The PIs will use a systematic approach to elucidate the processes that control sea ice thickness and areal extent on seasonal and longer timescales. First, they will identify responses that are robust among current comprehensive global climate models (GCMs). Next, they will develop fundamental theories to explain these responses, beginning with the consideration of physical first principles, continuing through simulations with a hierarchy of models with increasing levels of complexity, and culminating in tests of the theories with satellite-derived observations. A principal tool will be an idealized atmosphere?sea ice GCM, which helps bridge the gap between understanding of fundamental processes and simulations with comprehensive GCMs. Using this and other models, they will test the theories developed for Arctic sea ice retreat in the broader context of sea ice changes in a continuum of climates spanning the full range from a completely ice-covered to a completely ice-free planet. These theories will be used to interpret observed sea ice changes and to evaluate the aspects of current GCM projections that differ among the models, such as the rate of 21st-century Arctic sea ice retreat. Specific questions to be addressed include: ? How do interactions between sea ice and other components of the climate system determine the seasonal cycle of hemispheric sea ice cover? ? What physical principles govern the sensitivity of sea ice to climate changes?
The proposed research will elucidate some of the most fundamental questions about the dynamics of sea ice and climate. It will bridge the gap between state-of-the-art GCM simulations and fundamental physical understanding, thereby providing metrics for model assessment, suggesting ways for model improvement, and reducing the uncertainty in the prediction of future sea ice retreat. By exploring sea ice retreat in a wide range of climates, it will place past and possible future Arctic sea ice changes in a broader context.