Decreased global rainfall during the past Ice Age

Abstract

WE have made a synthesis of atmospheric conditions for 20,000 yr BP based on fairly standard palaeoclimatological data and procedures. From mean zonal conditions at 20° latitude intervals from 80°N to 80°S we have computed radiative heating rates and the energy balance of atmospheric columns. We find that net cooling of the atmosphere by radiative processes was smaller at 20,000 yr BP than it is now. The total heating of the air, which sums to zero in the annual average except during periods of climatic change, is the sum of heating by latent heat liberation, heating by conduction and convection from the boundary layer, and radiative cooling (see for example Fig. 7 in ref. 1). Because the change in radiative cooling is comparable to boundary layer heating itself, we postulate that the decrease in cooling is accompanied primarily by a decrease in heating from rainfall and therefore that global rainfall was smaller at 20,000 yr BP than it is now. Examination of the magnitudes of the three components suggests that the decrease in rainfall was about 10%.