700 Million B.C.T. - Iceball Earth

Around 700 million years ago, the Earth became an iceball.

     During the 1960s, geologists discovered rocks about 700 million years old all over the world bore the signature of rough treatment from glaciers.  The Soviet scientists, M. I. Budyko, proposed one possible cause: runaway global cooling.  According to the theory, bright, white polar ice sheets reflect more of the sun's heat and light back into space than do darker land masses or open water.  So as the ice sheets grow during an Ice Age, they exert a feedback effect that further cools the world.  The bigger they get, the more cooling they cause and so the more the ice sheets grow.  Budyko's theoretical models of the Earth's climate suggested that this feedback could pass a point of no return, leaving the planet to freeze over.

     On the resulting Iceball Earth, ice was everywhere: even the oceans were frozen.  Except for a few organisms clinging on or around volcanoes, no life could survive.  The temperature around the world was an average minus 40 degrees centigrade.  It was extremely cold.

     How then could Iceball Earth have shaken off its Arctic glaze and return to being the liquid blue planet that we know today?  The answer may lie in the volcanoes.   Volcanoes thrusting through the ice would have continued to disgorge gases, mostly carbon dioxide.  Carbon dioxide is a greenhouse gas that causes global warming.  Today, volcanic carbon dioxide is kept in check by natural processes such as chemical weathering of rocks, which removes the gas from the atmosphere in the form of carbonate minerals.  However, on Iceball Earth, weathering would be suppressed because there would be no rain to wash the carbon dioxide from the skies, and no exposed rocks to react with it.  So volcanic carbon dioxide would accumulate gradually in the atmospher and warm the planet. 

     At some point, there would be enough of it to break the reign of ice, and the seas would thaw.  Calculations suggest that a huge amount of carbon dioxide is needed to do this: about 350 times the amount in today's atmosphere.  So once melting began, temperatures would soar and the planet would gravitate toward becoming a hothouse.