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 Eurasian continents: in this sense, the last ice age ended about 10,000 years ago.

Many glacial periods have occurred during the last few million years, initially at 40,000-year frequency but more recently at 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 Jean de Charpentier of the former extent of the Swiss Grimsel glacier). No single person invented the idea.

Between 1825 and 1833, Charpentier assembled evidence in support of the concept. In 1836 Charpentier convinced Louis Agassiz of the theory, and Agassiz published it in his book ƒtude sur les glaciers (Study of Glaciers) of 1840.

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.

The earliest hypothesized ice age is believed to have occurred around 2.7 to 2.3 billion (109) years ago during the early Proterozoic Age.

The earliest well-documented ice age, and probably the most severe of the last 1 billion 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, during the Late Ordovician Period.

A minor ice age occurred from 460 to 430 million years ago, during the Late Ordovician Period.

There were extensive polar ice caps at intervals from 350 to 260 million years ago, during the Carboniferous and early Permian Periods, associated with the Karoo Ice Age.

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.

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. 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 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, almost tropical, 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. For example, an article in Nature argues that the current interglacial might be most analogous to a previous interglacial that lasted 28,000 years. Nonetheless, fear of a new glacial period starting soon does exist. (Global Cooling)

However, many now believe that anthropogenic (manmade) forcing from increased "greenhouse gases" would outweigh any Milankovitch (orbital) forcing; and some recent considerations of the orbital forcing have even argued that in the absence of human perturbations the present interglacial could potentially last 50,000 years.

Snowball Earth

Cold Water Tossed on 'Snowball Earth' Theory   Live Science - October 6, 2011
Although increasing levels of greenhouse gases in the Earth's atmosphere these days are seen as a harbinger of doom, millions of years ago they may have rescued the planet from a deep freeze. Some researchers believe that at points in our planet's history - at least two, possibly three times - ice blanketed its surface, down to the equator and across the oceans, forming a "Snowball Earth." But new research raises questions about whether a surge in carbon dioxide - one of the greenhouse gases responsible for modern, human-caused global warming - could have been responsible for the big thaw that followed the most recent Snowball Earth, about 635 million years ago. And if there wasn't a greenhouse effect big enough to melt the thick veil of ice, perhaps, the researchers suggest, Earth may not have been a big, icy snowball at the time.

The cause of ice ages remains controversial for both 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 landmasses 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.

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. 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.

During the most recent North American glaciation, the Wisconsin glaciation (70,000 to 10,000 years ago), ice sheets extended to about 45 degrees north latitude.

This Wisconsinian glaciation left widespread impacts on the North American landscape. The Great Lakes and the Finger Lakes were carved by ice deepening old valleys. Most of the lakes in Minnesota and Wisconsin were gouged out by glaciers and later filled with glacial meltwaters.

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.

The Driftless Zone, around the junction of Wisconsin, Minnesota, and Iowa, was not covered by glaciers.

There have been four major periods of glaciation in the Earth's past. The first, and possibly most severe, may have occurred from 800 Ma to 600 Ma (million years ago, 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 also that the end of this cold period was responsible for the subsequent Cambrian Explosion, a time of rapid diversification of multicelled life during the Cambrian era. However, this theory is recent and controversial.

A minor series of glaciations occurred from 460 Ma to 430 Ma. There were extensive glaciations from 350 to 250 Ma. The present Pleistocene ice age has seen more or less extensive glaciation on 40,000 and later, 100,000 year cycles. The last glacial period ended about 10,000 years ago.

Ice Age   Wikipedia

Cracking the Ice Age NOVA

Chauvet Cave France's Magical Ice Age Art - National Geographic

Humble moss helped to cool Earth and spurred on life   BBC - February 2, 2012
Primitive moss-like plants could have triggered the cooling of the Earth some 470 million years ago, say researchers. A study published in Nature Geoscience may help explain why temperatures gradually began to fall, culminating in a series of "mini ice ages". Until now it had been thought that the process of global cooling began 100 million years later, when larger plants and trees emerged. The simple plants' interactions with rocks are believed to be the cause. The humble moss has created the climate which we enjoy today.

