Causes of The Karoo Ice Age
The evolution of land plants with the onset of the Devonian period, began a long term increase in planetary oxygen levels. Large tree ferns, growing to 20 meters high, were secondary dominant to the large arborescent Lycopods (30–40 metres high) of the Carboniferous coal forests that flourished in equatorial swamps stretching from Appalachia to Poland, and later on the flanks of the Urals. Oxygen levels reached anything up to 35%, and global carbon dioxide got below the 300 parts per million level which is today associated with glacial periods. This reduction in the greenhouse effect was coupled with lignin and cellulose (as tree trunks and other vegetation debris) accumulating and being buried in the great Carboniferous Coal Measures. The reduction of carbon dioxide levels in the atmosphere, would be enough to begin the process of changing polar climates, leading to cooler summers which could not melt the previous winter's snow accumulations. The growth in snowfields to 6 metres deep would create sufficient pressure to convert the lower levels to ice.
Earth's increased planetary albedo produced by the expanding ice sheets would lead to positive feedback loops, spreading the ice sheets still further, until the process hit limit. Falling global temperatures would eventually limit plant growth, and the rising levels of oxygen would increase the frequency of fire-storms because damp plant matter could burn. Both these effects return carbon dioxide to the atmosphere, reversing the "snowball" effect and forcing greenhouse warming, with CO2 levels rising to 300 parts per million in the following Permian period. Over a longer period the evolution of termites, whose stomachs provided an anoxic environment for methanogenic lignin digesting bacteria, prevented further burial of carbon, returning carbon to the air as the greenhouse gas methane.
Once these factors brought a halt and a small reversal in the spread of ice sheets, the lower planetary albedo resulting from the fall in size of the glaciated areas would have been enough for warmer summers and winters and thus limit the depth of snowfields in areas from which the glaciers expanded. Rising sea levels produced by global warming drowned the large areas of flatland where previously anoxic swamps assisted in burial and removal of carbon (as coal). With a smaller area for deposition of carbon, more carbon dioxide was returned to the atmosphere, further warming the planet. By 250 million years ago, planet Earth had returned to a percentage of oxygen similar to that found today.
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