An energy balance climate model is used to study climatic sensitivity to surface boundary conditions and to hypothetical atmospheric properties of the Last Glacial Maximum (LGM) of 18 ka. Imposition of Climate: Long-Range Investigation, Mapping, and Prediction (CLIMAP) sea and land ice and CLIMAP sea surface temperatures (SSTs) gives a global mean atmospheric freely predicted SSTs, sea ice, and seasonal land snow cover causes global mean cooling of 3.2 ¿C, northern hemisphere sea ice extent close to the CLIMAP amounts, but relatively little response in the southern hemisphere. Reducing atmospheric CO2 from 280 to 200 parts per million by volume (ppmv), applying the base case LGM aerosol increase scenario of L. D. D. Harvey (1988a), or decreasing the effective cloud droplet radius over oceans by 4% with constant liquid water content (LWC), as suggested by data from polar ice cores, gives global mean atmospheric coolings of 1.5 ¿C, 2.2 ¿C, and 0.9 ¿C, respectively. When any one of these atmospheric changes is combined with imposed CLIMAP land glacier ice, the global mean cooling ranges from 4.0 ¿C to 4.7 ¿C and sea ice extent close to the CLIMAP 18-ka amounts occurs in both hemispheres.

If all three of the above atmospheric chagnes are combined with CLIMAP 18-ka land ice, then unrealistically large cooling and sea ice extent occurs. This can be used to constrain estimats of the increase of atmospheric aerosols and of the increase in cloud reflectivity due to changes in cloud droplet radius during the LGM, or may indicate that a stabilizing negative feedback is needed. One possible feedback is a temperature-LWC feedback. This feedback is characterized by the parameter f≡(1/L)∂L/∂T, where L=LWC. Values of 0.03 to 0.05 K-1 are required to give realistic LGM simulations in the presence of the combined CO2, aerosol, and cloud optics forcings, whereas no cloud optics feedback is needed if the aerosol and cloud optics forcings were much smaller than assumed here. Applying the same model to CO2 doubling (320 to 640 ppmv) with f ranging from 0 to 0.05 K-1 gives a global mean surface air temperature response of 2.2--3.0 ¿C. ¿ American Geophysical Union 1989