Orbital forcing and Eocene continental temperatures

The ability to define terrestrial paleotemperatures is a key issue for unde rstanding past climate states and the processes that produced them. Paleote mperatures are defined, ideally, by proxy data interpretations, and support ed by theoretical modeling results that provide physical explanations for t he temperatures. Such explanations are especially critical for paleoclimate s substantially warmer than present, for which we have no modem or recent e xamples. 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 be tween 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 extr eme values of orbital parameters (within the range of calculated Pleistocen e values) produces a wide range of continental temperatures. These include cooler Northern Hemisphere continental summer temperatures and milder winte r temperatures by up to 30%, and an annual temperature range reduced by 75% , relative to temperatures produced at the same location with modern orbita l configuration. Temperature responses to the specified orbital forcing are most evident at middle and high latitudes. The results produced with one s pecification of extreme orbital configurations are more similar to proxy da ta interpretations than any previous results of these temperature parameter s. We suggest that orbital variation and resulting insolation responses mus t be included in paleoclimate modeling studies that aim to explain proxy da ta and paleoclimate conditions for specific times in Earth's history. (C) 1 998 Elsevier Science B.V. All rights reserved.