Ice Ages

The Milankovitch Astronomical Theory best explains regular climatic fluctuations. The theory is based on three variations in the position of Earth relative to the Sun: the eccentricity (elongation or circularity of the shape) of Earth's orbit, the tilt of Earth's axis toward or away from the Sun, and the degree of wobble of Earth's axis of rotation. The total effect of these changes causes one region of Earth—latitude 60° to 70° north, near the Arctic Circle—to receive low amounts of summer radiation about once every 100,000 years. These cool summer periods last several hundred to several thousand years and thus provide sufficient time to allow snowfields to expand and merge into glaciers in this area, signaling the beginning of a glaciation.

When glaciers expand during an ice age, the sea level drops because the water that forms glaciers ultimately comes from the oceans (see Water Cycle). Global sea level affects the overall temperature of the planet because solar radiation, or heat, is better absorbed by water than by land. When sea levels are low, more land surface becomes exposed. Since the land is not able to absorb as much solar radiation as the water can, the overall average temperature of the planet decreases, or cools, and may contribute to the onset of an ice age.

A map showing Earth during an ice age would look very different from a map of the modern world. During the Wisconsin glaciation of 115,000 to 10,000 years ago, two ice sheets, the Laurentide and the Cordilleran, covered the northern two-thirds of North America, including most of Canada, with ice. Other parts of the world, including Eurasia and parts of the North Atlantic Ocean, were also blanketed in sheets of ice.

The Laurentide continental ice sheet extended from the eastern edge of the Rocky Mountains to Greenland. The separate Cordilleran Ice Sheet was composed of mountain ice caps and valley glaciers that flowed onto the surrounding lowlands in parts of northern Alaska, in parts of the Sierra Nevada, and in the Cascade Range and the Rocky Mountains as far south as New Mexico. Where the continental shelf between Alaska and Siberia was uncovered, the Bering land bridge formed. In northern Eurasia, continental ice extended from Great Britain eastward to Scandinavia and Siberia. Separate mountain glacial systems covered the Alps, the Himalayas, and the Andes. The extensive ice sheets on Antarctica and Greenland did not expand very much during each glaciation. Sea ice grew worldwide, particularly in the North Atlantic Ocean.