There's plankton calcification too. Though that mechanism might break if increased atmospheric CO2 increases the acidity of the oceans...

Here's some joy: the last comparable period of sudden warming, which may have been down to a methane-release event, took place so rapidly that the geological record can't resolve a timescale (the resolution's about 1000 years). And it took 100,000 years to settle back down again.

'Welcome to the Hot Earth: Home of modern Humanity. Remember our old combustion-using forebears in the Age of Waste? Back then, they had something called "snow"...'