FIGURE 3.2 Seasonal temperature and mean-annual net precipitation [∆(P-E)] changes in the (a) Northern and (b) Southern Hemispheres, calculated by a general circulation/climate model, for the period 18 ka to present [abscissa in thousands of years before present (K.Y.B.P. = ka); from Kutzbach and Guetter, 1988].

Glacial Coverage At the height of the last glaciation, vast areas of North American and Europe were covered with thick sheets of glacier ice (Figure 3.1). In addition, expanded mountain glaciers at this time covered smaller areas of Eurasia, North America, South America, Africa, and New Zealand. In sum, about 30 percent of the present land area was glaciated 18 ka (Flint, 1971).

Most of the areas covered were poleward of 40°N, with most of the rest being poleward of 40°S. Mountain glaciers expanded at all latitudes, especially in North America, South America, and Africa, and despite limited areal extent might have been important in affecting global chemical weathering rates.

FIGURE 3.3 Areas of subaerial shelf exposure at 18 ka (by assuming a 130-m sea-level lowstand).

Shelf Exposure As the Wisconsinan ice sheets grew, sea level fell, progressively exposing more and more of the continental shelf region (Figure 3.3), which then became subject to glaciation at high latitudes and chemical weathering processes at lower latitudes. At full exposure, the continental shelves represent about 13 percent of the total land area of the planet; the majority of this was exposed at the glacial maximum. Interestingly, this ''new land" would have appeared preferentially in the tropics; approximately 40 percent of the total shelf area falls between 20° north and south of the equator (Figure 3.4).

Some of the more important shelf emergences include the Amazon and southeastern South American shelves, the perimeter of the Gulf of Mexico, and a large area of southwestern Asia and Oceania. Today, land areas adjacent to