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surface temperature
Page 153
18
Conclusions
Despite (1) uncertainties in the interpretation of the climate
record, (2) severe limitations on the predictive capabilities of
the models, (3) masking effects of the natural variability of the
models, and (4) masking effects of the natural variability of the
climatic system, there is clear evidence and wide agreement among
members of the atmospheric sciences community and the members of
this panel about several basic facts:
1. The atmospheric concentration of CO2 has increased by at least 25 percent
since preindustrial times and is currently increasing at about 0.5
percent per year.
2. The atmospheric concentration of CH4 has doubled during that period and is
increasing at about 0.9 percent per year.
3. Atmospheric concentrations of CFCs, which are a result
of industrial activities and have been released into the atmosphere
in quantity only since World War II, are currently increasing at
about 4 percent per year.
4. Items 1, 2, and 3 are primarily direct consequences of
human activities.
5. Current interpretations of temperature records reveal
that the global average temperature has increased between 0.3°
and 0.6°C (0.5° and 1.1°F) during the last century.
As a result, the panel concludes that there is a reasonable
chance of the following:
1. In the absence of greater human effort to the contrary,
greenhouse gas concentrations equivalent to a doubling of the
preindustrial level of CO2 will
occur by the middle of the next century.
2. The sensitivity of the climatic system to greenhouse
gases is such that the equivalent of doubling CO2 could ultimately increase the average
global temperature by somewhere between 1° and 5°C
(1.8° and 9°F). This range is slightly broader than those
used by other groups. Prudence dictates that the uncertainties in
the science base call for wider rather than narrower ranges of
projected temperatures for use in policy choices. In the panel's
view, this range expresses much less unwarranted faith in the
numbers produced by GCMs than does a narrow range.
Page 154
3. The transfer of heat to the deep ocean occurs more
slowly than the transfer of heat within the atmosphere or within
the upper layers of the ocean. The resulting transient period, or
''lag," means that the global average surface temperature at any
time is lower than the temperature that would prevail after all the
redistribution had been completed. The greater the response, the
faster the warming; however, the increase in the warming rate is
less than proportional to the climate sensitivity, so it will take
longer for the full warming to appear. At the time of equivalent
CO2 doubling, for example, the
global average surface temperature may be as little as one-half the
ultimate equilibrium temperature associated with those
concentrations.
4. A rise in sea level may accompany global warming,
possibly in the range of 0 to 60 cm (0 to 24 inches) for the
temperature range listed above. This range allows for uncertainties
in estimates of current sea level change. Zero is included in the
range not only because of the uncertainties but also because
precipitation in the Antarctic functions to partially offset
thermal expansion.
5. A wide range of potentially amplifying or moderating
feedbacks have been suggested that involve atmospheric composition
and climatic changes. Examples include increased CH4 emissions as the permafrost melts,
increased carbon uptake by plants at higher CO2 concentrations, increased summer drying
of continental interiors, increased continental precipitation in
winter, increased hurricane frequency and/or intensity, and many
more potential changes and surprises. Convincing quantitative
demonstrations and confirmations of these and other potential
changes are lacking, and there is no evidence that any of the
changes are imminent, but none of them are precluded.
