Exceptional Polar Amplification of Eocene Climate Under Modest Greenhouse Gas Forcing

Eocene palaeoclimates provide imperfect but useful analogs for the future with respect to equilibrium responses towards higher-than-present atmospheric CO2. Parallel temperature and CO2 reconstructions from middle Eocene terrestrial sediments that infill a kimberlite diatreme in subarctic Canada enable detailed assessments of regional climate sensitivity for the interval immediately post-dating 48 Mya. These sediments have exceptional preservation of botanical fossils, revealing a humid-temperate forest ecosystem with abundant Metasequoia (dawn redwood). Pollen assemblages and oxygen stable isotopes from wood cellulose indicate mean annual temperatures (MATs) at least 17 ̊C warmer than present, and coldest months more than 25 ̊C. Stomatal indices from Metasequoia foliage constrain atmospheric CO2 to 450-1000 ppm, with a median of 630 ppm. The reconstructed MATs are ~10 ̊C warmer than predicted by a global Earth system sensitivity of 6 ̊C per CO2 doubling, demonstrating that exceptional polar amplification characterized middle Eocene climates even in absence of cryospheric feedbacks (i.e. coldest months consistently >0 ̊C). Fundamental changes to synoptic circulation may explain the inferred magnitude of high-latitude warming and concomitant increase of precipitation, for which direct analogies exist in contemporary global warming: deepening lows and enhanced cyclogenesis over the Arctic Ocean, slowing of the polar jet stream, and widening of Hadley cells. Altered circulation patterns during greenhouse climate states may explain why climate models have difficulty capturing fully the conditions implied by proxies under moderately elevated, but not extreme (i.e., <1000 ppm), atmospheric CO2 concentrations.