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.
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.