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:For the animated movie, see Ice Age (movie).- For the band, see Ice Age (band).
An ice age is a period of long-term downturn in the temperature of Earth's climate, resulting in an expansion of the polar ice caps and mountain glaciers (" glaciation"). 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, and of the last 4 Myr, 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 'glacial periods' for colder periods during ice ages and 'interglacial' for the warmer periods.
1 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.
2 Major periods of glaciation
There have been four major periods of glaciation in the Earth's past. The first, and possibly most severe, may have occurred from 800 to 600 million years ago (the cryogenian in the late Proterozoic Age) and it has been suggested that it produced a Snowball EarthThe Snowball Earth also known as the Varangian glaciation is a recent hypothesis, largely formulated by Paul F. Hoffman, Sturgis Hooper Professor of Geology at Harvard University. It proposes that the ice age which took place in the Precambrian was so sev in which the earth iced over completely. It has been suggested that the end of this cold period was responsible for the subsequent Cambrian ExplosionThe Cambrian Explosion is the radiation of animal phyla that started about 570 million years ago, which is 30 million years after the beginning of the Cambrian geologic period, and lasted between 5 and 17 million years through much of the early Cambrian., though this theory is recent and controversial. A minor series of glaciations occurred from 460 to 430 million years ago. There were extensive glaciations from 350 million years before present to 250 million. The present PleistoceneThe Pleistocene epoch is part of the geologic timescale, usually dated as 1. 6 million to 10,000 years before present, with the end date expressed in radiocarbon years. It covers most of the latest period of repeated glaciation, up to and including the Yo ice age has seen more or less extensive glaciation on 40,000 and 100,000 year cycles. The last glacial period ended about 10,000 years ago.
3 Interglacials
In between periods of glaciation, there are multi-million year periods of more temperate climate, but also within these above mentioned periods (or at least within the last one), temperate and severe periods occur. The colder periods are called 'glacial periods', the warmer periods 'interglacials', such as the Eemian interglacial eraThe Eemian interglacial era Sangamon era in North America) is the second-to-latest interglacial era of the Ice age. It began about 131,000 years ago, consisted of an early warm period of about 3,000 to 4,000 years duration, a rapid cooling and then a much.
We are in an interglacial period now, the last retreat ending about 10,000 years ago. There appears to be a folk wisdom that "the typical interglacial period lasts ~12,000 years" but this is hard to substantiate from the evidence of ice core records. Nonetheless, this led to some fear of a new glacial period starting soon, a global coolingGlobal cooling is (or perhaps more appropriately was) a concern that the Earth may be ending its current warm period, the climate will cool, and perhaps even begin the glaciation of an ice age. Note: An obsolete geology meaning referring to the Earth's su concern. Many now believe that anthropogenic forcing from increased " greenhouse gases" would outweigh any MilankovitchMilankovitch cycles is the name given to the collective effect of changes in the Earth's movements upon its climate. The eccentricity, axial tilt, and precession of the Earth's orbit vary in several patterns, resulting in 100,000 year ice age cycles of th (orbital) forcing; and more recent consideration of the orbital forcing suggests that even in the absence of human perturbation the present interglacial would last at least 50,000 years.
