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In general, the Earth seems to have been ice free even in high latitudes except during relatively rare glacial maximums such as the one from which we emerged from 10 to 15 thousand years ago.

The causes of glaciations have been much debated ever since the phenomenon was clearly identified in the 17th century. Modern theories tend to revolve around periodic oscillations in the Earth's orbit; hypothecised periodic changes in solar output; and/or the effects of continental masses drifting into polar regions where Antarctica currently resides.

Known periods of glaciation include the Huronian (2400Ma-2100Ma), Sturtian-Varangian (950Ma-570Ma), Andean-Saharan (450Ma-420Ma), Karoo (360Ma-260Ma), Cenozoic (30Ma-Present). These can be further divided by location and time (eg: the names Riss (180,000 - 130,000 years bp) and Wurm (70,000-10,000 years bp) refer specifically to glaciation at the headwaters of the Donau / Rhine rivers.) Not every year in each interval was a time of complete or even partial glaciation. The best studied glaciation, that of the recent past, appears to have taken place in at least four separate ice incursions and retreats. Unfortunately the scouring action of each glaciation tends to remove most of the evidence of prior icesheets almost completely, except in regions where the later sheet doesn't achieve full coverage. It is probable that glacial periods other than those above have been overlooked because of the paucity of exposed rocks from high latitudes from older time periods. The Varanger glaciation was especially severe and may have extended to the Equator. This has led to a recent 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 hypothesis that the Earth froze over completely in the late ProterozoicIn geology, the Proterozoic is an eon prior to the first abundant complex life on earth. Classically, the boundary between the Proterozoic and the Paleozoic was set at the base of the Cambrian period when the first fossils of animals known as trilobites a, then thawed very rapidly as trapped waterDrinking water This article focuses on water as we experience it every day. The water (molecule) article describes water from a scientific and technical perspective. Water is an abundant substance on Earth. It exists in many forms, such as sea, rain, and and carbon dioxideCarbon dioxide is an atmospheric gas composed of one carbon and two oxygen atoms. One of the best known of chemical compounds, it is frequently called by its formula: :CO (pronunciation: "see oh two") Carbon dioxide results from the combustion of organic were returned to Earth's atmosphereEarth's atmosphere is the layer of gases surrounding the planet Earth and retained by the Earth's gravity. It consists of nitrogen (78. 1% by volume) and oxygen (20. 9%), with small amounts of argon (0. 9%), carbon dioxide (variable, but around 0. 035%),. An alternative hypothesis, sometimes called Slushball Earth , maintains that the Equator at least was ice-free.

An ice age is sometimes used to refer to a single glaciation, or to an entire period of repeated glaciations such as the recent 30 million years of the Cenozoic period, especially the Pleistocene glaciations.