The carbon cycle – warm blankets and snowballs

During the earth’s very early history, around 4 billion years ago, the sun radiated 25 – 30% less heat than it does today.  If the earth had the same blanket of gases as it does today, the earth should have been c.20C colder.  However, there is evidence (the presence of water and the development of life, 3.8 billion years ago) that the earth was not this cold, and was in fact warmer than today.  The reason for this lies in the carbon cycle.

The early earth was apparently covered by a warm blanket of carbon dioxide gas, acting as a blanket, trapping a proportion of the heat of the sun and re-radiating it back to the surface of the earth.  This carbon dioxide was itself part of a carbon cycle: carbon dioxide was dissolved in rainwater to become carbonic acid; this rained down to the earth’s surface; it weathered silicate rocks to make bicarbonate ions; which in turn entered the sea water as bioavailable calcium taken up by shellfish to make their shells; these in turn were deposited at the bottom of the seas as limestone.  This carbon cycle was triggered by the enlargement of the continents, making more material available to be weathered by the carbonic acid and thus more carbon to be taken out of the atmosphere and deposited in the earth’s crust.

As the continents continued to expand, the reduction of carbon dioxide in the atmosphere caused the planet to cool and more polar ice to be created, in a negative feedback loop.  As the continents continued to expand, so the planet contiuned to cool, and the ice expanded further away from the poles.  When the ice reached the tropics, the planet entered into a tipping point, and the ice closed over the entire planet, meeting at the equator.  This has happened twice in the earth’s history, at 2.2 billion and 700 million years ago, during periods known as ‘snowball earth’. 

Again the carbon cycle played a key role here, this time as a positive feedback loop, reversing the earth’s snowball.  Volcanoes continued to spew carbon dioxide into the atmosphere, but the time the land was so covered by ice that the carbonic acid could not weather the rock and be deposited in the earth’s crust.  The carbon dioxide thus built up in the atmosphere, developing as a warming blanket that eventually absorbed and re-radiated enough energy back to the earth’s surface to start to melt the ice and reverse the snowball. 

I find it fascinating that the lime products I am using to renovate my house are in fact carbon stores from billions of years ago, in the same way that coal is a carbon store.  However, there is a key difference between these two materials.  When coal is burnt to produce heat, the carbon inside it is released as carbon dioxide which serves to warm the planet as a blanket gas.  When limestone is burnt to produce lime, carbon dioxide is also produced, however, the lime reabsorbs this carbon when it is used, and allowed to harden. 

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