONS OF GREENHOUSE WARMING
an increase in greenhouse gas concentrations equivalent to a doubling of the
preindustrial level of atmospheric CO2 would produce global average tem-
perature 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°FJ. 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 panel performed its analyses and developed recommenda-
tions 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. Following the report
is an appendix called "Questions and Answers About Greenhouse Warming,"
which discusses relevant issues in a format the panel believes may be especially
useful to the reader.
Representative terms from entire chapter:
average temperature
