Sensitivity of Late-Glacial and Holocene Climates to the Combined Effects of Orbital Parameter Changes and Lower Boundary Condition Changes: “Snapshot” Simulations with A General Circulation Model for 18, 9, AND 6 ka BP

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Abstract
Sensitivity experiments can be used to illustrate the response of the general circulation to prescribed changes in lower boundary conditions (such as ocean temperature) or external forcing conditions (such as solar radiation). The climatic record from the late-glacial and the Holocene provides examples for both types of prescribed change experiments. A number of general circulation model experiments have been carried out. These are reviewed.At 18 ka 8P, orbital parameter values were very much like those of today, but the lower boundary conditions (ocean temperature, ice-sheet extent, etc.) were very different. The change in ocean temperature, and ice-sheet extent and thickness, were prescribed from the results of the Climate: Long-range Investigation Mapping and Prediction (CLIMAP) project.At 9 ka BP, orbital parameter values were very different from present, leading to increased radiation in July and decreased radiation in January (compared to present). The North American ice sheet still covered a significant area, so that lower boundary conditions also differed from the present ones. The combined and individual effects of these prescribed changes on the general circulation are reviewed, particularly in the context of changes of the monsoon circulation.At 6 ka BP, the solar radiation distribution differed from that of today in much the same fashion as at 9 ka BP, although the magnitude of the change was reduced. Lower boundary conditions were probably very similar to those of today.A series of experimental results from 18, 9, and 6 ka BP are presented as “snapshot” estimates of the paleoclimate of those times. The results are based upon simulations with the community climate model of the National Center for Atmospheric Research.