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negative feedbacks that could limit the continued buildup of CO2 in the atmosphere or, alternatively, the intensity of global warming in the presence of increased concentrations of greenhouse gases. A negative feedback effect related to an increase in global cloud cover appears now as the most likely way for the earth to escape any serious global warming. Henderson-Sellers and McGuffie (1989), in an analog study using historical data, found a positive correlation between temperature and cloud cover. However, their data covered only a small fraction of the earth's surface and, as noted above, a majority of GCM runs have predicted a decrease in global cloud cover, rather than an increase, to accompany global warming. Lindzen (1990) has argued that a negative feedback effect will arise as enhanced convection carries additional heat upward to be radiated into space from the upper troposphere and intensified subsidence between the convective cloud towers thins the water vapor shield between the surface and the top of the atmosphere. However, his calculations do not take account of the fact that convective clouds introduce large amounts of water into the upper troposphere, much of it as ice from anvil clouds. Randall (1989) has noted the need for improved parameterization of the high stratiform clouds induced by deep convection, which could be the basis of improved assessments of suggestions such as those of Lindzen. It is possible that the human race will bum up nearly all the available fossil fuels in the next 200 to 300 years. Comparing the atmospheric and fossil fuel reservoirs (table 1) shows that, in theory, such rapid consumption of fossil fuels could push the atmospheric CO2 concentration to almost unimaginable levels, say 1500 or 2000 ppm. The processes of rock weathering and advection into the deep ocean are much too slow to offset the effects of large increases in the global rate of fossil fuel consumption. Interaction with the biosphere poses the only possible limit in such a case. In particular, according to Sellers and McCarthy (1990), ". . . land vegetation is now thought likely to dominate the short-term biospheric response to increasing atmospheric CO2'', Some persons have argued that very large increases in atmospheric CO2 will be self-limiting because the growth of vegetation will accelerate in response to the increased concentrations of CO2, There have been numerous investigations of the impacts of increased concentrations of CO2 on plant growth, but mostly by persons interested in the potential impacts on crop yields (e.g., Waggoner 1983). In general, the effects are modest and some crops, com for example, become saturated at CO2 concentrations above 500 ppm. That is, further increases in the atmospheric CO2 concentration will produce no further acceleration in their growth rates. However, Idso (1991) has provided some experimental evidence that the growth rates of trees are much more sensitive to increases in CO2 than are the growth rates of crops. He reports that production of wood in one species increased by a factor of 2.8 when trees were exposed to a CO2 concentration of 600 ppm. As trees account for well over half of the biospheric exchanges between the atmosphere and the earth's surface (both land and water), their sensitivity to increases in CO2 concentrations could be very important. Idso's (1991) calculations indicate that, even in the face of a doubling of the present rate of fossil fuel emissions" trees could limit the concentration of atmospheric CO2 to 700 ppm. There is much uncertainty associated with his numbers but, clearly, the potential of CO2 removal by growing trees is important enough to justify further study. If fossil fuel use continues to increase and accelerated tree growth fails to prevent a rise in the concentration of atmospheric CO2 to over 1,000 ppm, could such a concentration lead to a runaway greenhouse effect, say another 6 DC warming over .md above the 3 DC predicted by some models for a doubling to 600 ppm? Fortunately, the answer appears to be "No." If the atmosphere becomes totally opaque in the CO2 absorption bands, further additions will have no direct effect and at that point the feedback effects also would begin to stabilize. The atmosphere is already almost opaque in the middle of the CO2 absorption bands, and increases in its concentration beyond 600 ppm will affect mainly the fringes of the bands. Therefore the additional warming for each unit increase in CO2 concentration will drop off asymptotically toward zero, as noted by Arrhenius (1896). This comforting argument does not apply to additions of other greenhouse gases to the atmosphere, because each one has its own absorption bands. Natural physical processes will limit the duration as well as the intensity of global warming. Increased rock weathering could draw down the excess atmospheric CO2 over a few thousand years, but the dissolution of excess CO2 in the ocean promises a quicker end to the enhanced greenhouse effect. The relative magnitudes of the various carbon reservoirs (table I) suggest that the ocean depths can absorb without strain all the carbon in the world's fossil fuel reserves. Once the fossil fuel is exhausted, the atmospheric CO2 concentration will begin to decrease as CO2 continues to dissolve in the ocean's mixed layer and be advected into the depths. Advection of CO2-enriched seawater into the deep ocean promises all eventual end to the enhanced greenhouse effect. It will take a few hundred years for enough upwelling and subsidence of ocean water to occur to bring the concentration down close to that of today, so global warming, if it proves real, will last through 10 or 20 generations of human beings. Note that, for reasons discussed earlier, the enhanced greenhouse warming would not relax its grip appreciably until the concentration of CO2 fell to near the level of today, while the thermal inertia of the ocean would prove as effective in prolonging global warming as in tempering its onset. It appears that the slight enhancement of carbon in the ocean, as excess atmospheric CO2 is dissolved in the mixed layer and advected into the depths, will eventually be brought down by increased deposition of organic matter and shells on the sea floor. That process would also require centuries to work itself out. Roberts (1987) estimated that the "cleanup" of the ocean would not be complete before the year 3000. 114