Regional climate modeling studies of western North America under early Eocene conditions

This dissertation explores the regional climate of western North America during the early Eocene (∼50-55 Ma). The early Eocene was the warmest part of the Cenozoic era and is considered to be a possible analog of the future climate that could result from anthropogenic climate change. The primary goal of this research is to use a regional climate model (RCM) to examine the sensitivity of the early Eocene climate to changes in various forcing factors, including greenhouse gas concentrations, land cover types, and orbital parameters, in order to gain a better understanding of the causes of this warm climate at a regional scale.;The first sensitivity study examines the regional climates resulting from CO2 levels of 560 ppm and 2240 ppm. While the results suggest that CO2 levels were most likely at least as high as 2240 ppm, this high concentration did not yield a regional climate that matches regional proxy data in all aspects. The second sensitivity study investigates the role of land cover in determining the regional climate. The results here suggest that adding lakes to the region brings the model results into closer agreement with proxy data estimates, and indeed, it seems critical to include these more realistic land cover types to capture the climate indicated by the proxy climate data. However, this important inclusion again did not produce a precise match between model output and regional proxy data. The third sensitivity study tests how cyclical changes in orbit may have influenced the regional climate. Results of this third experiment suggest that an orbit resulting in minimal seasonality brings the model results into closer agreement with proxy data estimates, though as in the previous studies, the modeled regional climate does not match regional proxy data in all aspects.