The Snowball Earth hypothesis refers to the idea that at some point during the Earth’s history, specifically during the Cryogenian period (850 to 630 million years ago), the entire surface was frozen over, including the oceans. The Snowball Earth hypothesis is controversial among paleontologists, but many believe it helps explain the presence of glacial deposits at tropical latitudes from this period, as well as other unusual aspects of the Cryogenian geological record. All scientists agree that there were huge glaciations in the Cryogenian period, the disagreement is on whether they became global in extent.
The Snowball Earth scenario, if it did indeed occur, is thought to have been initiated by the drifting of continents into an almost exclusively equatorial configuration. This would have caused the rapid weathering of continental rocks, which would have then absorbed large amounts of atmospheric carbon dioxide. As carbon dioxide is a greenhouse gas that plays a crucial role in keeping the Earth warm, these depletion would have caused runaway ice accumulation. As glaciers covered the Earth, they would have raised the Earth’s albedo (reflectivity), reflecting light energy back into space, further accelerating cooling. The eventual result would have been a planet entirely covered in ice, with equatorial temperatures similar to present-day Antarctica.
To dispel the deep freeze of the Snowball Earth would have required copious amounts of carbon dioxide. As silicate and carbonate rocks, the normal sources of carbon dioxide, would have been entirely covered up, the gas would have had to come from volcanic explosions. Over periods of time in the millions of years, volcanic explosions would belch enough CO2 into the atmosphere to initiate global warming. This warming would cause ice around the equator to melt and expose land, lowering the Earth’s albedo and causing positive feedback of warming.
Some scientists believe a “Slushball Earth” hypothesis is more realistic than Snowball Earth, where a region of thin-ice or ice-free ocean would exist around the equator. This is necessary for an active hydrological cycle, and indeed, some geological deposits from the time indicate the presence of one.
The Snowball Earth is sometimes cited as holding back the evolution of multicellular life, but the truth is, we don’t know whether this is true or not. Unicellular life certainly existed billions of years before the Cryogenian and survived through it, perhaps in deep-sea hydrothermal vents and other refugia. The survival of Earthly life through 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.