Fig. 1.

Constraints on climate sensitivity for the globally warm Cretaceous and early Paleogene (125–40 Mya). (A) Tropical sea surface temperature (SST) records (15–29). Boxes represent studies with high sampling density. The red band corresponds to estimated tropical SST under an atmospheric CO₂ concentration of 1,000 ppm and a climate sensitivity (ΔT_2×) of 3°C per CO₂ doubling. Temperatures above this band probably represent times when climate sensitivity exceeded 3°C (see text). (B) Atmospheric CO₂ records (compilation from ref. 2). Dashed lines correspond to present-day CO₂ (387 ppm; A.D. 2009) and 1,000 ppm CO₂. Records from the boron proxy are excluded because they are likely unreliable (30); records from the goethite and nahcolite proxies are excluded because they are not yet widely applied.