Changes in the sea ice seasonal cycle in response to climate change

It has been widely noted that sea ice retreats faster in summer than winter in the Northern Hemisphere, both in observations and in projections from state-of-the-art climate models. Explanations for why the wintertime sea ice cover should be less sensitive to global warming have been proposed. However, in the Southern Hemisphere sea ice retreat is fastest in winter in climate model projections. Here we address two central questions: (1) Why does Northern Hemisphere sea ice extent retreat fastest in summer in response to warming? (2) Why does Southern Hemisphere sea ice extent retreat fastest in winter in response to warming? The proposed explanation primarily involves the shape of coastlines and eddy heat fluxes in the atmosphere, with vertical heat conduction in sea ice and the surface albedo feedback also playing a role. These ingredients are incorporated into a fundamental physical theory, and we demonstrate that the theory holds in widely ranging climates simulated with a hierarchy of models.