Page 164 - Earth's Climate Past and Future

140 PART III • Orbital-Scale Climate Change

40° N FIGURE 8-3 Vegetation in North
Africa Vegetation across the northern
Mediterranean part of Africa ranges from rain forest near
the equator to savanna and grassland in
30° the Sahel to desert scrub vegetation in
Sahara desert the Sahara. This pattern reflects the
and desert scrub
diminishing northward reach of summer
monsoon moisture from the tropical
20° Atlantic. (Adapted from J. F. Griffiths,
Climates of Africa [Amsterdam: Elsevier,
Grass
1972]. Inset photos courtesy of Tom Smith,
10° Tree savannah University of Virginia.)

Rain forest
0°

10° S
20° W 10° 0° 10° 20° 30° E

For position only

Strong Stronger
summer Strong Weaker
radiation summer winter
radiation radiation
Solar
radiation Modern Stronger Stronger
seasonal summer winter
cycle Weaker monsoon monsoon
winter
radiation
Weak Land Land
A Winter Summer Winter B C

FIGURE 8-4 The orbital monsoon hypothesis (A) Departures from the modern seasonal
cycle of solar radiation have driven stronger monsoon circulations in the past. (B) Greater
summer radiation intensified the wet summer monsoon, while (C) decreased winter insolation
intensified the dry winter monsoon. (Adapted from J. E. Kutzbach and T. Webb III, “Late
Quaternary Climatic and Vegetational Change in Eastern North America: Concepts, Models, and
Data,” in Quaternary Landscapes, ed. L. C. K. Shane and E. J. Cushing [Minneapolis: University of
Minnesota Press, 1991].)