The global distribution of July climate has been simulated with a two-level atmospheric general circulation model using the surface boundary conditions of sea-surface temperature, ice-sheet topography and surface albedo assembled by CLIMAP for 18 000 years before present. These conditions respresent an approximate doubling of the ice-covered surface area of the earth, a 1°C decrease of the average sea-surface temperature, and an increase of the average surface albedo from 0.14 to 0.22. Compared with the simulation of present July conditions, the ice-age atmosphere is found to have been substantially cooler and drier, especially over the continents of the Northern Hemisphere, corresponding to an enhanced anticyclonic circulation over the major ice sheets and a general weakening of the summer monsoonal circulation. The midlatitude westerlies are strengthened and systematically displaced southward in the vicinity of the major ice sheets, along with an equatorward shift in the zones of maximum eddy activity.
On a global basis the July ice-age surface air temperature is 4.9°C lower than today's (5.8°C over the ice-free continents), while the global cloudiness and relative humidity show relatively small decreases. Ice-age precipitation in the Northern Hemisphere is about 20% below that simulated for today's July, with reduced convective rainfall over the continents accounting for most of the reduction. The intensity of the ice-age tropical Hadley circulation and the associated transports by the mean meridional circulation are reduced to about two-thirds of their present simulated values in response to reduced meridional gradients of net heating and moisture deficit. These results are in general agreement with those found from more simplified models and diagnostic calculations, and are verified at least in terms of the surface temperature by the available independent paleoclimatic data. Significant disagreement exists, however, with the temperature, pressure and circulation found in a previous study by Williams using the NCAR GCM with different ice-age boundary conditions, including in particular somewhat lower tropical sea-surface temperatures. The sensitivity of the simulated ice-age climate to such boundary condition changes needs further research, and more complete simulations are needed to establish the annual course of the ice-age climate.