As apex predators in the Arctic, he explains, polar bears sit at the top of the marine food chain. Monitoring their range, movement, and physiology makes them useful barometers for gauging larger environmental changes – in this case, global warming. And because they rely so heavily on the integrity of the food chain that supports them, they represent an early-warning network for changes that may be taking place with links father down the chain.
A US Geological Survey analysis of polar-bear population studies, published in 2010, put the number of polar bears Arctic-wide at some 24,600 animals. Some 5,000 roam the islands making up the Canadian Archipelago. About 11,900 animals are spread along the continental coasts open to the Arctic Ocean. Some 7,700 live around bays and basins where all the sea-ice tends to be seasonal, instead of a mixture of seasonal and multi-year ice. Hudson Bay falls into this category.
The bay boasts high biological productivity, so polar bears can gorge themselves in anticipation of the melt season. But the bay's melt season also has experienced some of the fastest growth.
Between 1979 and 2009, the melt season Arctic-wide expanded by at least 20 days, according to a study published four years ago by scientists from NASA and the National Snow and Ice Data Center in Boulder, Colo. That's a pace of 6.6 days per decade.
Hudson Bay, by contrast, has seen the melt season grow by more than 10 days per decade – among the highest rates for any region in the Arctic. It's also one of the southernmost habitats for polar bears, so in effect they live on a climatological knife's edge compared with their northern kin. Any effects of warming are likely to show up there earlier than at higher latitudes, Cherry says.