The Ice Age

What do scientists mean when they refer to THE Ice Age?

Usually, they mean the last one, the Wisconsinan, which occurred near the end of the Pleistocene epoch.

The phrase "ice age" is often used in the disciplines of paleontology, archeology, ecology, and related fields. But what exactly is meant by this term? Ice ages may be simply defined as periods of exteme cooling, with the most pronounced effects occuring at higher latitudes. The most obvious effect is the "locking up" of great quantities of water in the form of ice. Since there is only a limited amount of free water on the earth at any time, locking up huge amounts of it will have an overall impact on sea levels, effectively lowering them.

In periods of protracted cooling, known as glaciations, ice masses grow and coalesce into huge sheets (ice caps) that may be more than a mile thick. For example, the small ice cap centered over Greenland at present is the last remnant of several coalesced caps that covered all of Canada and much of the northern tier of the U.S. at the height of the (Wisconsinan) glaciation 20,000 years ago.

A number of ice ages (glaciations) have occurred through geological time, including ones that took place hundreds of millions of years ago in the Carboniferous Period, the late Ordovician, and the pre-Cambrian. However, prior to the Pleistocene, extreme cooling was a rare event.

The Pleistocene has been marked by many phases of very cold temperatures alternating with comparatively warm ones. Traditionally, it was thought that there were four principal cold stages, called "the" ice ages; these alternated with much warmer spans (interglaciations), the high points of which are sometimes referred to as "climatic optima."

The world during the Wisconsinan glaciation.

The world today.

Glacial (ice age) and Interglacial (predominately warm) stages in North America:

Name Climate Time Spanned
(years ago)
Wisconsinan ice age 75,000 - 10,000
Sangamonian predominantly warm 120,000 - 75,000
Illinoisan ice age 170,000 - 120,000
Yarmouthian predominantly warm 230,000 - 170,000
Kansan ice age 480,000 - 230,000
Aftonian predominantly warm 600,000 - 480,000
Nebraskan ice age 800,000 - 600,000
pre-Nebraskan predominantly warm 1,600,000 - 800,000

We now know that the cold stages were not uniformly cold, nor were the warmer spans continuously warm. In fact, there is evidence that at least two dozen warm-cold cycles have occurred during the past 1.6 million years, and that some of the changeovers occurred within the period of a century or so. From the standpoint of Quaternary extinctions, it is interesting to note that only the close of the last major glaciation can be correlated with widespread mammal extinctions at high latitudes.