An ice age is a stage of long-term slump in the temperature of Earth's climate, resulting in an extension of the continental ice sheets, polar ice sheets and mountain glaciers. Glaciological, ice age is often used to mean a period of ice sheets in the northern and southern hemispheres; by this description we are still in an ice. More colloquially, when speaking of the last few million years, ice age is used to pass on to colder periods with wide 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 previous, glaciological, sense; and use the term 'glacial periods' for colder periods during ice ages and 'interglacial' for the heater periods.
During the last few million years, there have been many hostile periods, going on initially at 40,000-year frequency but more freshly at 100,000-year frequencies. These are the best deliberate. There have been four main ice ages in the advance history.
Origin of ice age theory
The idea that, in the past, glaciers had been far wider was folk knowledge in some alpine regions of Europe. No single person made-up the idea. Between 1825 and 1833, Jean de Charpentier assembled verification in support of this idea. In 1836 Charpentier influenced Louis Agassiz of the theory, and Agassiz published it in his book Étude sur les glaciers of 1840.
At this near the beginning stage of knowledge, what were being deliberate were the glacial periods within the past few hundred thousand years, during the present ice age. The far previous ice ages' very survival was unsuspected.