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The Incredible Shrinking Polar Bears

In Canada's Hudson Bay, a long-term study confirms they are losing weight and bearing fewer cubs as global warming melts away their icy habitat. Is this a preview of what other populations of polar bears will soon be facing?

  • Jim Morrison
  • Feb 01, 2004

BY MARCH when a mother polar bear departs her earthen den with nursing cubs into subzero temperatures along Canada's Hudson Bay, she has fasted for eight months and lost more than half her weight, as much as 400 pounds. After a week or two of acclimation around the den, she heads to the frozen water.

Fortunately, her emergence onto the sea ice along the continental shelf coincides with the birth of ringed seals, her primary prey.

From April until summer when the ice breaks up in the bay and the seals disappear into open water, she is in a desperate race against time to pack on enough pounds of fat to get her through the long summer and fall fast. If she becomes too lean, she'll stop producing milk and her cubs will die.

Hunting is hard work. With her massive limbs and wide paws, evolved for prowling the ice and swimming, she expends twice the energy to walk than most other mammals. So she stalks her prey slowly, relying on her extraordinary sense of smell--she can sniff seals in their subnivean lairs up to a mile away. Often, she will remain motionless on her stomach for long stretches beside a breathing hole, waiting for a seal to surface.

Polar bears, the largest terrestrial carnivores in the world, abandoned the land for this harsh life between 200,000 and 300,000 years ago. Taxonomists have since named them Ursus maritimus, the "bear of the sea." The creatures have been known to swim as far as 60 miles in a day. But they're really the bear of the ice. The shifting floes of the Arctic region are home to these massive animals. In areas of Hudson Bay, however, the ice is now abandoning them, threatening the long-term survival of the polar bears that have survived for eons in the region. Their plight is a preview of the challenges other polar bear populations that live farther north will face in the coming decades.

Global warming, experts believe, is causing the ice pack to melt an average of two weeks earlier each July than 20 years ago. In one study, Josefino Comiso, a researcher at NASA's Goddard Space Flight Center in Maryland, used satellite data to track trends in minimum sea ice cover and temperature over the Arctic from 1978 to 2000. Last year, he reported that the perennial sea ice is melting faster than previously thought due to rising temperatures and interactions between ice, ocean and the atmosphere that accelerate the melting process.


CUBS STAY CLOSE to their mothers until they are about two and a half years old. Mothers weigh between 300 and 350 pounds when they leave their dens in spring and need to put on another 100 to 200 pounds before the ice thaws. "For a polar bear," says a researcher, "fat is where it's at."

The timing of the early ice melt couldn't be worse for polar bears, especially pregnant and nursing females and their cubs. In Hudson Bay, fat-rich ringed seal pups wean in May and then emerge from their lairs. A month later, adult seals haul out on the ice to molt, offering bears a second course. But when the ice disappears early, so does the bears' larded buffet.

"Nobody really knows what percentage of the fat a bear stores and uses through the year is captured in the spring," says Ian Stirling, a Canadian Wildlife Service scientist who has studied polar bears for more than 30 years. "It's certainly well over 50 percent and may be as high as 70 or 75 percent. All we know is it's terribly important."

Stirling and his colleagues also know that polar bears along the Hudson Bay are lighter and in poorer condition than they were 20 years ago. They know because their study of the bears--looking at a single population for more than two decades--is unique in its longevity. For each week the ice breaks up earlier, the researchers have found, the bears come ashore 22 pounds lighter. And if climate models created in recent years by several scientists in the United States and Great Britain prove to be correct, the area will be three to five degrees warmer within 50 years. With every degree increase, the ice breakup will occur one week earlier.

The 1,200 bears of the western Hudson Bay live at one of the southernmost extremes of the species' range. As the planet continues to warm, what happens there will no doubt happen farther north to the rest of the estimated 25,000 bears that live in 20 distinct circumpolar populations. Already, ice conditions in the Beaufort Sea off the north coast of Alaska, home to another major polar bear population, are changing dramatically.

The longer summers in the Hudson Bay area are particularly challenging for pregnant and nursing females and their cubs. Mothers weigh between 300 and 350 pounds after leaving the den and need to gain between 100 and 200 pounds before the thaw arrives. Healthy pregnant females often gain more than 400 pounds of fat alone. So a mother may spend half her time hunting, catching a seal every few days. She can gorge on more than 60 pounds of seal meat at a single sitting. (A typical 1,200-pound male bear can down 150 pounds at a time.)

