Is a runaway greenhouse effect underway?

[Odin]

Originally posted by chegitz guevara
I thought limestone was the fossilized remains of diatoms.

basically its a mix of calcareous protist shells. In the Precambrian there was much less in the way of biomineralization than today, most of the limestone just precipitated inorganically. The start of widepread biomineralization (I think it was about 1 billion years ago during the protistian radiation if I remenber right) just made the process more effecient (and may have caused the 3 "Snowball Earth" events in the late Precambrian). Also, the spread of the first land plants made the whole cycle more efficent by increasing the weathering rates, causing the huge drop in atmospheric CO2 in the Carboniferous (with some help from the erosion of the freshly risen southern Appalachians and Urals), and bringing the oxygen levels up to 35%, the highst in Earth's history.