Variations on a Pathway to an Early Eocene Climate

The climate of the early Eocene was characterized by much higher temperatures and a smaller equator-to-pole surface temperature gradient than today. Comprehensive models have been reasonably successful in simulating that annual average. However, good simulations seasonal variations, particular warmer Arctic winters over land, proven more difficult. Further, while increased greenhouse gases seems necessary to achieve an climate, it is unclear whether there unique combination factors leads agreement with all available proxies. Here we use very flexible General Circulation Model examine sensitivity modeled differences CO2 concentration, land properties, ocean heat transport, cloud extent thickness. Even absence ice or changes cloudiness, increasing concentration polar-amplified change because water vapor levels combined lack convection at high latitudes, nonlinear dependence longwave radiation on amplifying increase winter land. Additional low clouds generally decrease summer further (except levels). An capacity, plausible given large vegetation, also decreases seasonality. Thus, different combinations factors—high levels, low-level clouds, capacity—can lead simulation within proxy uncertainty range majority data.