In most areas, polar bear cubs are weaned at about two and a half years of age. In some years, fewer than half of those cubs may live to become adults. The main cause of death is a lack of food. "Lighter cubs have lower survival rates and light cubs come from leaner pregnant females," says Andrew Derocher, a University of Alberta scientist who has studied bears for 20 years. "For a polar bear, fat is where it's at."

"If all the ice goes in Hudson Bay, as the forecasts predict, then there won't be any polar bears there," Stirling says. "And there's no place they can go. People often ask, 'Can't they just go farther north?' But the answer is no. That habitat is already occupied by other polar bears." While some bears do cross the North Pole, the conditions for survival there are so extreme that the region is inhospitable even for them. "For the long term, I think the situation looks very bad for polar bears," he adds.

The connection between global warming and the bears' deteriorating condition could be proved only because of Stirling's desire to build a unique database about the lives of polar bears--and because of geographic luck. The Hudson Bay bears are the most studied population in the world, thanks to their accessibility and Stirling's remarkable ability to acquire funding year after year. "The Hudson Bay database exists because of the determination of a single individual," Derocher says. "Ian's study has provided us with insights not possible from other polar bear populations."

Other populations move across large expanses of moving ice. But along the bay, the entire population comes ashore in one concentrated area for a few months annually. That makes conducting research there efficient--a necessity for cash-strapped researchers. Over more than two decades, Stirling and his associates have captured 80 percent of the adult bears in the area. Each has been tagged, measured, weighed, checked for fat.

The luck came in the location of the research. As it happens, the western area of Hudson Bay has been a hot spot--more affected by global warming than nearby regions so the changes there have been more dramatic. While statistics show that the world generally is getting warmer, temperature changes vary by location. Southeastern Hudson Bay, for example, actually became cooler over the last few decades, though recently the area has been warming as well.

In the early 1980s, Stirling began noticing that bears seemed lighter. Cubs were taking longer to wean so females were reproducing less frequently. There was no Eureka moment, he says, just a gradual realization that a trend seemed to be developing.

He got another clue after Mount Pinatubo in the Philippines blew its top in 1991 and the particulate matter it sent into the atmosphere cooled the planet. The following summer the ice on Hudson Bay melted almost a month later, extending the bears' hunting season in the process. The effect was dramatic. The bears were heavier, had more cubs and more of the youngsters survived those brutal first years. But that was still only a piece of a tricky, subtle puzzle.

"It took 20 years to have enough data to determine that this was happening," Stirling says, "because there always is quite a bit of annual fluctuation. You can't detect an underlying trend such as this unless you've been able to look at it for a long time."


TO COOL OFF, a bear will lie flat on the ice or roll around on the cold ground.

Because polar bears are at the top of the food chain, their plight provides a strong, early warning about the stress climate change is putting on the entire Arctic region. "Usually when you take large predators out of an ecosystem you get major changes. But exactly what those would be I don't think anybody would say," says Stirling. "Clearly, there would be huge changes in the Arctic ecosystem without polar bears."

Stirling is now nearing retirement age and says it's time for some of the younger researchers to step up to the plate. In Derocher and Nick Lunn, his colleague at the Canadian Wildlife Service, he has two eager disciples determined to carry on his work. "There's no other data like this on any Arctic species. If you let it go, even for a year, it's gone. You can't fill that gap," says Lunn, who wants to spend more time studying the reproduction of individual animals over their lifespans. Initially, he notes, it may be that not every bear will be affected in the same way by the warming trend.

Derocher, meanwhile, plans to further study the effects of climate change on bear populations. "I believe," he says, "that a better understanding of interactions between sea ice, polar bears and their prey are the key issues for understanding how the bears will respond to climate change over the coming years." He also hopes to continue investigating the effects of toxic chemicals on the bears and other Arctic wildlife.

These days, Derocher spends a lot of time thinking about what it's like to be a polar bear, walking on the sea ice, using smell to find dinner during the howling Arctic night when the temperature is minus 40 degrees F. "You can't help but respect an animal that makes a living in this kind of environment," he says. "It's a scenario that humans don't relate to very well."

