Terrestrial Conditions at the Last Glacial Maximum and CLIMAP Sea-Surface Temperature Estimates: Are They Consistent?

CLIMAP (1981, “Seasonal Reconstruction of the Earth's Surface at the Last Glacial Maximum,” Geological Society of America Map and Chart Series MC-36) boundary conditions were used as inputs to the GISS general circulation model, and the last glacial maximum (LGM) climate was simulated for six model years. The simulation was compared with snow line depression and pollen-inferred temperature data at low latitudes, specifically for Hawaii, Colombia, East Africa, and New Guinea. The model does not produced as much cooling at low latitudes as is implied by the terrestrial evidence. An alternative experiment in which the CLIMAP sea-surface temperatures were uniformly lowered by 2°C produces a better fit to the land data although in Hawaii model temperatures are still too warm. The relatively warm CLIMAP tropical sea-surface temperatures also provide for only a slight decrease in the hydrologic cycle in the model, in contrast to both evidence of LGM tropical aridity and the results of the experiment with colder ocean temperatures. With the CLIMAP sea-surface temperatures, the LGM global annual mean surface air temperature is 3.6°C colder than at present; if the ocean temperatures were allowed to cool in conformity with the model's radiation balance, the LGM simulation would be 5°–6°C colder than today, and in better agreement with the tropical land evidence.