Most worryingly, the climate models that we depend on as a species to predict our future have largely failed to predict our sultry ancient past. And though the gulf is narrowing, and models are catching up, even those that come close to reproducing the hothouse of the early Eocene require injecting 16 times the modern level of CO2 into the air to achieve it--far beyond the rather meager doubling or tripling of CO2 indicated by the rock record.