Volcanoes erupt, venting CO2 from the earth’s crust into the atmosphere, where it dissolves in rainwater to form carbonic acid that dissolves silicate rocks flushing bicarbonate into river systems and back into the sea.

In the sea, marine organisms use the bicarbonate to construct shells. These fall to the bottom of the ocean to form sediment on the seabed. The famous white cliffs of Dover are an example of the rocks this forms.

These dense seabed plates eventually move under light plates, dragging the carbon back into the molten heat of the core where the carbon is cooked into CO2.

The exchange of carbon between rocks, ocean and atmosphere cycles over 100 times the amount of carbon found in our terrestrial ecosystems every 1 million years.

Without this long-term cycle, the plant would turn into a giant snowball.

I now feel sufficiently unimportant, and convinced that on a geologically relevant timescale (i.e. more time that any of us can ever imagine), even if we ruin the earth for own habitation the planet itself will continue to recycle itself to support life in the future. 

(The Emerald Planet, Beerling 2007)