Snowball Earth Hypothesis

Thawing Earth

Over time scales of millions of years the amount of carbon dioxide (a known greenhouse gas) in the ocean-atmosphere system is adjusted to sustain a balance between its supply by volcanoes, both on land and in the ocean, and its elimination by chemical weathering reactions with silicate rocks, which changes the carbon dioxide to calcium carbonate which is subsequently buried in sediment (Hoffman, 2002).

On a snowball Earth this balance brakes down, there is still volcanic activity releasing carbon dioxide but the conversion of carbon dioxide to calcium carbonate is halted since gas exchange between the ocean and atmosphere are reduced (Hoffman, 1999). Over a period of millions of years the carbon dioxide accumulates in the atmosphere until the green house gas effect is so great that it over powers the albedo of a snowball Earth (Hoffman, 1999). It is expected that 0.12 bar of carbon dioxide (about 350 times the present concentration) would have been necessary to overcome the albedo of a snowball Earth (NASA, 2001). As the Earth thaws and equatorial regions become free of ice, the Earth's albedo decreases, resulting in even warmer temperatures as a consequence the ice melts faster and faster, this negative feedback continues until the Earth is completely free of ice. Raymond Pierrehumbert's estimates show that once Earth starts to thaw it will become free of ice in just a few hundred years. At the end of a thawing period the Earth's tropical sea-surface temperature reaches 50C (Hoffman, 2002).