Cold Water Tossed on 'Snowball Earth' Theory   Live Science - October 6, 2011
Although increasing levels of greenhouse gases in the Earth's atmosphere these days are seen as a harbinger of doom, millions of years ago they may have rescued the planet from a deep freeze. Some researchers believe that at points in our planet's history - at least two, possibly three times - ice blanketed its surface, down to the equator and across the oceans, forming a "Snowball Earth." But new research raises questions about whether a surge in carbon dioxide - one of the greenhouse gases responsible for modern, human-caused global warming - could have been responsible for the big thaw that followed the most recent Snowball Earth, about 635 million years ago. And if there wasn't a greenhouse effect big enough to melt the thick veil of ice, perhaps, the researchers suggest, Earth may not have been a big, icy snowball at the time.

Scientists to Unearth Ice Age Secrets from Preserved Tree Rings   Science Daily - April 6, 2010
Oxford University is involved in a research project to unearth 30,000 year old climate records, before they are lost forever. The rings of preserved kauri trees, hidden in New Zealand's peat bogs, hold the secret to climate fluctuations spanning back to the end of the last Ice Age.

Search for ice sheet 'tipping point'   PhysOrg - January 13, 2010
A new study examines how ice sheets, such as the West Antarctic Ice Sheet, could become unstable as the world warms.

Underground gases tell the story of ice ages -- and America's split jet stream   PhysOrg - January 13, 2010
Deep underground aquifers in the American Southwest contain gases that tell of the region's ancient climate, and support a growing consensus that the jet stream over North America was once split in two.

Bering Strait Influenced Ice Age Climate Patterns Worldwide   Science Daily - January 12, 2010

In a vivid example of how a small geographic feature can have far-reaching impacts on climate, new research shows that water levels in the Bering Strait helped drive global climate patterns during ice age episodes dating back more than 100,000 years.

Big Freeze: Earth Could Plunge into Sudden Ice Age   Live Science - December 2, 2009

Big Freeze Plunged Europe Into Ice Age in Months   Science Daily - December 1, 2009

Last Ice Age happened in less than year say scientists   Scotsman.com - August 3, 2008
The last ice age, 13,000 years ago took hold in just one year, more than ten times quicker than previously believed, scientists have warned. Rather than a gradual cooling over a decade, the ice age plunged Europe into the deep freeze, German Research Centre for Geosciences at Potsdam said. Cold, stormy conditions caused by an abrupt shift in atmospheric circulation froze the continent almost instantly during the Younger Dryas less than 13,000 years ago Ð a very recent period on a geological scale. The new findings will add to fears of a serious risk of this happening again in the UK and western Europe Ð and soon. Dr Achim Brauer, of the GFZ (GeoForschungs Zentrum) German Research Centre for Geosciences at Potsdam, and colleagues analyzed annual layers of sediments, called "varves", from a German crater lake. Each varve records a single year, allowing annual climate records from the region to be reconstructed.

Oldest Known DNA Found in Greenland Ice Core   National Geographic - July 5, 2007
The oldest known strands of DNA have been recovered from frozen mud taken from the base of Greenland's ice sheet, according to a new study. The discovery could rewrite what was thought about Greenland's ecological pastÑand could alter current predictions about how global warming will affect the island's ice.

New Ice Core Reveals 800,000 Years of Climate History   National Geographic - July 5, 2007
Earth's polar temperature has swung wildly by as much as 15 degrees Celsius (27 degrees Fahrenheit) over the last 800,000 years, an Antarctic ice core has revealed.

Bone-Crushing Wolves Roamed Alaska During Ice Age   National Geographic - June 22, 2007
Gray wolves that roamed Alaska during the last ice age were built to tackle prey much larger than themselves and devour them completely bones and all a new study says. The ancient wolves had short snouts, strong jaws, and massive canine teeth unlike those on any wolves today.

Antarctic Icebergs Teeming With Life, Study Says National Geographic - June 22, 2007
The oblong chunks of free-floating ice are hotspots for ocean denizens, a new study says. Anecdotal scientific observations suggest marine plants, shrimplike crustaceans, and seabirds big players in the ocean food chain congregate on and around the chunks of ice. Icebergs are proliferating in the Antarctic as rising temperatures shrink and split the continent's ice shelves, leading scientists to wonder what effect this has on the marine environment.

