Modeling early Paleogene climate: From the top of the atmosphere to the bottom of the ocean

This dissertation addresses critical issues in early Paleogene paleoclimatology. This study's goal is to develop a more general and deeper understanding of climate by focusing on a time interval for which climate proxies and models have consistently disagreed. Climate proxies provide intriguing evidence of a “greenhouse” early Paleogene world: greenhouse gas (GHG) concentrations were higher than modern, midlatitude continental interiors were above freezing year-round, crocodiles lived at high latitudes, and vertical and meridional oceanic thermal gradients were small. The geologic record indicates that the climate system's response to increased GHG concentrations is to warm the poles and deep oceans and increase winter temperatures without raising tropical sea surface temperatures (SSTs), counter to every existing theory and model.; Results from the uncoupled atmospheric general circulation model (GCM) experiments described in Part I demonstrate that (1) changes to land surface characteristics have important consequences at regional but not larger scales; (2) specification of warm polar SSTs implies approximately double modern ocean heat transport and does little to warm continental interiors during winter; (3) midlatitude continental interior temperatures are at least as sensitive—if not more—to changes in tropical SSTs as to extratropical SSTs; (4) testable predictions for past wind-driven ocean currents can be made from existing atmospheric GCM output.; Results from Part II demonstrate that (1) a fully coupled GCM can be efficiently integrated to equilibrium for both a “degraded” modern case and an early Paleogene case; (2) the early Paleogene simulation produces temperature gradients very similar to modern, suggesting that it is unlikely that increased meridional heat transport caused early Paleogene small temperature gradients; (3) this simulation produces unrealistically cold continental interior temperatures, showing that a complete treatment of the ocean does not solve the equable climate “problem”; (4) atmospheric transport statistics, residual heat budgets, radiation budgets, and precipitation patterns remain virtually unchanged between the modern and early Paleogene case. The degree to which the early Paleogene simulation produces climate statistics that are very similar to modern, despite drastically changed initial and boundary conditions, raises serious questions about climate models' ability to reproduce climates demonstrably different than modern.