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CHAPTER 12 • Last Glacial Maximum 223

60° 120° 180°E Climate model simulations suggest that the main
N
reason for this colder, drier glacial climate in Asia was
Sea ice a much stronger high-pressure cell in Siberia during
Ice sheet
winter that produced an increased outflow of cold, dry
air southward (Figure 12–16B). In addition, icy condi-
tions in the North Atlantic cut off much of the moisture
source for the Asian interior.
60° Permafrost The effects of these harsh winters in northern Asia
and tundra
extended into southeastern Asia. Today the influence of
Grass
steppe Mixed the warm, moist summer monsoon allows forests to
with grass and forest Ocean grow along the Pacific coasts of China and Japan. At the
scattered glacial maximum, the stronger and more persistent win-
30° trees Forest ter monsoon sent cold air southward from Siberia and
A Glacial maximum (observed) pushed the northern forest limit to the south (see
60° 120° 180°E Figure 12–16A). The North Pacific was also colder than
N it is today, with much more extensive winter sea ice
Sea ice
along the coast of Asia and in the Bering Sea because of
Ice sheet cold Siberian air blowing out over the ocean.

Winter
60° monsoon 12-9 Climate Changes Far from the Northern
Ice Sheets
Farther from the direct influence of the great northern
hemisphere ice sheets, climate changes were less dra-
Ocean matic. In these regions the lower glacial concentrations
30° of CO and methane were probably the major cause of
2
B Glacial maximum (model) the climate changes (Chapter 10).
Large changes occurred in the Antarctic, where
FIGURE 12-16 Glacial northern Asia was treeless (A) At the CLIMAP estimated that the winter limit of sea ice
last glacial maximum, Asia was covered by permafrost and expanded northward by several degrees of latitude in the
tundra in the north and steppe in the south, with little forest. far-southern Atlantic and Indian oceans (Figure 12–17).
(B) Climate models indicate that this distribution of vegetation Later analyses have reduced these limits only very
resulted from a much stronger and colder winter high-pressure slightly. Associated with this shift in sea ice was a north-
cell in northern Asia, with stronger cold winds blowing to the ward displacement of the region of strongest upwelling
south. (A: Adapted from V. P. Grichuk, “Late Pleistocene Vegetation and highest surface-water productivity, but productivity
History,” in Late Quaternary Environments of the Soviet Union, ed. A. A. decreased in regions nearer Antarctica where the cover
Velichko et al. [Minneapolis: University of Minnesota Press, 1984]. of sea ice persisted longer during summer.
B: Adapted from J. E. Kutzbach et al., “Simulated Climatic Changes: The record of climate change on arid southern
Results of the COHMAP Climate Model Experiments,” in Global hemisphere continents remains sparse. Expanded desert
Climates Since the Last Glacial Maximum, ed. H. E. Wright et al. dunes in Australia (see Figure 12–5) suggest an even
[Minneapolis: University of Minnesota Press, 1993].)
more arid climate and an intensification of the modern
counterclockwise wind flow. Lake levels and pollen data
Forests were completely absent from the entire northern also provide supporting evidence for a drier interval
part of Asia, an indication that this region had an even near the glacial maximum. One factor that contributed
harsher continental climate than it does today. to greater glacial aridity in northern Australia was the
The harsh winter cold and sparse snow cover caused withdrawal of the ocean from a vast area just to its north
the ground to freeze tens of meters deep, forming (see Figure 12–2). Another factor was the lower concen-
permafrost, but in most regions the surface layer trations of greenhouse gases in the atmosphere. A
thawed in summer and allowed grasses and herbs to pro- southward shift in the circum-Antarctic storm belt
liferate. South of the year-round and seasonal per- might also have led to drying of southern Australia.
mafrost were extensive grassy steppes (Figure 12–16A). Climate in much of South America is heavily influ-
The greater area covered by tundra and steppe (rather enced by winds from nearby oceans. Most ocean mois-
than modern forest) made the ground surface much ture is dropped in the Amazon rain forest and along the
more reflective in the snowy season and further cooled eastern flanks of the Andes. Scattered records from the
this region. Amazon Basin hint at the possibility that the rain forest