Virginia journalist Jim Morrison writes regularly about natural history for Smithsonian and several other magazines.

World Class Navigators

When it comes to establishing home ranges, polar bears are the undisputed champions of the bear world. While a grizzly's territory in the Rocky Mountains might encompass as much as 1,000 square miles, in some cases a polar bear's range in the Arctic might extend across 100,000 or more square miles, depending upon the availability of ice and food. "They are good navigators," says Canadian Wildlife Service scientist Ian Stirling, "although we do not yet know how they do it." He and other bear biologists do know that the animals have remarkable capabilities. Using satellite technology, researchers tracked one female polar bear for four months as she traveled from the Beaufort Sea coast in Alaska to northern Greenland, passing within 150 miles of the North Pole along the way, a distance of about 3,000 miles.

Promoting Climate Change Solutions

NWF's Climate Change and Wildlife Program documents the threats global warming poses to animal and plant species, including polar bears. It also promotes ways that government and citizens can contribute to solutions. For more information about the NWF program, including a list of steps each of us can take to help reduce greenhouse gas pollution, see

A Good News, Bad News Story

Canadian scientist Andrew Derocher returned recently from six years of studying polar bears on Norway's Svalbard archipelago. There, he found another challenge facing the animals: pollution. Toxic chemicals, including flame retardants, PCBs and pesticides such as DDT, repeatedly evaporate, rise and then fall to the ground, leapfrogging across the globe. Eventually, they ride northbound winds, migrating to the Arctic Circle. Though some of these toxics were banned decades ago, they remain present in the Arctic environment, building up in ice and ocean sediment.

Over time, the chemicals accumulate in the fat of animals, especially predators at the top of the food chain. For instance, high levels of PCBs have been found in killer whales, seals, bottlenose dolphins and belugas--and in Svalbard's polar bears. Derocher found that the levels of contaminants in bears are much higher in Svalbard as a result of pollution from nearby Russian sources than they are in bears in western Hudson Bay.

The scientist determined that Svalbard bears with elevated levels of contaminants suffer from compromised immune systems. And while he believes pollution levels in the animals are declining, he remains concerned.

"It's a good news, bad news story," he says. "The good news is we seem to be better at reducing levels of these chemicals. The bad news is we're not 100 percent efficient at doing that. So right now, it's really hard to quantify what the long-term effects of such chemicals are on wildlife." - Jim Morrison

Fact, Not Fiction

For years, politicians have been calling for stronger scientific evidence showing the phenomenon that's melting the polar bear's icy realm is linked to human activities. Now two of the nation's leading atmospheric scientists have declared there is no longer any doubt: People are having measurable impacts on global climate, and the affects are likely to get worse.

Writing in the journal Science, Thomas Karl, director of NOAA's National Climatic Data Center, and Kevin Trenberth, head of the Climate Analysis Section of the National Center for Atmospheric Research (NCAR), paint a detailed picture of the climatic changes expected to wallop the Earth in coming years. From atmospheric observations and multiple computer models, Karl and Trenberth conclude that there is a 90 percent probability global temperatures will rise by 3.1 to 8.9 degrees F during the next century. As a result, they predict extreme weather events, such as flooding and drought, will become more prevalent.

The researchers site greenhouse gases as the largest human influence on global climate. By their estimates, carbon dioxide levels in the atmosphere have risen by 31 percent since preindustrial times. Even if carbon dioxide emissions were slashed, they say, temperatures would increase by approximately 0.9 degrees F in the coming decades. "Given what has happened to date and is projected in the future, significant and further climate change is guaranteed," they write.

They also warn that if current greenhouse gas emissions continue, the world will face the fastest rate of climate change in 10,000 years. The researchers are calling for more research to determine the local and worldwide impacts of such dramatic temperature fluctuations and for international cooperation in studying and monitoring global warming in the coming years. "Climate change is truly a global issue, one that may prove to be humanity's greatest challenge," they write. "It is very unlikely to be adequately addressed without greatly improved international cooperation and action."

In the United States last fall, a bill that would have set mandatory limits to hold industry accountable for reducing greenhouse gas emissions died on the Senate floor. However, many conservationists say the close 55–43 vote on that climate legislation--the first of its kind--signals the political climate on this issue is beginning to shift in the nation's capital. - Rene Ebersole

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