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The snowball earth theory is one that says that at one point in the past the earth was completely covered in ice. The time at which this phenomenon of almost complete glaciation was supposed to occur was about 600 million years ago. The "snowball" lasted a long time and could have existed for 10 million years.

The process that creates the snowball is one of positive feedback. It has been calculated that if the earth?s glaciers reach a certain point there will be a runaway freezing process. This will be because the glacial ice has a high albedo and so there will be less light absorbed. The earth will then cool down more and glaciation will continue to advance. The earth could have become completely glaciated. Temperatures of -40 degrees C would have been common. It is unlikely that much complex life could have survived.

The most commonly accepted way of escaping from the snowball involves volcanoes. The volcanic eruptions produce greenhouse gases. The greenhouse gases are not removed because of the lack of photosynthesising plants. These increase the greenhouse effect, and so warm the earth up again. Photosynthesizing organisms would increase in numbers when the snowball began to melt. This would use up the carbon dioxide in the atmosphere. Eventually the carbon dioxide would reach a low, stable level at which the releases were equal to the uptakes at a temperature closer to what we have today.

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