A step-change in the date of sea-ice breakup in western Hudson Bay

Over the last four decades there has been a trend to earlier summer breakup of the sea ice in western Hudson Bay, Canada. This sea ice is critical for the polar bears that use it for hunting; with the earlier breakup believed to be a factor in the declining health of the regional polar bear population. Analysis of the change to earlier breakup using passive microwave satellite data is problematic due to currently unquantifiable systematic errors between different satellites. Analysis using Canadian sea ice charts from 1971 to 2008 shows that the change to earlier breakup is best represented by a 12 day step. This step occurs between 1988 and 1989 with no significant trend before or after the step. Although not as great as the three-week gradual change suggested by previous studies this change is still significant. An increase in regional south-westerly winds during the first three weeks of June and a corresponding increase in surface temperature are shown to be likely contributing factors to this earlier breakup. It remains to be seen whether these changes in atmospheric circulation might be ascribed to human actions, or simply natural climate variability.

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