When the carbon in the worlds oceans exceeds  310 gigitons which will happen about 2100, it will have reached the levels necessary to duplicate the 5 former world extinctions. One of those was the end of the Permian, that nice warm, wet time. It is though to take 10,000 years to recover as some of the carbon goes to the bottom of the ocean and is taken out of the loop. Previous carbon buildup was much slower than the 200 years we are in the middle of, so that is an unknown.
D.H. Rothman, Thresholds of Catastrophe in the Earths Systems.
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