General circulation model experiments at 3000-year intervals for the past 18 000 years were made to estimate the magnitude, timing, and pattern of the climatic response to prescribed changes of orbital parameters (date of perihelion, axial tilt, eccentricity) and glacial-age lower boundary conditions (ice sheets, land albedo, sea ice and sea surface temperature). The experiments used the Community Climate Model (CCM) of the National Center for Atmospheric Research (NCAR). The response of monsoon circulations and tropical precipitation to the orbitally produced solar radiation changes was much larger than the response to changes of glacial-age boundary conditions. The continental interior of Eurasia was 2–4 K warmer in summer, and summer monsoon precipitation of North Africa-South Asia was increased by 10–20% between 12 000 and 6000 yr BP (before present) when perihelion occurred during northern summer (rather than in winter as now) and the earth's axial tilt was larger than now. Southern Hemisphere summer monsoons were weaker during the same period. In northern midlatitudes, glacial-age features such as the North American ice shed exerted a strong influence on the climate until 9000 yr BP. Much of the climatic change of the period 12 000 to 6000 yr BP can be described as an amplified (weakened) seasonal cycle in response to the larger (smaller) seasonal radiation extremes of the Northern (Southern) Hemisphere. Summers were warmer and winters colder in Northern Hemisphere lands, but there was little change in annual average temperature. However, because of the nonlinear relationship between saturation vapor pressure and temperature, the sensitivity of the hydrologic cycle to orbital parameter changes was larger in summer than in winter (and in the tropics rather than high latitudes); in the northern tropics, this led to a net increase in estimated annual average precipitation and precipitation minus evaporation. Many features of the results are in agreement with geologic evidence.

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