Disappearing by Degrees
by Mary-Russell Roberson

Along the western coast of Hudson Bay in Canada, female polar bears (Ursus maritimus) and their new cubs come out of their dens in late February or early March. The mama bear hasn't eaten in about eight months. If she and her nursing cubs are to survive, she must get food and get it fast. Food means seals, and the place to get seals is out on the sea ice. The female keeps her cubs near the den for a few days so they can practice walking, then she leads them toward Hudson Bay.

Polar bear
As global warming heats up the Arctic, polar bears in western Hudson Bay, Canada, are losing their icy hunting grounds. (Dave Olsen/USFWS)

"She's gonna get out there and try to kill as many seals as she can," says Andrew Derocher, a polar bear researcher and professor of biological sciences at the University of Alberta. "If she doesn't get enough to eat, soon she'll stop nursing and the cubs will expire very quickly."

Although polar bears can swim, they rarely hunt in open water. Instead, they wait patiently on the edge of the ice for a seal to come up for air. For the first few weeks after females and cubs arrive on the sea ice, hunting can be difficult. Adult seals are experienced in the ways of evading hungry polar bears. But in April, ringed seal pups (Phoca hispida) and bearded seal pups (Erignathus barbatus) are weaned and take to the sea. These six-week-old pups weigh more than 200 pounds (almost as big as adults) and are about 50 percent fat. "They are like huge fat packets," says Nick Lunn, a research scientist with the Canadian Wildlife Service. "Polar bears have this opportunity with all these naive young seal pups out there. It's a feast—like kids in the candy store."

For the next two to three months, polar bears eat seals and build up their fat reserves. No one has yet quantified exactly how important this spring feast is to the western Hudson Bay bears, but Derocher estimates that the bears take in 70 to 80 percent of their yearly caloric allotment from April to June. When the ice on the bay breaks up and melts, the bears come ashore. Females and their cubs go farther inland and stay out of the way of potentially aggressive males, which keep closer to the coast. During the summer, bears may eat a few berries here or there but for the most part, they fast, living off the fat reserves they built up during the spring seal feast. In the fall, all the bears except the pregnant females, which are holed up in dens, head back out on the ice.

The spring feast is especially important for females that are nursing last year's cubs, and for pregnant females. Nursing females must consume enough to sustain not only themselves, but their cubs as well. And pregnant females, after mating in the spring, will gestate, bear, and nurse their cubs over a period of more than eight months in a den on land, and go without eating while other polar bears are out hunting seals.

The ice in Hudson Bay typically begins to break up sometime in May or June; the bay is usually ice-free by late July or early August. The timing varies from year to year depending on the weather. Derocher and Lunn, working with Ian Stirling of the Canadian Wildlife Service, have found that when the ice breaks up early, bears come ashore in poorer condition, with less fat than usual. Females give birth to fewer cubs and the cubs are smaller and less likely to survive. When the ice breaks up later, bears come ashore in better condition and females produce more and larger cubs. A female in really good condition might weigh 900 pounds, of which 450 pounds might be fat.

An example of a good year was 1992. That year was particularly cold, because the June 1991 eruption of Mt. Pinatubo in the Philippines spread tons of sulfuric acid particles, which reflected heat from the sun, around the Earth's stratosphere. An El Niño event also helped keep temperatures down. The ice in Hudson Bay broke up three weeks later than average, and bears were in very good condition when they began their summer fast. As a result, more cubs than usual were born and survived in 1992 and 1993. Researchers still call those bears Mt. Pinatubo bears.

Polar bear sow and cubs
When ice is scarce, female polar bears are unable to consume enough fat to nurse their cubs. (Steve Amstrup/USFWS)

A population of polar bears can survive a bad year or two. But lately, there have been more bad years than good for some bears. Researchers have discovered a trend in western Hudson Bay of earlier and earlier breakup. On average, the ice breaks up 2.5 weeks sooner now than 30 years ago. Earlier breakups cut short the spring feast that is so important to the yearly cycle of the Hudson Bay bears.

