A climate model is used to simulate, for an idealized Pangean continent, the changes of temperature, wind, and precipitation produced by changes in earth’s orbital parameters. The calculation uses an orbit of moderate eccentricity (0.025). Enhanced summer and winter monsoon circulations and increased summer rainfall and runoff are simulated for the hemisphere where perihelion occurs in summer and aphelion in winter. The increases in rainfall and runoff are about 25% or more, relative to the average climate. These increases occur in the tropical and subtropical belt extending from the equator to about 40° latitude in some regions. These same regions would experience 25% or more reductions in rainfall and runoff in the opposite phase of the precession cycle. These model results suggest that rainfall and runoff would undergo cyclic changes of ± 25% or more with periods of 23,000 years over a substantial fraction of the Pangean tropics and subtropics.

Abstract Atmospheric general circulation models (AGCMs) can be used to simulate climatic patterns—past, present, or future— provided that certain boundary and atmospheric conditions are specified. Examples of relevant boundary conditions are the solar radiation at the top of the atmosphere, composition of the atmosphere, height of the lower boundary (land, mountains, ice sheets), and characteristics of the lower boundary (albedo, roughness, sea-ice location, ocean temperature). Given this information, AGCMs simulate many (but not all) features of the present climate (Pitcher and others, 1983). These models are also used to estimate future conditions; for example, the sensitivity of the climate to increased concentration of carbon dioxide in the atmosphere (Washington and Meehl, 1984). AGCMs have been used to simulate past climates in situations where geologic evidence or astronomical theory provides estimates of all or most of the appropriate past boundary conditions. Another application of AGCMs is to estimate the sensitivity of the simulated climate to likely changes of a single boundary condition (solar radiation, presence/absence of ice sheets, etc). Simulations of the climate of the last glacial maximum are reported by Alyea (1972), Williams and others (1974), Gates (1976a, b), Manabe and Hahn (1977), Hansen and others (1984), Rind and Peteet (1985, 1986), and Manabe and Broccoli (1985). Rind and others (1986) have studied the impact of cold North Atlantic sea-surface temperatures on the climate, with implications for understanding the Younger Dryas (11-10 ka). Climate simulations for 9 ka are found in Kutzbach (1981), Kutzbach and Otto-Bliesner (1982), Kutzbach and Guetter (1984), and Webb and others (1985). Kutzbach and Guetter (1986) report simulations for the period 18 ka to present at 3,000-year intervals; the experiments combine changes in solar radiation, atmospheric composition, and lower boundary conditions. Barry (1983) lists many of these climatic simulations of the last glacial maximum and provides an overview of the ice-age climatology of North America based upon observations.

Abstract The Laurentide ice sheet expanded and retreated as part of the response of the climate system to the changes in solar radiation that are determined by the Earth’s orbital variations (Berger and others, 1984). In turn, the area, height, and reflectivity of the ice sheet influenced the climate of the Northern Hemisphere. During the last deglaciation the changing size of the ice sheet and the changing latitudinal and seasonal distribution of solar radiation (Ruddiman and McIntyre, 1981) were a continuously varying set of boundary conditions for the climate of eastern North America. Fossil-pollen data from eastern North America record the past 18,000 years of vegetation changes that occurred in response to the consequent climatic changes (Jacobson and others, this volume). The purpose of our study is to examine how the changes in the boundary conditions during the past 18,000 years have governed the climatic changes of eastern North America that accompanied deglaciation and are recorded in the fossil-pollen data. Kutzbach (this volume) and Kutzbach and Guetter (1986) used a general circulation model of the atmosphere—the Community Climate Model of the National Center for Atmospheric Research (NCAR CCM)—to simulate the response of climate to changing boundary conditions during the past 18,000 years. Models like the NCAR CCM illustrate the physically consistent response of individual climate variables to changes in the boundary conditions (Kutzbach, 1985). One focus of our paper is to use the model simulations to examine not only how atmospheric circulation and climate changed in eastern North America during d?aciation but also how these changes were controlled by the changing area, height, and reflectivity of the ice sheet and by the changes in the latitudinal and seasonal distribution of solar radiation. Thorough tests of such models are needed, however, before they can be routinely used to describe the nature and causes of past climatic changes (Webb and Wigley, 1985).