Consistent changes in the sea ice seasonal cycle in response to global warming

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 retreats fastest in winter in climate model projections. Here we show that the inter-hemispheric differences can be attributed to differences in coastline geometry. After accounting for this geometry, we find that the sea ice changes simulated in both hemispheres in most climate models are consistent with sea ice retreat being fastest in winter in the absence of landmasses. These results demonstrate that despite the widely differing rates of ice retreat among climate model projections, the seasonal structure of the sea ice retreat is robust among the models and uniform in both hemispheres.