An ice age is a period of long-term downturn in the temperature of Earth'Climate" title ="Climate">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.
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 . 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.
Major periods of glaciation
There have been four major periods of glaciation in the Earth'Cryogenian" title ="Cryogenian">cryogenian in the late Proterozoic Age) and it has been suggested that it produced a Snowball Earth in which the earth iced over completely. It has been suggested that the end of this cold period was responsible for the subsequent Cambrian Explosion, 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 Pleistocene 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.
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'Eemian_interglacial_era" title ="Eemian interglacial era">Eemian interglacial era.
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 cooling concern. Many now believe that anthropogenic forcing from increased "greenhouse gases" would outweigh any Milankovitch (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.
Causes of ice ages
The cause of ice ages remain controversial, but the general consensus is that it is a combination of up to three different factors: atmospheric composition (particularly the fraction of CO2 and methane), changes in the Earth'Sun" title ="Sun">Sun known as Milankovitch cycles (and possibly the Sun'Galaxy" title ="Galaxy">galaxy), and the arrangement of the continents.
The first of these three factors is probably responsible for much of the change, especially for the first ice age. The "Snowball Earth" hypothesis maintains that the severe freezing in the late Proterozoic was both caused and ended by changes in CO2 levels in the atmosphere. However, the other two factors do matter.
An abundance of land in the arctic and antarctic circles appears to be a necessity for an ice age, probably because the land masses provide space on which snow and ice can accumulate during cooler times and thus trigger positive feedback processes like albedo changes. The Earth'Earth" title ="Earth">Earths orbit and the change of albedo may influence the occurrence of glacial and interglacial phases - this was first explained by the theory of Milutin Milankovic.
The present ice ages are the most studied and best understood, particularly the last 400,000 years, since this is the period covered by ice cores that record atmospheric composition and proxies for temperature and ice volume. Within this period, the match of ice age frequencies to the Milankovic orbital forcing periods is so good that orbital forcing is the generally accepted explanation. The amount of solar influx is calculated to vary as much as 25% (from 400 watts/square meter to 500 watts/square meter at 65 degrees north latitude) in response to the combined effects of the changing distance to the sun, the precession of the Earth's axis, and the changing tilt of the Earth's axis ( see graph at  ). Some workers believe that the strength of the orbital forcing appears to be too small to trigger glaciations, but feedback mechanisms like CO2 may explain this mismatch.
While Milankovic forcing predicts that cyclic changes in the Earth'Orbit#Orbital_parameters" title ="Orbit">orbital parameters can be expressed in the glaciation record, additional explanations are necessary to explain which cycles are observed to be most important in the timing of glacial periods. In particular, during the last 800 thousand years the dominant glacial oscillation has been 100 thousand years, which corresponds to changes in Earth'Eccentricity" title ="Eccentricity">eccentricity and inclination, and yet is by far the weakest of the three frequencies predicted by Milankovic. During the period 3.0 - 0.8 million years ago the dominant pattern of glaciation corresponded to the 41 thousand year period of changes in Earth's obliquity. The reasons for preferring one frequency to another are poorly understood and an active area of current research, but the answer probably relates to some form of resonance in the Earth's climate system.
The "traditional" Milankovitch explanation struggles to explain the dominance of the 100,000 year cycle over the last 8 cycles. Richard A. Muller and Gordon J. MacDonald    and others have pointed out that those calculations are for a two-dimensional orbit of Earth but the three-dimensional orbit also has a 100 thousand year cycle of orbital inclination. They proposed that these variations in orbital inclination lead to variations in insolation, as the earth moves in and out of dust clouds. Although this is a different mechanism to the traditional view, the "predicted" periods over the last 400,000 years are nearly the same. The Muller and MacDonald theory, in turn, has been challenged by Rial . Ruddiman has established a plausible model that explains the 100,000 cycle by the modulating effect of eccentricity (weak 100,000 year cycle) on precession (23,000 year cycle) combined with greenhouse gas feedbacks in the 41,000 and 23,000 year cycles.
Recent glacial and interglacial phases
The last glacial and interglacial phases of the Pleistocene are named, from most recent to most distant, as follows (names before the ''BCE" title ="BCE">BCE. In the UK, Eastern Europe and the Alps yet other names are used; see Geology of the United Kingdom for UK names):
| Wisconsinan /|
Weichsel or Vistula / Würm
|glacial period||15 – 70|
|Sangamon / Eemian||interglacial||70 – 130|
|Illinoian / Saale / Riß||glacial||130 – 180|
|Yarmouth / Holstein||interglacial||180 – 230|
|Kansan / Elster / Mindel||glacial||230 – 300|
|Aftonian / Cromer||interglacial||300 – 330|
|Nebraskan / Elbe / Günz||glacial||330 – 470|
|— / Waalian||interglacial||470 – 540|
|— / Donau II||glacial||540 – 550|
|— / Tiglian||interglacial||550 – 585|
|— / Donau I||glacial||585 – 600|
The end of the last glaciation also corresponds quite closely to the development of permanent human settlements and agriculture, and it is possible that there is a connection between the two events.
Glaciation in North America
The Wisconsinan glaciation has had a considerable effect on the landscape of the Northern Hemisphere. In North America the Great Lakes and the Finger Lakes were carved by ice deepening old valleys. The old Teays River drainage system was radically altered and largely reshaped into the Ohio River drainage system. Other rivers were dammed and diverted to new channels, such as the Niagara, which formed a dramatic waterfall and gorge, when the waterflow encountered a limestone escarpment. Another similar waterfall near Syracuse, New York is now dry. Long Island was formed from glacial till, and the watersheds of Canada were so severely disrupted that they are still sorting themselves out -- the plethora of lakes on the Canadian Shield in northern Canada can be almost entirely attributed to the action of the ice. As the ice retreated and the rock dust dried, winds carried the material hundreds of miles, forming beds of loess many dozens of feet thick in the Missouri Valley. Before the theory of ice ages, such catastrophic changes were usually attributed to floods.
- Little Ice Age
- Genesee River: Glacial Geology - Relief maps of some glacial landforms and drainage alterations in western NY.
- Imbrie, John and Katherine Palmer Imbrie. Ice ages: Solving the Mystery. Cambridge, Massachusetts: Harvard University Press, 1979, 1986 (reprint). ISBN 089490020X; ISBN 0894900153; ISBN 0674440757.
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