What were the ice ages?
In the nineteenth century, Louis Agassiz helped compile the evidence that large parts of the Earth have been periodically covered with advancing and retreating ice sheets. The last ice age ended between 14,000 and 10,000 years ago. The climate of the most recent 10,000 years has been remarkably stable, at least by comparison to the wild climate shifts being deduced from analysis of the last 250,000 years of ice cores from the glaciers of Greenland. During that period there have been 35 major climate shifts that affected Greenland.
During the most recent ice age, when glaciers covered most of Wisconsin, Michigan, and New England, global mean temperatures were about 10°F cooler than at pre sent.
During the Pleistocene era, extending back 600,000 years, there were four major glacial periods lasting 30,000 to 100,000 years. But in the interglacial periods, the Earth’s temperature averaged 5°F warmer than today.
Scientists from the Lamont-Doherty Earth Observatory at Columbia University figure that the Earth is 8,000 years overdue for a major climate change. Instead of global warming, they are thinking of a possible ice age, and one that could start with little warning. The magnitude of such shifts in the past have been equivalent to changing the climate of Atlanta to that of Duluth, Minnesota.
While ice ages have played a dramatic role in the Earth’s climate system over the past million or so years, they are relatively rare when the entire geological record is examined. Ice ages have accounted for less than one percent of the last 600 million years. The geological record suggests the first large-scale glacial epoch occurred about 2.3 billion years ago.