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Carbon Dioxide and Climate: A Scientific Assessment
ice decreases; this lowers the mean global albedo and increases the fraction of solar radiation absorbed. Estimates of this effect lead to a further decrease of λ by between 0.1 and 0.9 W m−2 K−1 with 0.3 a likely value. Some uncertainty in albedo feedback also arises from cloud effects discussed in the next section. Taking into consideration all the above direct effects and feedbacks, we estimate λ to be 1.7±0.8 W m−2 K−1 and hence ΔT for doubled CO2 to lie in the range of 1.6 to 4.5 K, with 2.4 K a likely value.
3.2 CLOUD EFFECTS
Most clouds are efficient reflectors of solar radiation and at the same time efficient absorbers (and emitters) of terrestrial infrared radiation. Clouds thus produce two opposite effects: as cloud amount and hence reflection increase, the solar radiation available to heat the system decreases, but the decreased upward infrared radiation at the tropopause and downward radiation from the base of the clouds raises the temperature of the earth’s surface and troposphere.
Because the change of solar absorption dominates, the net result of increased low cloudiness, and very likely also middle cloudiness, is to lower the temperature of the system. The net effect of an increased amount of high cirrus clouds is less certain because their radiative characteristics are sensitive to height, thickness, and microphysical structure. Present estimates are that they raise the temperature of the earth’s surface and the troposphere.
It follows that if a rise in global temperature results in an increased amount of low or middle clouds, there is a negative feedback, and if a rise in global temperature results in an increased amount of high clouds, there is a positive feedback. The effect of cloud albedo by itself gives a negative feedback. Thus if clouds at all levels were increased by 1 percent, the atmosphere-earth system would absorb about 0.3 m−2 less solar radiation and lose about 0.5 W m−2 less thermal radiation. The net effect would be a cooling of about 0.4 W m−2, or, if this occurred together with a doubling of CO2, a decrease of ΔQ from 4.0 to 3.6 W m−2.
How important the overall cloud effects are is, however, an extremely difficult question to answer. The cloud distribution is a product of the entire climate system, in which many other feedbacks are involved. Trustworthy answers can be obtained only through comprehensive numerical modeling of the general circulations of the atmosphere and oceans together with validation by comparison of the observed with the model-produced cloud types and amounts. Unfortunately, cloud observations in sufficient detail for accurate validation of models are not available at present.
Since individual clouds are below the grid scale of the general circulation models, ways must be found to relate the total cloud amount in a grid box to