© Madison Stevens/Polar Bears International
12/7/2017 7:02:10 PM
A Better Ice Year for Churchill's Polar Bears
But what does it mean?
By Dr. Steven C. Amstrup, Chief Scientist
At this time last year, western Hudson Bay had just frozen, making it the latest freeze-up in 38 years for Churchill’s polar bears. After weeks of anxiously watching daily ice reports, I could finally relax my crossed fingers, knowing the bears were back on the ice, hunting seals.
This year, I breathed a sigh of relief as the bears returned to the ice more than a month earlier than they did in 2016—thanks to colder temperatures and favorable wind patterns that created a frozen band along the coast. By early November, this band was wide enough and thick enough to allow the bears to start to move offshore.
This year’s freeze-up pattern allowed Churchill’s polar bears to head onto a rim of ice along the shore, even as most of Hudson Bay remained open water—illustrating that the pattern of where ice appears first (or disappears last) can alter whether bears will use it.
But to track long-term trends in freeze-up and break-up across the Arctic, we need a consistent definition of ice free.
The National Snow and Ice Data Center considers an area ice covered when the ice extent is 15%. Under normal circumstances, polar bears prefer ice extents greater than 50%, but as the ice melts in summer, they will continue to hunt on less consolidated ice. I have seen bears in Alaska hunting on very small patches of melting ice; observations of radio-collared polar bears in western Hudson Bay suggest they will cling to ice until the extent crosses below 30%. Therefore, in an attempt to view ice as polar bears do, we have used a threshold of 30% coverage when examining multiyear patterns of break-up and freeze-up.
Despite a slowing in the freeze rate after mid-November, the figure below shows that this year’s freeze-up in western Hudson Bay (when the extent of sea ice crossed the 30% line) was the third earliest in the last decade, and the ice-free period the second shortest. Clearly this was a good news year for polar bears in this part of the world.
The big picture
However, looking across the entire 38-year satellite record, the figure reminds us that although this year may have been more beneficial to the bears than last, it was not all that great when compared to the historical pattern. The gray lines in the figure, which represent ice cycles of years before 2007, illustrate that in most prior years the ice broke up later and froze earlier than in recent years.
In fact, the average ice-free period during the last decade was 27 days longer than during the first decade of the satellite record, meaning polar bears are now food-deprived for nearly a month longer than they were back then.
Looking beyond Hudson Bay, we see more troubling trends. In the Chukchi sea of northwest Alaska, this year’s sea ice freeze is a month behind the trend of even recent years. Freeze-up around Svalbard this year is the 3rd or 4th slowest on record, lacking enough ice for pregnant female bears to travel to their traditional denning areas on Hopen Island. Arctic-wide, sea ice is in a tie for the lowest early December extent ever recorded and the long-term trend in summer sea ice continues downward at about 13% per decade. These longer term and circumpolar comparisons are vital to understanding global warming and its effects on sea ice, and make it clear that this year’s freeze-up in western Hudson Bay doesn’t detract from threats to the global future of polar bears!
Global warming doesn’t mean every year will be a bit warmer than the year before. Rather, it means the long-term average is rising. Even as the average rises, some years will be a bit cooler than the one before, some a bit warmer. The natural variation we’ve always had in the climate system will continue! As with temperatures, the figure shows, each year’s sea ice cycle also can be different than the year before, even as the long-term average sea ice extent declines. To understand the problem polar bears face, we need to step back from a focus on this year’s freeze in western Hudson Bay and look at the long-term trend.
A wider perspective
Without halting the rise in CO2 and other greenhouse gases, we cannot stop sea ice from declining. This means that the frequency of “bad ice years” can only increase as long as greenhouse gas levels rise. In the short term, some years may be better for bears than others, but if humans don’t stop pumping ever more CO2 into the atmosphere, polar bears ultimately will disappear.
Last year at this time, when I wrote that the western Hudson Bay freeze was the latest recorded in the 38-year satellite record, I expressed hope that this year’s freeze would be much earlier. My hopes panned out, and natural variation in the climate system gave Churchill’s bears a reprieve—this year. This reprieve is evidence that it is still possible to assure polar bears persist over much of their range, and encourages us at PBI to work ever harder to do so. The critical question, however, is whether we can alter human behaviors in time to turn that possibility into reality!
This chart shows the annual cycle of sea ice break-up and freeze-up in western Hudson Bay during the 38-year satellite record. The ice-free season begins when fractional ice cover crosses the 30% dotted line on its downward summer path, and ends when the rising concentration path again crosses the 30% line in autumn. In 2017, these dates were June 30 and November 21. The resulting ice-free period of 144 days was 8 days shorter than the mean of the most recent decade, which is good for polar bears. But it was 19 days longer than the mean of the first decade of the satellite record, illustrating the progressive lengthening of the ice-free period as the world warms. Note that the spread in dates when both the falling and rising lines cross the 30% threshold illustrate there is much annual variability in freeze-up and break-up dates. Note also, the gray lines showing the pattern during years before 2007 reveal how much shorter the ice-free period used to be.