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CHAPTER 8
Insolation Control
of Monsoons
Monsoon circulations exist on Earth today because the land responds to
seasonal changes in solar radiation much more quickly than the ocean does.
The reason for these differing rates of response is that the land has a far smaller
capacity for storing heat than the ocean does. In this chapter we will examine
evidence that insolation variations over orbital time scales have been the
primary driver of summer monsoons. Variations in insolation produced large
changes in heating of tropical landmasses and in the strength of summer mon-
soons at the 23,000-year cycle of precession. Just 11,000 years ago, strong sum-
mer insolation drove a vigorous monsoon circulation in North Africa, and the
southern part of today’s Sahara Desert was dotted with lakes, seasonally flow-
ing rivers, and grassland vegetation. Similar changes in monsoon strength have
occurred throughout the last several hundred thousand years, with especially
large changes 200 Myr ago across the northern tropics of Pangaea.
