be accurately tested by reference to actual paleoenvironmental data from Africa and Eurasia and the surrounding oceans. With the availability of greenhouse gas records and known orbitally controlled changes in solar radiation, along with known changes in orography, volcanism, coastlines, and ocean gateways, models have proven to be remarkably accurate in simulating past climates. However, challenges remain in accurately simulating the waxing and waning of ice sheets and the effects of glacial climates on tropical climates due to the complex interactions of several critical factors—the tropical forcing of monsoons by precession changes and forcing due to high-latitude climate changes, CO2 changes, sea-level changes, and deep-ocean circulation changes.

Time Window 4: Continuing Large Glacial/Interglacial Cycles (0.5-0 Ma)

The last several glacial/interglacial cycles, and in particular the climate of the last 150,000 years, is relatively well documented worldwide, although details of the spatial and temporal variability of climate on the continents are scant except for the past 20,000 years. The previous interglacial, the period around 125 ka, has been the subject of detailed study, as has the subsequent onset of glaciation around 115 ka, the LGM around 22 ka, and the subsequent warming trends culminating in the mid-Holocene, around 9-6 ka. The changes in forcing are relatively well known—primarily the orbitally-forced changes in insolation and the changing levels of CO2 and methane. This period then offers unique opportunities for detailed time-space simulation of climate, the comparison of the simulated climate with observations, and the subsequent analysis of potential linkages with hominin evolution and dispersal.

There are H. erectus fossils in China as recent as 250 ka and it should be possible to simulate the climate in Asia at this time and in particular the intensity of both summer and winter monsoons. Both H. neanderthalensis and H. sapiens appear in the period directly before or slightly into the penultimate glaciation (190-130 ka), and it should be possible to simulate the climate and vegetation of this period in considerable detail. There is evidence of megadroughts in tropical Africa between 135 ka and 90 ka, a period that preceded the dispersal of humans out of Africa around 60 ka and their widespread movements thereafter. It will be possible to use climate models, focusing primarily on the known orbital forcing and known glacial boundary condition forcing (ice sheets, sea level, CO2 level) to simulate the period of the past 125 ka with considerable accuracy and make detailed comparisons with the observations, both spatial and temporal. Millennial-scale changes in climate are also well documented in both polar and tropical latitudes during much of this most recent period, and offer the unique opportunity to study the possible causes of these events and the possible effects on ecosystems and humans.



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