State dependent climate sensitivity and polar amplification

Two of the main questions in climate dynamics are: (1) what is the change in global mean surface temperature caused by a doubling of atmospheric carbon dioxide levels, and (2) what is the change equator-to-pole temperature gradient for a given global mean temperature change. One hope is that paleoclimate records can provide key constraints on these two parameters which can constrain predictions of future climate change derived from climate models. In this talks I summarize work on this topic incorporating paleoclimate data and models spanning about 40 millions years from the Eocene, Oligocene, and Miocene. While we find that constraints on the lower bounds on climate sensitivity can be derived from this kind of study, the fact that climate sensitivity can be dependent on climate state and the forcing factors in the past are poorly known may be insurmountable obstacles to further tightening the likely bounds of modern or future climate sensitivity. On the other hand, polar amplification of global warming, and more generally the overall pattern of associated temperature change, appears to be more robust and stationary across a range of climate states and hence a more meaningful target for modeling. The change in temperature gradient for given mean surface temperature change may be a more productive avenue for continued investigation in models and data as it relies less on knowledge of climate change forcing factors and it is of direct relevance for tightening the range of possible future regional climate predictions.

Speaker: Matthew Huber, Purdue University

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