Snowball Earth

The Snowball Earth hypothesis proposes that during one or more of Earth's icehouse climates, Earth's surface became entirely or nearly entirely frozen at least once, sometime earlier than 650 Mya (million years ago). Proponents of the hypothesis argue that it best explains sedimentary deposits generally regarded as of glacial origin at tropical palaeolatitudes and other enigmatic features in the geological record. Opponents of the hypothesis contest the implications of the geological evidence for global glaciation and the geophysical feasibility of an ice- or slush-covered ocean and emphasize the difficulty of escaping an all-frozen condition. A number of unanswered questions remain, including whether the Earth was a full snowball, or a 'slushball' with a thin equatorial band of open (or seasonally open) water. The snowball-Earth episodes are proposed to have occurred before the sudden radiation of multicellular bioforms, known as the Cambrian explosion. The most recent snowball episode may have triggered the evolution of multicellularity. Another, much earlier and longer snowball episode, the Huronian glaciation, which would have occurred 2400 to 2100 Mya, may have been triggered by the first appearance of oxygen in the atmosphere, the 'Great Oxygenation Event'. Long before the idea of a global glaciation was established, a series of discoveries began to accumulate evidence for ancient Precambrian glaciations. The first of these discoveries was published in 1871 by J. Thomson who found ancient glacier-reworked material (tillite) in Islay, Scotland. Similar findings followed in Australia (1884) and India (1887). A fourth and very illustrative finding that came to be known as 'Reusch's Moraine' was reported by Hans Reusch in northern Norway in 1891. Many other findings followed, but their understanding was hampered by the rejection of continental drift. Sir Douglas Mawson (1882–1958), an Australian geologist and Antarctic explorer, spent much of his career studying the Neoproterozoic stratigraphy of South Australia, where he identified thick and extensive glacial sediments and late in his career speculated about the possibility of global glaciation. Mawson's ideas of global glaciation, however, were based on the mistaken assumption that the geographic position of Australia, and that of other continents where low-latitude glacial deposits are found, has remained constant through time. With the advancement of the continental drift hypothesis, and eventually plate tectonic theory, came an easier explanation for the glaciogenic sediments—they were deposited at a time when the continents were at higher latitudes. In 1964, the idea of global-scale glaciation reemerged when W. Brian Harland published a paper in which he presented palaeomagnetic data showing that glacial tillites in Svalbard and Greenland were deposited at tropical latitudes. From this palaeomagnetic data, and the sedimentological evidence that the glacial sediments interrupt successions of rocks commonly associated with tropical to temperate latitudes, he argued for an ice age that was so extreme that it resulted in the deposition of marine glacial rocks in the tropics. In the 1960s, Mikhail Budyko, a Russian climatologist, developed a simple energy-balance climate model to investigate the effect of ice cover on global climate. Using this model, Budyko found that if ice sheets advanced far enough out of the polar regions, a feedback loop ensued where the increased reflectiveness (albedo) of the ice led to further cooling and the formation of more ice, until the entire Earth was covered in ice and stabilized in a new ice-covered equilibrium. While Budyko's model showed that this ice-albedo stability could happen, he concluded that it had in fact never happened, because his model offered no way to escape from such a feedback loop. In 1971, Aron Faegre, an American physicist, showed that a similar energy-balance model predicted three stable global climates, one of which was snowball earth.