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CHAPTER 15 • Humans and Preindustrial Climate 277

Ice volume America, and North America. This change altered the
18
Dust (δ O) carbon-isotopic composition of the carbon left behind in
More Less Max. Min.
0 soils and in the teeth of grazing animals on all four conti-
13
nents. The trend toward heavier δ C values in North
Africa (Figure 15–5 center) indicates a change from C3
vegetation (trees and shrubs) toward C4 vegetation
(warm-season grasses). Greenhouse experiments suggest
that such a shift could have happened because CO con-
1 2
centrations fell past a threshold of 600–500 ppm over
several million years. The fact that a gradual change from
C3 to C4 carbon occurred on all four continents suggests
falling atmospheric CO concentrations as a common
2
explanation. As this trend developed, global climate was
2 cooling, and ice sheets were growing (Figure 15–5 right).
Climate modeling points to several factors that may
Myr ago have driven a long-term drying in Africa. One factor
was construction of volcanic plateaus in east Africa over
the last 30 million years. Model simulations show that
prior to plateau construction, moisture-bearing winds
3
blew from the Indian Ocean westward into East Africa
(Figure 15–6). As the plateaus took form, this influx of
moisture was choked off, and the vegetation shifted
from forests to grasslands across much of the region
where most hominin remains are found but not across
4 the rest of Africa. Another factor that may have con-
Carbon
Grassland Woodland tributed to this regional drying was a cooling of the

FIGURE 15-5 Long-term changes in African dust and
vegetation Over the last 4.5 Myr, (left) increasing amounts of
dust were blown from North Africa to the tropical Atlantic, Inland sea
and (center) vegetation cover in many regions gradually shifted
away from trees and shrubs toward warm-season grasses.
(Right) These changes occurred during a time of global
cooling. (Adapted from P. B. deMenocal, “Plio-Pleistocene
African Climate,” Science 270 [1995]: 53–59.) Tibet

also have increased because the spring winds that lift
and carry the fine debris became stronger.
A long-term drying trend in North Africa is also
evident in a sediment sequence from the Atlantic Ocean
just offshore of the Sahara Desert that contains a his-
FIGURE 15-6 Tectonic effects on African climate Over
tory of pollen changes over portions of the last 3.7 Myr.
the last 20 Myr, East African plateaus became drier because of
Gradually decreasing amounts of forest pollen and
local uplift in eastern Africa and cooling of the western Indian
increasing amounts of savanna and desert scrub pollen
Ocean (orange), uplift of the Tibetan Plateau (yellow), and
indicate a progressive trend toward dry-adapted vegeta-
shrinkage of an inland sea in west-central Asia (blue).
tion. Other shorter-term pollen records from the East (Adapted from P. Sepulchre et al., “East African
African plateau also reveal a long-term trend toward Aridification and Late Neogene Uplift,” Science 313 [2007]:
more open vegetation. 1419–23, W. F. Ruddiman and J. E. Kutzbach, “Plateau Uplift
A long-term decrease in atmospheric CO could also and Climate Change,” Scientific American 264 [1991]: 66–75,
2
have been a factor in the change in vegetation. Over this and G. Ramstein et al., “Effect of Orogeny, Plate Motion, and
interval, a major vegetation shift was underway not just in Land-Sea Distribution on Eurasian Climate over the Past 30
East Africa but also in parts of southern Asia, South Million Years,” Nature 386 [1997]: 786–95.)