Britannica Sources used in this answer

Why is summer sunlight at high northern latitudes important for the growth of ice sheets?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Map showing the extent of the Laurentide, Cordilleran, and Greenland ice sheets during the Wisconsin glaciation, roughly 100,000 to 11,000 years ago. The Laurentide Ice Sheet covers most of present-day Canada and parts of the northern United States in light blue, the Cordilleran Ice Sheet covers the Pacific Northwest in teal, and the Greenland Ice Sheet is marked in purple. Modern international and state or provincial boundaries are outlined for reference. — Pleistocene Epoch Ice sheets during the Wisconsin glacial episode penetrated deep into North America.

In the context of the Pleistocene Epoch, summer sunlight—specifically the amount of solar radiation (insolation) received at high northern latitudes—is the critical factor in the growth and decay of continental ice sheets.1

According to the astronomical theory of climate cycles, glaciations are hypothesized to begin during periods of low summer insolation. When summer sunlight is weak, temperatures remain cool enough to prevent the complete melting of the previous winter's snow. If the winter snowpack persists through the summer season, it can accumulate over time, eventually leading to the development and expansion of large-scale ice sheets.1

This process is amplified by albedo feedback: as snow and ice cover increase, the Earth's surface reflects more solar radiation back into the atmosphere, which causes further cooling and promotes additional ice growth. Thus, the "pacemaker" of the ice ages is the systematic variation in Earth's orbit, which dictates whether high-latitude summers are cool enough to sustain the growth of ice.1