The role of downwelling longwave radiation in the response of sea ice to increasing CO2

Equable climates are periods during which the high latitudes are extremely warm, and the equator to pole temperature gradient is very small. Recent work by D. Abbot and E. Tziperman suggests that strong positive cloud and water vapor feedbacks can produce equable climates when the CO2 concentration is high enough. Motivated by their work, we analyze the joint variability of the hydrological and energy cycles of the atmosphere and lower boundary in the Arctic, with CO2 at four times the preindustrial concentration. Emphasis is on the radiative effects of water vapor and clouds and the role downwelling longwave radiation. The goal of our work is to gain a better understanding of how these processes operate in the Arctic environment, especially with elevated concentrations of CO2. We compare simulations performed with the Community Earth System Model (CESM), and the SP-CESM, in which the standard parameterizations are replaced with a cloud resolving model. Results show significant Arctic sea ice differences between the two models. The mechanisms that lead to the different model results are analyzed.