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The Snowball Earth hypothesis suggests that the entire Earth was frozen during the Cryogenian period, with some scientists believing in a “Slushball Earth” scenario instead. The depletion of atmospheric carbon dioxide caused by continental drift is thought to have initiated the freeze, which could only be reversed by volcanic explosions releasing enough CO2 to cause global warming. The impact on the evolution of multicellular life is unknown.
The Snowball Earth hypothesis refers to the idea that at some point during the Earth’s history, particularly during the Cryogenian period (850 to 630 million years ago), the entire surface was frozen, including the oceans. The Snowball Earth hypothesis is controversial among paleontologists, but many believe it helps explain the presence of glacial deposits in tropical latitudes from this period, as well as other unusual aspects of the cryogenic geological record. All scientists agree that there were huge glaciations in the Cryogenian period, the disagreement is that they became global in extent.
The Snowball Earth scenario, if it actually occurred, is thought to have been initiated by continental drift in an almost exclusively equatorial configuration. This would have caused continental rocks to rapidly break down, which would then have absorbed large amounts of atmospheric carbon dioxide. Since carbon dioxide is a greenhouse gas that plays a crucial role in keeping the Earth warm, this depletion would have caused ice to build up unchecked. As glaciers covered the Earth, they would have raised the Earth’s albedo (reflectivity), reflecting light energy back into space, further accelerating cooling. The end result would have been a planet entirely covered in ice, with equatorial temperatures similar to present-day Antarctica.
Copious amounts of carbon dioxide would have been required to dispel the deep freeze of snowball Earth. Since silicate and carbonate rocks, the normal sources of carbon dioxide, would have been entirely covered, the gas would have had to come from volcanic explosions. Over periods of time in the millions of years, volcanic explosions would spew enough CO2 into the atmosphere to kickstart global warming. This warming would melt the ice around the equator and expose the earth, lowering the earth’s albedo and causing positive feedback of the warming.
Some scientists believe that a “Slushball Earth” hypothesis is more realistic than Snowball Earth, where a region of thin or ice-free ocean would exist around the equator. This is necessary for an active hydrological cycle, in fact some geological deposits of the time indicate its presence.
It is sometimes said that Snowball Earth holds back the evolution of multicellular life, but the truth is that we don’t know whether this is true or not. Single-celled life certainly existed billions of years before the Cryogenian and survived through it, possibly in deep-sea hydrothermal vents and other refuges. The survival of terrestrial life during the Snowball Earth period is sometimes cited as an argument that life could exist in the ice-covered oceans of outer Solar System bodies such as the Jovian moon Europa.
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