"It's long been thought that when the planet warms, as it did during the PETM, the rate of rock weathering on land, which absorbs CO2 from the atmosphere, increases," Donald E. Penman, a postdoctoral fellow and geologist at Yale University, said in a news release. "This draws down CO2 and cools the planet back down again."
http://www.upi.com/Science_News/2016/07/26/Scientists-find-evidence-of-planetary-carbon-release-valve/9661469539726/
