Orbital forcing and Eocene continental temperatures

The ability to define terrestrial paleotemperatures is a key issue for understanding past climate states and the processes that produced them. Paleotemperatures are defined, ideally, by proxy data interpretations, and supported by theoretical modeling results that provide physical explanations for the temperatures. Such explanations are especially critical for paleoclimates substantially warmer than present, for which we have no modern or recent examples. Model results and geologic data describing continental mean annual temperatures for the warm Early Eocene Earth as well as other time periods are generally in agreement. However, there are persistent discrepancies between climate model results and proxy data interpretations for temperature seasonality estimates. In this paper we test the idea that orbital forcing and resulting distributions of solar radiation played a role in determining continental interior temperatures during the Eocene. Specification of extreme values of orbital parameters (within the range of calculated Pleistocene values) produces a wide range of continental temperatures. These include cooler Northern Hemisphere continental summer temperatures and milder winter temperatures by up to 30%, and an annual temperature range reduced by 75%, relative to temperatures produced at the same location with modern orbital configuration. Temperature responses to the specified orbital forcing are most evident at middle and high latitudes. The results produced with one specification of extreme orbital configurations are more similar to proxy data interpretations than any previous results of these temperature parameters. We suggest that orbital variation and resulting insolation responses must be included in paleoclimate modeling studies that aim to explain proxy data and paleoclimate conditions for specific times in Earthʹs history.