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The general term "ice age" or, more precisely, "glacial age" denotes a geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in an expansion of continental ice sheets, polar ice sheets and alpine glaciers. Within a long-term ice age, individual pulses of extra cold climate are termed "glaciations". Glaciologically, ice age implies the presence of extensive ice sheets in the northern and southern hemispheres;[1] by this definition we are still in an ice age (because the Greenland and Antarctic ice sheets still exist).[2] More colloquially, when speaking of the last few million years, "the" ice age refers to the most recent colder period with extensive ice sheets over the North American and Eurasian continents in this sense, the most recent ice age peaked, in its Last Glacial Maximum about 20,000 years ago. This article will use the term ice age in the former, glaciological, sense glacials for colder periods during ice ages and interglacials for the warmer periods. The idea that in the past glaciers had been far more extensive was folk knowledge in some alpine regions of Europe Imbrie and Imbrie (1979) quote a woodcutter by the name of Jean-Pierre Perraudin[3] telling Jean de Charpentier of the former extent of the Grimsel glacier in the Swiss Alps.[4] Macdougall (2004) claims the person was a Swiss engineer named Ignaz Venetz,[5] but no single person invented the idea.[6] Between 1825 and 1833, Charpentier assembled evidence in support of the concept. In 1836 Charpentier, Venetz and Karl Friedrich Schimper convinced Louis Agassiz, and Agassiz published the hypothesis in his book Étude sur les glaciers (Study on Glaciers) of 1840.[7] According to Macdougall (2004), Charpentier and Venetz disapproved of the ideas of Agassiz who extended their work claiming that most continents were once covered by ice. At this early stage of knowledge, what was being studied were the glacial periods within the past few hundred thousand years, during the current ice age. The existence of ancient ice ages was as yet unsuspected.
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