Diamond mines, Arctic forests, and Eocene polar amplification under modest greenhouse gas forcing
Eocene palaeoclimates are natural experiments on equilibrium climate response to higher-than-present atmospheric CO2 in the absence of large cryospheric feedbacks. Climate models commonly underestimate the magnitude of Eocene warmth relative to proxy data, especially in high-latitude continental interiors. However, a shortcoming of the continental proxy record is that CO2 is rarely reconstructed in parallel with other palaeoclimate information, complicating assessment of regional climate sensitivity and data-model fidelity. I will present reconstructions of temperature, precipitation, and CO2 from middle Eocene (~38 Myrs ago) terrestrial sediments that infill a kimberlite diatreme in subarctic Canada. Exceptionally preserved fossil plants reveal a humid-temperate forest ecosystem with mean annual precipitation ~1000 mm wetter than present. Mean annual temperatures (MATs) derived from pollen assemblages and wood cellulose d18O are >17 °C warmer than present. Median atmospheric CO2 inferred from stomatal indices and a gas-exchange model are 630 and 430 ppm, respectively. Reconstructed MATs are at least 6 °C warmer than predicted from Eocene climate models constrained by 560-1120 ppm CO2, underscoring the presence of exceptionally high amplification of subarctic warming. Fundamental changes to synoptic circulation may help explain the amplification, for which analogies exist in contemporary global warming: stronger cyclogenesis over the Arctic Ocean, slowing of the polar jet stream, and widening of Hadley cells.