Early Eocene low orography and high methane enhance Arctic warming via polar stratospheric clouds
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Proxy data suggest that the early Eocene (~56–47.8 million years ago) was characterised by a much weaker equator-to-pole temperature gradient than today. However, general circulation models consistently underestimate high-latitude temperatures indicated by proxy records, suggesting that they may miss important processes. Previous studies hypothesised that wintertime polar stratospheric clouds may have played an important role in Arctic warming through greenhouse forcing but these studies did not consider the effects of atmospheric chemistry or the early Eocene topography. Here, we examine these factors using a high-top atmospheric model with interactive chemistry. The lower orography in the low-to-mid-latitude Northern Hemisphere early Eocene weakens the stratospheric circulation which, in combination with sufficiently high methane concentrations, leads to a substantial increase in polar stratospheric clouds in the Arctic winter. Furthermore, an increase in early Eocene polar stratospheric clouds due to a 16- to 64-fold higher than pre-industrial methane concentration results in a radiative forcing larger than the direct greenhouse effect from the methane itself. This polar stratospheric cloud-induced radiative forcing could cause up to 7.4 K of Arctic surface warming. These results point to the potential for nonlinear interactions between individual forcings.
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Acknowledgements: We acknowledge the support from the Australian Research Council (CE170100023, D.D., M.J., S.C.S., K.J.M., A.S.G; FL150100035, S.C.S.; DP180100048, K.J.M.) and the Australian Government Research Training Program Scholarship (D.D). Computational resources were provided by the National Computational Infrastructure (NCI) National Facility at the Australian National University through awards under the Merit Allocation Scheme and the University of New South Wales high-performance computing (HPC) at NCI Scheme. The CESM project is supported primarily by the US National Science Foundation (NSF). This material is based upon work supported by the National Center for Atmospheric Research, which is a major facility sponsored by the NSF under cooperative agreement number 1852977. We thank C. K. West and the other anonymous reviewers for valuable suggestions.
Funder: Australian Research Council Centre of Excellence for Climate Extremes (CE170100023) Australian Government Research Training Program Scholarship
Funder: Australian Research Council Centre of Excellence for Climate Extremes (CE170100023) Australian Research Council (FL150100035)
Funder: Australian Research Council Centre of Excellence for Climate Extremes (CE170100023) Australian Research Council (DP180100048)
Funder: Australian Research Council Centre of Excellence for Climate Extremes (CE170100023)
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1752-0908
