- For the animated movie, see Ice Age (movie). For the band, see Ice Age (band).
Variations in CO2, temperature and dust from the Vostok
ice core over the last 400 000 years
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 ("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 (because the Greenland and Antarctic ice sheets still exist). 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 at 40–100,000 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 . 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 were unsuspected.
Major ice ages
There have been at least four major ice ages in the Earth's past. The earliest hypothesized ice age is believed to have occurred around 2700 to 2300 million years ago during the early Proterozoic Age. The earliest well-documented ice age, and probably the most severe of the last 1000 million years, occurred from 800 to 600 million years ago (the Cryogenian period) and it has been suggested that it produced a Snowball Earth in which permanent sea ice extended to or very near the equator. It has been suggested that the end of this ice age was responsible for the subsequent Cambrian Explosion, though this theory is recent and controversial. A minor ice age occurred from 460 to 430 million years ago. There was an extensive ice age from 350 to 260 million years ago.
The present ice age began 40 million years ago with the growth of an ice sheet in Antarctica, but intensified during the Pleistocene (starting around 3 million years ago) with the spread of ice sheets in the Northern Hemisphere. Since then, the world has seen cycles of glaciation with ice sheets advancing and retreating on 40,000 and 100,000 year time scales. The last glacial period ended about 10,000 years ago. A map is available showing estimated ice extent and coastline changes during the last glacial period.
The timing of ice ages throughout geologic history is in part controlled by the position of the continental plates on the surface of the Earth. When landmasses are concentrated near the polar regions, there is an increased chance for snow and ice to accumulate. Small changes in solar energy can tip the balance between summers in which the winter snow mass completely melts and summers in which the winter snow persists until the following winter. See the web site Paleomap Project for images of the polar landmass distributions through time. Due to the positions of Greenland, Antarctica, and the northern portions of Europe, Asia, and North America in polar regions, the Earth today is considered to be prone to ice age glaciations.
Evidence for ice ages comes in various forms, including rock scouring and scratching, glacial moraines, drumlins, valley cutting, and the deposition of till or tillites and glacial erratics. Successive glaciations tend to distort and erase the geological evidence, making it difficult to interpret. It took some time for the current theory to be worked out. Analyses of ice cores and ocean sediment cores unambiguously show the record of glacials and interglacials over the past few million years.
In between ice ages, there are multi-million year periods of more temperate climate, but also within the ice ages (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 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 (ie. manmade) 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, both for the large-scale ice age periods and the smaller ebb and flow of glacial/interglacial periods within an ice age. 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's orbit around the Sun known as Milankovitch cycles (and possibly the Sun's orbit around the 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 within 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's orbit does not have a great effect on the long term causation of ice ages, but does seem to dictate the pattern of multiple freezings and thawings that take place within the current ice age. The complex pattern of changes in Earth's 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 glacial/interglacial frequencies to the Milankovic orbital forcing periods is so good that orbital forcing is the generally accepted explanation. 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 can change and significantly redistribute the sunlight received by the Earth. Of particular importance are changes in the tilt of the Earth's axis, which impact the intensity of seasons. For example, the amount of solar influx in July at 65 degrees north latitude is calculated to vary by as much as 25% (from 400 W/m2 to 500 W/m2, see graph at ). It is widely believed that ice sheets advance when summers become too mild to melt all of the accumulated snowfall from the previous winter. 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's 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/interglacial periods. In particular, during the last 800 thousand years the dominant inter/glacial oscillation has been 100 thousand years, which corresponds to changes in Earth's eccentricity and orbital 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 (tilt of the axis). 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 known dust bands in the solar system. 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 .
Another worker, Ruddiman has suggested 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. And yet another theory has been advanced by Peter Huybers who argued that the 41,000 year cycle has always been dominant, but that the Earth has entered a mode of climate behavior where only the 2nd or 3rd cycle triggers an ice age. This would imply that the 100,000 year periodicity is really an illusion created by averaging together cycles lasting 80 and 120 thousand years. This theory is consistent with the existing uncertainties in dating, but not widely accepted at present (Nature 434, 2005, ).
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 '/' are North America, names after it Northern European, dates in thousand years BCE. In the UK, Eastern Europe and the Alps yet other names are used; see Geology of the United Kingdom for UK names):
The end of the last glacial 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. Isostatic rebound continues to reshape the Great Lakes and other areas formerly under the weight of the ice sheets.