| Summary: | Proxy data suggest that the early Eocene (similar to 56-47.8 million years ago) was characterized 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 hypothesized 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. 1852977
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