3
Physical Processes Important for Climate and Climate Modeling

In order to assess the climatic effects of increased atmospheric concentrations of CO2, we consider first the primary physical processes that influence the climatic system as a whole. These processes are best studied in simple models whose physical characteristics may readily be comprehended. The understanding derived from these studies enables one better to assess the performance of the three-dimensional circulation models on which accurate estimates must be based.

3.1
RADIATIVE HEATING

3.1.1.
Direct Radiative Effects

An increase of the CO2 concentration in the atmosphere increases its absorption and emission of infrared radiation and also increases slightly its absorption of solar radiation. For a doubling of atmospheric CO2, the resulting change in net heating of the troposphere, oceans, and land (which is equivalent to a change in the net radiative flux at the tropopause) would amount to a global average of about ΔQ=4 W m2 if all other properties of the atmosphere remained unchanged. This quantity, ΔQ, has been obtained by several investigators, for example, by Ramanathan et al. (1979), who also compute its value as a function of latitude and season and give references to other CO2/climate calculations. The value 4W m−2 is obtained by several methods of calculating infrared radiative transfer. These methods have been directly tested against laboratory measurements and, indirectly, are found to



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