Page 268 - Earth's Climate Past and Future

244 PART IV • Deglacial Climate Changes

Precessional Lakes at high levels Lake Zimway-Shala FIGURE 13-16 Weakening
insolation (%) Meters above base level monsoons (A) Low-latitude summer
Min Max 0 20 40 60 0 50 100
0 0 insolation has slowly decreased since
reaching a maximum value 10,000
years ago. (B, C) The decrease in
2000 2000 summer insolation has weakened the
summer monsoons and caused lake
? levels in North Africa to fall. (B:
4000
Years ago 14 C years ago Overflow Street-Perrott, “Milankovitch Forcing of
4000
Adapted from J. E. Kutzbach and F. A.
Fluctuations in the Level of Tropical
6000
6000
Lakes,” Nature 317 [1985]: 130–34. C:
Adapted from R. Gillespie et al., “Post-
? glacial Arid Episodes in Ethiopia Have
8000 8000
Implications for Climate Prediction,”
? Nature 306 [1983]: 680–83.)
10,000 10,000
A B C

No-analog mixtures developed because each veg- impossible to analyze past vegetation changes by
etation type responded to a different combination of lumping pollen together into larger communities or
environmental variables from those that controlled the assemblages; each vegetation type has to be analyzed on
other types. These individualistic responses make it its own.

Spruce pollen Oak pollen
Observed Simulated Observed Simulated

0

6000

11,000
Years ago

14,000

16,000 FIGURE 13-17 Data-model vegetation
comparisons Pollen in lake sediments indicates
large-scale changes in the distribution of spruce and
oak pollen during the last deglaciation. Model
simulations of climate and vegetation reproduce
21,000
many but not all aspects of these observed patterns.
(Adapted from T. Webb III et al., “Late Quaternary
A B Climate Change in Eastern North America: A Comparison
Pollen percentages of Pollen-Derived Estimates with Climate Model Results,”
<1% 1–5% 5–20% >20% Quaternary Science Reviews 17 [1998]: 587–606.)