Melting Ice Means No Dinner for Polar Bears—So They’re Eating Each Other

For polar bears, melting ice means limited seal-hunting.
A polar bear and her cub smile for the camera at a German zoo in 2008. (Photo: Getty Images)
Mar 25, 2013· 6 MIN READ
David Kirby has been a professional journalist for 25 years. His third book, Death at Seaworld, was published in 2012.

Is climate change killing polar bears? Most scientists answer in the affirmative, but some people still seem to need convincing.

Changes to the timing of migration have resulted in polar bears spending progressively longer periods of time on land without access to sea ice and their marine mammal prey.

It may seem like a given that rising global temperatures reduce the integrity of Arctic sea ice, which in turn shortens the seal-hunting season for polar bears, who spend the long winter out on frozen waters waiting for prey to pop up through breathing holes in the ice.

But that dynamic, to the disbelief of most scientists, is still contested.

Some researchers, native Inuit, and the Canadian government all refute what seems so obvious to many experts on the Arctic and its giant apex predator: Melting ice means less time to hunt seals.

There is mounting evidence to support this contention.

The most recent data came last week in the Journal of Animal Ecology, where a new paper concluded that, in Hudson Bay, a shortened ice-pack season, caused by climate change, is limiting seal-hunting opportunities for bears in the region.

Every fall, polar bears gather on the edge of the bay, waiting for the ice pack to completely form. Over the winter, they hunt seals and store up fat reserves. In late spring, when the ice begins breaking up, they return to the onshore tundra, where there are no seals, to spend the summertime “melt season.”

This new study, however, found a “trend towards earlier arrival of polar bears on shore and later departure from land, which has been driven by climate-induced declines in the availability of sea ice,” the authors wrote.

“Changes to the timing of migration have resulted in polar bears spending progressively longer periods of time on land without access to sea ice and their marine mammal prey,” said the team of Canadian researchers led by University of Alberta biologist Seth Cherry. Investigators analyzed “sea ice dynamics” and compared them with polar bear migrations, tracked via data collected from satellite-linked collars, between 1991–1997 and 2004–2009.

“The links between increased atmospheric temperatures, sea ice dynamics, and the migratory behavior of an ice-dependent species emphasizes the importance of quantifying and monitoring relationships between migratory wildlife and environmental cues that may be altered by climate change,” they concluded.

From 1979 to 2009, the average melt season grew by at least 20 days, according to NASA and the National Snow and Ice Data Center—or 6.6 days per decade.

In Hudson Bay, at the southernmost edge of polar bear habitat, the melt season grew by more than 10 days each decade. The increased fasting caused by melting ice is most likely having a negative impact on reproduction and survival rates of at least some animals, with younger bears thought to be most at risk because they lack the body fat of adults.

Such data do nothing to deter the skeptics, chief among them the government of Canada, which claims that the polar bear population is not only stable, it has actually increased in recent years.

But according to the Polar Bear Specialist Group of the International Union for Conservation of Nature (IUCN), a closer examination reveals that half of the country’s 13 population groups are declining, and only three are stable. Two others lack sufficient data, leaving just one group, about two percent of the total population, with an increase, mostly due to reduced hunting quotas.

Canada also says its trade in polar bear parts remains at sustainable levels. But the government acknowledges that 3.75 percent of its bears are killed every year, even though “the maximum rate of population growth for polar bears is between 4% and 6% per year,” according to Humane Society International (HSI). “Even in healthy growing populations, an annual hunt quota of 3.75% would slow, and possibly even stop, that growth.”

Other skeptics have taken up the “polar bears are doing just fine” banner.

Rob Lyons, deputy editor of the U.K. magazine spiked, excoriated scientists claiming that polar bears are not only threatened, but their situation grows direr with shifting patterns of weather and ice.

“How did the polar bear, a vicious killing machine that is thriving, become the poster boy of climate-change alarmism?” Lyons’ headline asked. It was only the opening salvo.

“Before we start getting into a lather about the future of nature’s greatest land-based killing machines (sorry, I mean big furry canaries in the climate-change coalmine), it is worth noting that there are more optimistic voices around,” he wrote.

Lyons mentioned Susan Crockford of the University of Victoria, who wrote a paper, Ten Good Reasons Not To Worry About Polar Bears, for the Global Warming Policy Foundation. That foundation, “while open-minded on the contested science of global warming,” according to its website, “is deeply concerned about the costs and other implications of many of the policies currently being advocated.”

