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Because these factors are changing simultaneously on glacial time scales, it is important that we consider their combined effects on chemical weathering. This we do in the next section by speculating on weathering feedbacks with vegetation, the transport of fine-grained material across climatic zones, the coverage of key rock types by glaciers, and the effect of latitudinally specific land area changes. We then close with a summary and a discussion of the implications of these effects for glacial/interglacial changes in oceanic and atmospheric chemistry, and climate. Uncertainties in our knowledge of many important parameters are sufficiently large that we can draw few firm conclusions, but we hope to have identified important areas for further research.
FACTORS THAT INFLUENCE GLOBAL CHEMICAL WEATHERING
A large body of observational and model evidence is available on differences between the climate of the Wisconsinan glacial maximum of about 18 ka and the modern climate. Convergence of results from different techniques allows some comparisons to be made with confidence (e.g., CLIMAP, 1976; Manabe and Broccoli, 1985; Kutzbach and Guetter, 1986; Rind, 1987; COHMAP, 1988). Globally, these include (where trends are known more accurately than absolute changes) the following:
the ice-age world was a few degrees colder than today. Ice-age cooling was smallest in the tropics (≈ to 5°C), increased to the poles (≈ 5 to 10°C) and was quite large on the midlatitude ice sheets (tens of degrees Celsius);
lower ice-age temperatures reduced both precipitation and evaporation below modern values (by about 10 percent); and
any changes in globally averaged soil moisture were smaller than the uncertainties in our knowledge.
soil moisture was significantly less than today in a large area of Eurasia south and east of the Fennoscandian ice sheet, and in a smaller area of North American south of the Laurentide ice sheet. These drier areas probably developed because of the combined effects of anticyclonic circulation over the ice sheets and of the ice sheets acting as cold traps for water vapor and reducing precipitation nearby;
the southwestern United States was significantly wetter than today, probably owing to a southward shift in storm tracks caused by the topography of the Laurentide ice sheet;
other regional changes occurred: soil moisture was probably reduced in equatorial regions, causing reduction in extent of ice-age tropical rain forests, but increased in some areas at higher southern latitudes; and
precipitation over ice sheets was reduced substantially compared to modern precipitation in the previously glaciated regions.
The evolution from ice-age to modern climates was not monotonic (Figure 3.2). In particular, a peak in Northern Hemisphere summer insolation around 9 ka caused strengthened monsoonal circulation and large soil-moisture increases compared to today and to the glacial maximum across the northern monsoon belt (primarily Africa and Asia), with decreased monsoonal circulation and soil moisture over smaller areas of land in the Southern Hemisphere (Kutzbach and Guetter, 1986; COHMAP, 1988).
SOIL MOISTURE CHANGES
FIGURE 3.1 Changes in soil moisture, 18 ka relative to today. Areas marked "positive" were wetter 18 ka than today. Maximum extent of glacier ice during the last glacial episode is also shown (Kutzbach and Guetter, 1988; COHMAP, 1988; ice extent from Broecker, 1983).