| The Early Paleogene (∼65--45Ma) has long been recognized as a time of uniquely warm and equable climate. Proxy climatic data of varying types (e.g. floral, faunal, isotopic) suggest considerable warmth from pole to pole (global average, mean annual temperature = 17--21°C), a slightly reduced equator to pole temperature gradient, and low seasonality, particularly in mid-latitude continental interiors. The primary scientific question on which this dissertation is based is: How did the Early Paleogene climate come to exist? To subdivide the main question, Why was the Early Paleogene so warm? and Why was seasonality so low during the Early Paleogene? With the aim of answering these questions, my coauthors and I used global climate models to test the climate responses to several different forcing factors. The climate forcing factors that we tested were, detailed surface boundary conditions, increased greenhouse gas concentrations, warmer than present sea surface temperatures (SST), reduced planetary obliquity, and combinations thereof. We conducted the majority of our climate modeling experiments in "data ocean," fixed SST, model configurations but also conducted two fully coupled, Earth System Model simulations. Given the limited spatial coverage of most climate proxies, detailed land-surface boundary conditions and increased model resolution serve to significantly improve the match between model results and data. Detailed surface boundary conditions are important not only on land but also over the ocean. Spatially and temporally varying SST distributions for the Early Paleogene are not available from proxy data. Previously, researchers have employed zonally averaged SST fields to force data ocean simulations of Early Paleogene climate. We developed a method to generate spatially and temporally varying Early Paleogene SSTs with a fully coupled Earth System Model and utilized those detailed SSTs in our modeling experiments. Based on changes in the mass distribution of Earth over the last 120Ma, we hypothesized that reduced planetary obliquity is possible, and likely the root cause of equable climates, during the Early Paleogene. Climate model experiments incorporating our Early Paleogene SSTs, increased greenhouse gas concentrations and a reduced planetary obliquity of 18° produce the most detailed and accurate representation of Early Paleogene climate to date. |
