A very interesting time-period in the recent past is the mid-Pliocene (~3 Millions years ago). The configuration of the world looked much the same as today, CO2 was likely less than present day, but still the temperatures were much warmer and sea-level ~15-20m higher. If it is possible for the earth system to have such an extreme response to greenhouse gas concentrations no different from today’s, then it raises questions about the level of faith that can be put into climate model projections of only ~1 m of sea-level rise by the end of this century.