Earth's climate alternates between ice ages and greenhouse periods , during which there are no glaciers on the planet. Earth is currently in the Quaternary glaciation , known in popular terminology as the Ice Age. In the terminology of glaciology , ice age implies the presence of extensive ice sheets in both northern and southern hemispheres. The amount of heat trapping gases emitted into Earth's oceans and atmosphere are predicted to prevent the next glacial period, which otherwise would begin in around 50, years, and likely more glacial cycles.
In , Pierre Martel — , an engineer and geographer living in Geneva , visited the valley of Chamonix in the Alps of Savoy. He reported that the inhabitants of that valley attributed the dispersal of erratic boulders to the glaciers, saying that they had once extended much farther. In the carpenter and chamois hunter Jean-Pierre Perraudin — explained erratic boulders in the Val de Bagnes in the Swiss canton of Valais as being due to glaciers previously extending further. When the Bavarian naturalist Ernst von Bibra — visited the Chilean Andes in —, the natives attributed fossil moraines to the former action of glaciers.
Meanwhile, European scholars had begun to wonder what had caused the dispersal of erratic material. From the middle of the 18th century, some discussed ice as a means of transport. The Swedish mining expert Daniel Tilas — was, in , the first person to suggest drifting sea ice in order to explain the presence of erratic boulders in the Scandinavian and Baltic regions. He regarded glaciation as a regional phenomenon.
Only a few years later, the Danish-Norwegian geologist Jens Esmark — argued a sequence of worldwide ice ages. In a paper published in , Esmark proposed changes in climate as the cause of those glaciations. He attempted to show that they originated from changes in Earth's orbit.
Andersen In a paper published in , Bernhardi speculated about former polar ice caps reaching as far as the temperate zones of the globe. In , independently of these debates, the Swiss civil engineer Ignaz Venetz — explained the dispersal of erratic boulders in the Alps, the nearby Jura Mountains, and the North German Plain as being due to huge glaciers. When he read his paper before the Schweizerische Naturforschende Gesellschaft , most scientists remained sceptical.
De Charpentier transformed Venetz's idea into a theory with a glaciation limited to the Alps. His thoughts resembled Wahlenberg's theory. In fact, both men shared the same volcanistic, or in de Charpentier's case rather plutonistic assumptions, about the Earth's history. In , de Charpentier presented his paper before the Schweizerische Naturforschende Gesellschaft. He began to wonder where such masses of stone had come from.
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During the summer of he made some excursions to the Bavarian Alps. Schimper came to the conclusion that ice must have been the means of transport for the boulders in the alpine upland. In the winter of to he held some lectures in Munich. Schimper, de Charpentier and possibly Venetz convinced Agassiz that there had been a time of glaciation.
They mainly drew upon the preceding works of Venetz, de Charpentier and on their own fieldwork. Agassiz appears to have been already familiar with Bernhardi's paper at that time. The audience was very critical and some opposed to the new theory because it contradicted the established opinions on climatic history. Most contemporary scientists thought that the Earth had been gradually cooling down since its birth as a molten globe. In order to overcome this rejection, Agassiz embarked on geological fieldwork.
De Charpentier felt that Agassiz should have given him precedence as it was he who had introduced Agassiz to in-depth glacial research. All together, it took several decades until the ice age theory was fully accepted by scientists. This happened on an international scale in the second half of the s following the work of James Croll , including the publication of Climate and Time, in Their Geological Relations in , which provided a credible explanation for the causes of ice ages. Geological 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. Furthermore, this evidence was difficult to date exactly; early theories assumed that the glacials were short compared to the long interglacials.
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The advent of sediment and ice cores revealed the true situation: glacials are long, interglacials short. It took some time for the current theory to be worked out. The chemical evidence mainly consists of variations in the ratios of isotopes in fossils present in sediments and sedimentary rocks and ocean sediment cores. For the most recent glacial periods ice cores provide climate proxies from their ice, and atmospheric samples from included bubbles of air.
Because water containing heavier isotopes has a higher heat of evaporation , its proportion decreases with colder conditions. This evidence can be confounded, however, by other factors recorded by isotope ratios. The paleontological evidence consists of changes in the geographical distribution of fossils. During a glacial period cold-adapted organisms spread into lower latitudes, and organisms that prefer warmer conditions become extinct or are squeezed into lower latitudes. This evidence is also difficult to interpret because it requires 1 sequences of sediments covering a long period of time, over a wide range of latitudes and which are easily correlated; 2 ancient organisms which survive for several million years without change and whose temperature preferences are easily diagnosed; and 3 the finding of the relevant fossils.
Despite the difficulties, analysis of ice core and ocean sediment cores  has shown periods of glacials and interglacials over the past few million years. These also confirm the linkage between ice ages and continental crust phenomena such as glacial moraines, drumlins, and glacial erratics. Hence the continental crust phenomena are accepted as good evidence of earlier ice ages when they are found in layers created much earlier than the time range for which ice cores and ocean sediment cores are available. Outside these ages, the Earth seems to have been ice free even in high latitudes;   such periods are known as greenhouse periods.
Rocks from the earliest well established ice age, called the Huronian , formed around 2.
Marie to Sudbury, northeast of Lake Huron, with giant layers of now-lithified till beds, dropstones, varves, outwash, and scoured basement rocks. Correlative Huronian deposits have been found near Marquette, Michigan , and correlation has been made with Paleoproterozoic glacial deposits from Western Australia.
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The Huronian ice age was caused by the elimination of atmospheric methane , a greenhouse gas , during the Great Oxygenation Event. The next well-documented ice age, and probably the most severe of the last billion years, occurred from to million years ago the Cryogenian period and may have produced a Snowball Earth in which glacial ice sheets reached the equator,  possibly being ended by the accumulation of greenhouse gases such as CO 2 produced by volcanoes.
The Andean-Saharan occurred from to million years ago, during the Late Ordovician and the Silurian period. The evolution of land plants at the onset of the Devonian period caused a long term increase in planetary oxygen levels and reduction of CO 2 levels, which resulted in the late Paleozoic icehouse. Its former name, the Karoo glaciation, was named after the glacial tills found in the Karoo region of South Africa.
There were extensive polar ice caps at intervals from to million years ago in South Africa during the Carboniferous and early Permian Periods. Correlatives are known from Argentina, also in the center of the ancient supercontinent Gondwanaland. Since then, the world has seen cycles of glaciation with ice sheets advancing and retreating on 40, and ,year time scales called glacial periods , glacials or glacial advances, and interglacial periods, interglacials or glacial retreats.
The earth is currently in an interglacial, and the last glacial period ended about 10, years ago. All that remains of the continental ice sheets are the Greenland and Antarctic ice sheets and smaller glaciers such as on Baffin Island. The definition of the Quaternary as beginning 2. The term Late Cenozoic Ice Age is used to include this early phase.
The maximum extent of the ice is not maintained for the full interval. The scouring action of each glaciation tends to remove most of the evidence of prior ice sheets almost completely, except in regions where the later sheet does not achieve full coverage. Within the ice ages or at least within the current one , more temperate and more severe periods occur.
The colder periods are called glacial periods , the warmer periods interglacials , such as the Eemian Stage. Glacials are characterized by cooler and drier climates over most of the earth and large land and sea ice masses extending outward from the poles.
Mountain glaciers in otherwise unglaciated areas extend to lower elevations due to a lower snow line.