Page 179 - Earth's Climate Past and Future

CHAPTER 9

Insolation Control

of Ice Sheets

Ice sheets covered northern North America and Europe 20,000 years ago. The
present locations of Toronto, New York, Chicago, Seattle, and London were
buried under hundreds of meters of ice. Later, the ice melted, and the last
remnants disappeared by 6000 years ago, near the time human civilizations
came into existence. The fact that ice sheets first appeared in the northern
hemisphere within the last 3 Myr can be explained by very slow tectonic-scale
cooling (Part II), but the evidence that ice sheets grew and melted over much
shorter intervals of time requires a different explanation.
The driver of these shorter-term variations in the amount of ice is orbitally
driven insolation changes. In this chapter, we investigate how changes in sum-
mer insolation control the size of ice sheets by determining the rate of ice
melting or accumulation. We explore two lags that are important to under-
standing the ice response: the lag of slow-responding ice sheets behind the
insolation changes and the delayed depression of bedrock beneath the weight
of the overlying ice. Both these lags are thousands of years in length. Then we
examine past changes in ice volume based on evidence from oxygen isotopes
and coral reefs. Finally, we analyze how the ice sheet history over the last 3 Myr
compares with predictions from the theory of orbital control of ice volume.