Of course, the majority of experts do not consider the science “contested,” nor do they think the costs of countermeasures outweigh the benefits to society.

Crockford called polar bears a “conservation success story,” because hunting restrictions that began in 1973 helped increase the total population “from around 10,000 to between 20,000 and 25,000” today.

“The only population shown to have declined in recent years—a fall in numbers described by Crockford as ‘modest’—is the one in the western Hudson Bay area,” Lyons wrote, contradicting data provided by the Polar Bear Specialist Group. “Polar bears have actually shown a remarkable ability to survive and thrive after months without food.”

Lyons was skeptical whether the decline in Arctic ice was due to “manmade global warming,” and whether that decline was hurting the bears. Any claim that polar bears are threatened is merely “a cynical attempt at emotional blackmail, designed to short-circuit debate about climate change while adding cash to the overflowing coffers of multinational green mega-NGOs,” Lyons asserted. “Given the apparent health of most polar-bear populations, it’s time the whole fairytale about polar-bear extinction was put on ice.”

What Lyons calls a fantasy, most scientists, including at the U.S. Fish and Wildlife Service, consider a harsh, prophetic reality: In May of 2008, the Service listed polar bears as a threatened species.

Dr. Ian Stirling, a leading polar bear researcher, adjunct professor at the University of Alberta, and author of Polar Bears: A Natural History of a Threatened Species, told TakePart that sea ice is “very definitely” forming later and melting earlier. He added that climate change is behind such shifts and it is directly linked to bear survival.

As for Lyons, Crockford and the Canadian government, Stirling said their position “does not agree with what most scientists think,” pointing to a 2009 status report from the IUCN’s Polar Bear Specialist Group.

“It is quite clear climate change is affecting bears,” Stirling continued. “The basics are not complicated. Polar bears need ice for a platform from which to hunt seals and other marine mammals. The climate is warming and that is causing the sea ice to break up earlier and re-freeze later.”

In some populations, this change “is causing bears to lose valuable feeding time, lose body condition, have reduced reproduction and lower survival of cubs. There are several years of peer-reviewed scientific studies, based on decades of research, that document all this very clearly.”

Since 2010 alone, several papers have reached similar conclusions. Here are just a few:

- Molnár et al. (2010) created a model showing that male survival rates fell by a factor of approximately eight if fasting periods increased by two months, and that rapidly declining sea ice would also reduce female mating success.

- Rode et al. (2010) found that bears in the Beaufort Sea area had smaller skull and body size, and reduced litter mass and number of yearlings, as sea ice receded, supporting the claim that bears eat less during low-ice years.

- Durner et al. (2011) determined that polar bears can swim extraordinarily long distances, which may help them cope with reduced sea ice. But one female, followed during a low-ice period, was forced to swim hundreds of miles, losing 22 percent of her body weight and her yearling cub.

- Molnár et al. (2011) created another model showing that a spring break-up occurring just one month earlier could mean a significant drop in the number and size of polar bear litters. A break-up two months earlier could lead to catastrophic (100 percent) reproductive failure.

- Pagano et al. (2012) observed increased long-distance swimming in female bears over a six-year study period. Long-distance swimming probably demands more energy than walking on ice, and may be life threatening, they said.

The shortening of seal-hunting season has created other, unexpected problems.

Stirling and Ross described adult males killing and cannibalizing younger bears in Norway. Feeding was likely the primary motive, rather than sexually selected infanticide, the authors said, adding that as the bears’ preferred prey (ringed seals) become less accessible due to melting ice, cannibalism could increase. Smith et al., meanwhile, described the catastrophic impact on some birds after hungry polar bears ravaged their eggs and chicks when the fall ice formation was delayed.

Stirling, like the vast majority of scientists, is deeply dismayed by the contentions of Lyons and others who continue to insist that the bears are alright.

Dr. Naomi Rose of HSI, which has been deeply involved in the issue, called Lyons “a typical global warming skeptic” who has cast his lot with “trophy hunters and Canadian Inuit who profit from them. This attitude has been incredibly frustrating, not only to NGOs but to the scientists who work with the Inuit to gather data on the bears.”

Stirling, as one of those scientists, insists that time is running out.

“They cannot adapt sufficiently quickly to respond to the extremely rapid pace of climate warming,” he said of the bears. “It took them tens of thousands of years to get to where they are now; that can’t be reversed in a hundred or two. With no cessation of climate warming, in 1-200 years there will be very few polar bears remaining.”