Icebergs are 'ecological hotspot'   BBC - June 22, 2007
Drifting icebergs are "ecological hotspots" that enable the surrounding waters to absorb an increased volume of carbon dioxide, a study suggests

Snowball Fight Erupts over Frozen Earth Theory   MSNBC - May 7, 2007 The theory that the Earth long ago froze completely over, like a giant snowball, is challenged by new data from desert outcroppings in Oman. The geological measurements indicate that even as glaciers spread across all the continents 700 million years ago, warm spells with liquid water were still common.

How prehistoric farmers saved us from new Ice Age   Guardian - March 5, 2005
Ancient man saved the world from a new Ice Age. That is the startling conclusion of climate researchers who say man-made global warming is not a modern phenomenon and has been going on for thousands of years. Prehistoric farmers who slashed down trees and laid out the first rice paddies and wheatfields triggered major alterations to levels of greenhouse gases such as methane and carbon dioxide in the atmosphere, they say.

Unprecedented Ice Age Cave Art Discovered in U.K.   National Geographic - August 18, 2004
Vivid frescoes of stampeding bulls, horses, and other animals drawn by Stone Age artisans grace the walls of many European caves. The most spectacular examples are found in Altimera in Spain and Lascaux and Chauvet in France.

'Sistine Chapel of the Ice Age'   BBC - July 13, 2004

An English cave has been described as the "Sistine Chapel of the Ice Age" after the discovery of 80 engraved figures in its limestone ceiling. The discovery was at Creswell Crags. It comes a year after the initial discovery of 12 engraved figures, which were trumpeted as the earliest examples of prehistoric cave art in Britain. The new discoveries were made possible by the good natural light in April and June, rock art experts said.

Chipmunks Descended From Ancestors That Survived Last Ice Age   Science Daily - July 13, 2004
A careful analysis of observations by NASA's Chandra X-ray Observatory of a rare quadruple quasar has uncovered evidence that possibly a single star in a foreground galaxy magnified X-rays coming from the quasar. This discovery gives astronomers a new and extremely precise probe of the gas flow around the supermassive black hole that powers the quasar.

Did cosmic rays cause ice ages?   Physics Web - July 12, 2004
Ice ages could be caused by changes in the flux of cosmic rays hitting the Earth according to three physicists. Jasper Kirkby of CERN, Augusto Mangini of the University of Heidelberg and Richard Muller of the University of California at Berkeley suggest that the cosmic rays exert their influence through their effect on clouds. By challenging the established insolation theory of glacial cycles, the physicists are sure to encounter opposition from the geophysics community

Ice cores unlock climate secrets   BBC - June 9, 2004
Global climate patterns stretching back 740,000 years have been confirmed by a three-kilometre-long ice core drilled from the Antarctic. Analysis of the ice proves our planet has had eight ice ages during that period, punctuated by rather brief warm spells - one of which we enjoy today.

New Evidence Supports Three Major Glaciation Events In The Distant Past   Science Daily - April 22, 2004
Glaciers reached Cape Cod, Massachusetts, in the most recent ice age about 20,000 years ago. But much harsher ice ages hit the Earth in an ancient geological interval known as "the Cryogenian Period" between 750 and 600 million years ago. A team of geologists from China and the United States now report evidence of at least three ice ages during that ancient time.

Humans Sped to U.K. After Ice Age   National Geographic - November 3, 2003
Humans hotfooted it to Britain after the last ice age, scientists say. The new research, which challenges previous studies, suggests these early settlers advanced rapidly as the glaciers melted away.

Methane theory gets frosty response BBC - April 11, 2003
A hot theory about how the Ice Age ended has got a frosty response at a meeting of the leading European and American geoscience societies in France

Ice Age star map discovered   BBC - August 9, 2000
prehistoric map of the night sky has been discovered on the walls of the famous painted caves at Lascaux in central France. The map, which is thought to date back 16,500 years, shows three bright stars known today as the Summer Triangle. A map of the Pleiades star cluster has also been found among the Lascaux frescoes. And another pattern of stars, drawn 14,000 years ago, has been identified in a cave in Spain.