The Pliocene. A period of time 2-5 million years ago hosting carbon dioxide levels ranging from 350 to 405 parts per million and global average temperatures that were 2-3 degrees Celsius hotter than 1880s levels. The great ice sheets of Greenland and West Antarctica were feeble, if they existed at all. And seas were about 25-80 feet higher than today.

(CO2 hit above 401.84 parts per million on March 9, 2015, and above 403 parts per million on March 10 — levels that test the upper boundary of CO2 last seen during the Pliocene and entering a range more similar to the Miocene. Image source: The Keeling Curve.)

In the context of human warming, the amount of heat forcing we’ve added to the global atmosphere from carbon dioxide emissions alone has been hovering in the range of the Pliocene for the past two decades. A heat forcing that, if…