An ice age is a period of long-term downturn in the temperature of Earth's climate, resulting in an expansion of the continental ice sheets, polar ice sheets and mountain glaciers. Glaciologically, ice age is often used to mean a period of ice sheets in the northern and southern hemispheres; by this definition we are still in an ice age. More colloquially, when speaking of the last few million years, ice age is used to refer to colder periods with extensive ice sheets over the North American and European continents: in this sense, the last ice age ended about 10,000 years ago. This article will use the term ice age in the former, glaciological, sense; and use the term 'glacial periods' for colder periods during ice ages and 'interglacial' for the warmer periods.

During the last few million years, there have been many glacial periods, occurring initially at 40,000-year frequency but more recently at 100,000-year frequencies. These are the best studied. There have been four major ice ages in the further past.

Origin of ice age theory

The idea that, in the past, glaciers had been far more extensive was folk knowledge in some alpine regions of Europe (Imbrie and Imbrie, p25, quote a woodcutter telling de Charpentier of the former extent of the Swiss Grimsel glacier). No single person invented the idea [1]. Between 1825 and 1833, Jean de Charpentier assembled evidence in support of this idea. In 1836 Charpentier convinced Louis Agassiz of the theory, and Agassiz published it in his book Étude sur les glaciers of 1840.

At this early stage of knowledge, what were being studied were the glacial periods within the past few hundred thousand years, during the current ice age. The far earlier ice ages' very existence was unsuspected.

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