Because of extensive erosion, it is difficult to determine the extent of ice sheets during the various ice ages. But scientists do know some things about the last ice age—the Pleistocene Ice Age—a time when up to 10 percent of the Earth was covered (although not simultaneously) by often miles-high ice. At their greatest extent, the Northern Hemisphere glaciers and ice sheets covered most of Canada, all of New England, much of the upper Midwest, large areas of Alaska, most of Greenland, Iceland, Svalbard, and other arctic islands, Scandinavia, much of Great Britain and Ireland, and the northwestern part of the former Soviet Union. In the Southern Hemisphere, glaciers were much smaller, with the main effects being cooler and much drier weather.

During the last stage of the Ice Age (the Wisconsin stage in the United States), ice sheets covered parts of Eurasia and much of North America, extending as far south as Pennsylvania. As the climate warmed up, scientists estimate that sea level rose about 410 feet (125 meters), an average rate of an inch (2.5 centimeters) per year for roughly 5,000 years. Interestingly, although most of the huge northern ice sheets melted, the Antarctic ice sheet decreased by only 10 percent.