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242 PART IV • Deglacial Climate Changes

stronger monsoons across the entire north-tropical Geologic evidence supports this simulation (Figure
14
region of North Africa, southern Arabia, and southern 13–13B, C). Between 10,000 and 7500 C years ago,
14
Asia (Figure 13–13A). lake levels determined by C dating of lake muds were
substantially higher than they are today across most of
North Africa between 15° and 30°N, in the southern
half of Arabia, and over southeastern Asia. Lake Chad,
in northern Africa, expanded to 300,000 km , an area
2
comparable to the modern Caspian Sea.
Evidence in sediment cores from the Arabian Sea
Stronger summer indicates that stronger monsoon winds blew along the
monsoon coast of Somalia in eastern Africa, and along the south-
east coast of Arabia 9000 years ago (Figure 13–14).
A Model simulation These intensified winds drove water offshore and
caused upwelling, which was recorded by key species of
planktic foraminifera.
Scientists have also discovered evidence that ancient
rivers flowed across regions that today are hyperarid
desert. In central Arabia, a river flowed more than 500 km
northeastward through the modern Arabian desert. Many
parts of North Africa and Arabia that are now extremely
dry were once grassy river valleys dotted with freshwater
lakes and occupied by hippopotamuses, crocodiles, tur-
B Observations tles, rhinoceroses, giraffes, and buffaloes (see Figure 8–7).
Effective moisture (9000 years ago versus today) Although climate models and evidence from the
Greater Less Same geologic record confirm Kutzbach’s theory of stronger
north-tropical monsoons 10,000 years ago, a closer look
North African lakes Model simulations reveals a mismatch in amplitude. The increase in rain-
(% at high level) Moisture surplus (mm)
fall in the models was small compared to geologic
0 50 100 0 200 400 evidence from lake levels and pollen assemblages.
0 0
Subsequent modeling efforts have explored positive
feedback processes that could have amplified the small
3000 response simulated by early models.
One important feedback is the increase in recycling of
water vapor provided by evapotranspiration of water
6000 vapor by vegetation (companion Web site, p. 34). The ini-
14 C years ago 9000 Years ago 9000 tial increase in monsoon rains in the early models causes
trees to advance northward into grasslands, and grasses to
move northward into deserts. In the later models, the
Calender
and transfers it to the atmosphere through evapotranspi-
years 12,000 advancing vegetation draws more moisture out of the soil
ration. With more water vapor in the atmosphere, more
rain falls, especially farther to the north (Figure 13–15).
15,000
This positive moisture feedback from vegetation results in
model simulations with wetter landscapes and higher
18,000 18,000 lakes that are in closer (but still not full) agreement with
the geologic evidence. One feedback not yet incorporated
C Data-model comparison versus time in models is increased recycling of moisture from small
lakes and low swampy regions near rivers that are too
FIGURE 13-13 Tropical monsoon maximum Climate model
simulations of stronger summer monsoons in the north tropics small to be represented in climate-model grid boxes.
near 9,000 years ago agree with evidence in the climate record, After reaching a peak near 10,000 years ago, sum-
such as higher lake levels. (A and B: Adapted from COHMAP mer insolation values at lower latitudes of the northern
Members, “Climatic Changes of the Last 18,000 Years: hemisphere have fallen continuously (Figure 13–16A).
Observations and Model Simulation,” Science 241 [1988]: This decrease has occurred because Earth’s precessional
1043–52. C: Adapted from J. E. Kutzbach and F. A. Street-Perrott, motion has carried it from a June 21 position close
“Milankovitch Forcing of Fluctuations in the Level of Tropical to the Sun 10,000 years ago to a June 21 position far
Lakes,” Nature 317 [1985]: 130–34.) from the Sun today (Chapter 7).