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CHAPTER 13 • Climate During and Since the Last Deglaciation 243

0 FIGURE 13-14 Upwelling in the Arabian
Asia Modern
Sea Climate model simulations of stronger
40°N summer monsoons over India 9000 years
Tibetan ago are supported by evidence from ocean
Plateau cores indicating greater upwelling along the
Upwelling Depth in core (m) 1 coasts of East Africa and Arabia in response
to strong winds pushing surface waters
20°N Arabia India 9000 offshore. (A: Adapted from W. L. Prell,
years
ago “Monsoonal Climate of the Arabian Sea During
the Late Quaternary: A Response to Changing
Africa Glacial Solar Radiation,” in Milankovitch and Climate, ed.
2 maximum
20,000 years ago A. Berger et al. [Dordrecht: Reidel, 1984]. B:
0°N W. L. Prell, “Variation of Monsoonal Upwelling:
40° 60° 80° 100°E 10 20 30 40
A Modern summer monsoons B Upwelling species (%) A Response to Changing Solar Radiation,” in
Climate Processes and Climate Sensitivity, ed. J. E.
Hansen and T. Takahashi [Washington, DC:
American Geophysical Union, 1984].)

By 6000 years ago, summer insolation values in the tance from their glacial location in the far southeastern
northern tropics were still about 5% higher than the United States to their modern concentrations in mid-
modern levels but were falling toward modern values. Atlantic states (Figure 13–17B).
This slow decrease should have produced a correspond- The climate changes that occurred midway through
ing decline in the strength of the tropical monsoons. the deglaciation produced unusual mixtures of plants
14
Direct observations and C dates of lakes across North called no-analog vegetation because no similar
Africa confirm a major drop in water levels during the combination exists today. For example, spruce trees
last 9000 years (Figure 13–16B). Today, lakes are lower grew with hardwood deciduous trees (such as ash) in the
than they were between 9000 and 6000 years ago, and northern Midwest of the United States early in the
many have completely dried out. deglaciation, even though ash and other deciduous trees
Examined individually, most lake level histories in are rare today in regions where spruce trees grow.
North Africa and India show large and abrupt changes
during the overall transition to lower levels (Figure
13–16C). As was the case for the irregular rates of melt-
ing of northern ice sheets, these short-term changes
in lake levels represent a type of climate response that 6 Insolation
cannot be directly attributed to the smooth, gradual 9000 years ago
forcing provided by changes in summer insolation.

13-7 Warmer, Then Cooler North Polar Summers 4

At the glacial maximum, the main controls on climate at Precipitation (mm/day) With soil and
vegetation
high northern latitudes had been the regional cooling 2 feedback
effects of the ice sheets and the global cooling caused by Modern summer
monsoon rains
low CO (and methane) values. As deglaciation pro-
2
ceeded, rising summer insolation values increasingly
0
warmed land areas located far from the ice sheets and in 0° 5° 10° 15° 20° 25° 30°
Latitude (N)
time overcame the cooling effects of the shrinking ice
sheets. Summer insolation values reached a peak 10,000
FIGURE 13-15 Vegetation-moisture feedback Climate
years ago, with the ice sheets much smaller but still
model simulations indicate that higher summer insolation
present.
9000 years ago caused stronger summer monsoons and greater
Northward Shifts in Vegetation The last deglacia-
northward penetration of moisture into Africa. Additional
tion dramatically transformed the vegetation of the model experiments show that positive moisture feedback from
northern hemisphere continents. In North America, wetter soils and increased vegetation caused even greater
cold-tolerant spruce trees retreated from their glacial penetration of moisture into the continental interior. (Modified
position in the central United States to their modern from J. E. Kutzbach et al., “Vegetation and Soil Feedbacks on the
position in northeastern Canada (Figure 13–17A). Response of the African Monsoon to Orbital Forcing in the Early
Warm-tolerant trees such as oak moved a smaller dis- to Middle Holocene,” Nature 384 [1986]: 623–26.)