General circulation model simulation of mild nuclear winter effects

The climatic effects of an elevated uniform global layer of purely absorbing smoke of absorption optical depth 0.2 have been simulated using a version of the 9-level spectral model of McAvaney et al. (1978). The model was run at rhomboidal wave number 21 with convective adjustment, prognostic precipitation and soil hydrology, but fixed zonally averaged climatological cloud and fixed sea surface temperature, for constant January and July conditions with and without smoke absorption. Results show a reduction in convective rainfall in the tropics and monsoonal regions of the order of 50%, with diurnal average soil surface coolings of several degrees C except in those locations where the reduction in soil moisture is sufficient to effectively stop evaporation at the surface. In that case, small increases in temperature may occur. Results over Australia are consistent with the zonal mean picture. Run in a diurnal cycle mode, the model shows that daily maximum temperatures are more strongly affected, with soil surface coolings of the order of 2°-3° C in summer (with some local warmings) and 4°-6° C in winter. Overninght minimum temperatures cool by only 1°-2° C in both summer and winter. Possible effects of a lowering of sea surface temperature, variations in cloud cover, neglect of scattering by smoke, and infrared absorption and emission by the smoke are discussed.