© Dmytro Cherkasov
12/2/2019 6:49:15 PM
Unusual Freeze-up of Hudson Bay
This year, sea ice began to form on Hudson Bay and then stalled, with much of the bay still open water. Most, but not all, of the bears tracked by researchers are out on the band of ice along the shore. We turned to three polar bear scientists for their thoughts on the unusual freeze-up—and what it means in terms of the big picture for polar bears.
Dr. Andrew Derocher, professor of biological sciences at the University of Alberta and a long-time scientific advisor to Polar Bears International. He has studied the Western Hudson Bay population since 1984.
"The Hudson Bay freeze-up in 2019 has been unusually variable. Freeze-up started slowly and then a cold snap resulted in significant ice formation over the span of a week. There were indicators of an early freeze-up and the bears were starting to move offshore. The inland bears, largely moms with cubs, started for the coast.
"While Arctic sea ice is inherently variable, 2019 in Hudson Bay is an oddity. Sea ice levels are currently well below normal and recent temperatures are well above normal. The lack of intense cold has effectively stalled freeze-up. The conditions look better for sea ice this week, but it takes sustained cold to make polar bear habitat.
"We know from long-term research and monitoring that freeze-up dates affect polar bear survival rates. Which bears are most at risk? It's a common pattern in ecology: the young, the old, and the unwell are all more likely to die. Conditions are far from desperate this year because the Western Hudson Bay polar bears were in good shape in 2019. A late break-up this summer definitely helped but, even in the spring, it was clear the bears were doing better this year.
"Coming off the ice in good condition is a big benefit for all bears. It may mean that 2020 brings a bumper crop of cubs with pregnant females now nestled into their dens, with new cubs being born anytime now. We can hope for a good year ahead: the population desperately needs a good year. While one good year is great for the bears, it really takes four years of adequate conditions to have a newborn recruited to the population. Time will tell.
"Another benefit of being a bit chubbier this year is that the bears are buffered against a later freeze-up. It's been an on and off freeze-up so far. The bears have had enough of their holiday ashore and want to be back at work. Making a living eating seals is never easy, but it's a lot more difficult in a warming Arctic."
Dr. Steven Amstrup, chief scientist for Polar Bears International and Polar Bear Project Leader for the U.S. Geological Survey for 30 years.
"Summer is an increasingly difficult time for polar bears. As humans continue to warm the world the sea ice on which polar bears depend to catch their seal prey is becoming less and less available. Whereas changes in winter ice are occurring more slowly, the summer sea ice extent is rapidly diminishing, with breakup occurring earlier and freeze-up later.
"In Western Hudson Bay, the average ice-free period—during which polar bears are food deprived and losing nearly two pounds a day—is 28 days longer now than it was in the 1980s. Because the bears are, on average, 50 pounds lighter now as they await freeze-up, annual ice formation has become a focal point for Churchill bear watchers. As we wait however, it’s important to remember that even as the average date of freeze-up is delayed by warmer global temperatures, normal weather fluctuations among years continue as always.
"Contrasting this year’s freeze-up in Hudson Bay with that in 2018 provides a good example of this interannual variation. As the maps above show, by this time in 2018 much of Hudson Bay was covered by sea ice, whereas this year’s late November ice is confined to a narrow strip along the western coastline of the bay (map on left). Last year’s earlier freeze meant Hudson Bay bears could get offshore and resume feeding sooner than they are this year.
"This recent flip-flop in Hudson Bay freeze-up times was reversed around Svalbard, north of Norway. Last year, when Hudson Bay ice formed early, Svalbard freeze-up was the latest ever recorded. The autumn ice of 2018 didn’t reach Svalbard until 16 January of 2019.
"This fall, however, while freeze-up has been delayed in Hudson Bay, Svalbard saw an early freeze (by recent standards). The Hudson Bay and Svalbard freeze-up patterns of the last two years provide great examples of the difference between short-term natural variation and the human-forced trend of warming temperatures and declining sea ice. And, as we wait for this year’s freeze in Hudson Bay, we need to remember that the real crisis for polar bears is not the short-term variation among years, but the fact that average summer ice extent is declining by about 13% per decade, and if we continue to warm the world, all years soon will be poor ice years and polar bears ultimately will disappear."
Geoff York, senior director of conservation for Polar Bears International. He has studied polar bears across the circumpolar Arctic for more than 25 years.
"Arctic sea ice has become less predictable with an increased probability of bad ice years. That statement has become a reality across the Arctic, a reality that does not bode well for polar bears, Arctic communities, and ice-associated species.
"We are seeing it this year in the stalled ice formation for Hudson Bay, now well below the historic mean. We are also seeing it in the Chukchi and Bering seas between Alaska and Russia, with an historic low summer sea ice extent followed by a very slow and late freeze-up in these regions.
"The later sea ice forms, the less time it has to grow thick, and the easier it will be to break apart and melt the following year, setting in motion a negative feedback loop with long-term repercussions for the entire Arctic food web."