Persistent greenhouse conditions in Eocene North America point to lower climate sensitivity

Abstract Oxygen isotope values (δ 18 O) in benthic foraminifera indicate a marked cooling from the early to late Eocene in the higher latitudes where oceanic bottom water forms. This record is widely interpreted as an indication of global surface cooling, leading to the hypothesis of greater climate sensitivity to atmospheric CO 2 concentration in warmer than in cooler climate states. Here we extend the coexistence approach using phylogenetic trees to study Eocene climatic change in the Western Interior of North America based on assemblages of small reptiles. Our results capture and elaborate on climatic features previously inferred for the early Eocene, such as drying during the Palaeocene-Eocene Thermal Maximum, but also show that the late Eocene in mid-latitude North America was considerably warmer and wetter than previously thought. Accordingly, the net Eocene cooling trend in benthic foraminiferal δ 18 O might reflect not global but rather regional, high-latitude phenomena, and hypothesised extreme values of climate sensitivity to CO 2 doubling might be revised downward.