The bears are indeed losing weight. Canadian researchers have been studying the western Hudson Bay polar bear population for more than 30 years and have seen the condition of the bears decline. "Condition" is quantified with a formula that includes the bears' length and weight—it's very similar to the body mass index (BMI) used for humans. The weight used in the formula is adjusted to account for the fact that the bears may be fasting and losing weight when they are caught and measured. If a bear is caught before September 1, the scientists subtract 0.85 kg (or 1.87 pounds, which is the amount typical polar bears lose per day during fasting) from its weight for each day before September 1. Conversely, for bears caught after September 1, the researchers add 0.85 kg per day. The body condition index is averaged for bears of different age classes and sexes, and used to make comparisons from year to year. From the early 1980s to the early 2000s, Lunn says, the condition of female polar bears in western Hudson Bay has declined by 15 to 20 percent.

"The bears come ashore in poor condition because they haven't had as much opportunity to feed, and we're asking them to turn around and go into their fasting state earlier," Derocher says. "It's a double-edged sword; they're being cut on both sides."

When the condition of females declines, cubs suffer. "As the female starts to lose her nutritional status, she stops nursing," Derocher says. "The cubs have very little in the way of body fat—they quickly burn off their stores and die quite easily."

Lunn and others are in the midst of a polar bear census in western Hudson Bay. While the work is not yet complete, Lunn says the numbers so far suggest that the population size has fallen since censuses of the mid-1980s and mid-1990s. He says, "We've seen differences. We don't seem to see the same number of adult females and cubs as we used to in denning areas. The adult females and cubs we find tend to be closer to the coast than they used to be, perhaps because there are not as many males near the coast as there used to be. We're not seeing adult males in the same numbers as we used to. There are more problem bears in and around Churchill. Things are changing. It's clear something is going on in the Hudson Bay ecosystem."

What's going on is likely to continue if, as researchers believe, climate change is the driving force behind the changes.

Average air temperatures on Earth are increasing. Since the start of the Industrial Revolution in the mid-1800s, global temperatures have risen an average of 0.6°C (1.1°F), according to the report of the Arctic Climate Impact Assessment published in 2004. This report was the result of the work of hundreds of scientists from all over the world.

The overwhelming consensus among scientists is that these temperature increases are due primarily to man-made emissions of certain "greenhouse" gases, such as carbon dioxide. Greenhouse gases trap the heat of the sun in our atmosphere. Burning fossil fuels is the main way humans add greenhouse gases to the environment. Before the Industrial Revolution, the concentration of carbon dioxide in our atmosphere was about 280 parts per million. Today it's between 360 and 380 parts per million. That's a 35 percent increase in about 150 years.

Temperatures are not increasing evenly all over the globe, because factors that affect climate vary from place to place. Topography, wind patterns, ocean currents, and reflectivity of the surface, to name just a few, all affect climate. In the past few decades, temperatures in the Arctic have increased almost twice as fast as the rest of the world. One reason for this is that as ice and snow melt, they reveal darker soil or water underneath, which tends to absorb the sun's heat rather than reflect it as ice and snow do.

Ice on Beaufort Sea
Sea ice on the Beaufort Sea. (USFWS)

John Walsh, a professor of atmospheric science spending a sabbatical at the University of Alaska in Fairbanks, did a study of average annual temperatures north of 50°N—the latitude of Prague and Winnipeg. He found that between 1943 and 2002, temperatures of coastal regions increased by an average of 0.4°C (0.7°F) and the temperatures of inland regions increased 0.8°C (1.4°F). Within those averages, there is quite a bit of variation. For example, the northern coast of Alaska has warmed 2°C (3.6°F) since 1973.

Western Hudson Bay is one of the areas where temperatures have risen faster than average. May and June air temperatures in western Hudson Bay have increased at a rate of 0.2 to 0.3°C (0.4 to 0.5°F) per decade since 1950.

Warming temperatures in the Arctic mean less ice. Josefino Comiso, a senior research scientist at NASA's Goddard Space Flight Center, has used satellite data to study annual and perennial sea ice in the Arctic from 1978 to the present. Annual sea ice forms anew each winter, while perennial sea ice, which is on average more than ten feet thick, remains frozen all year long. Comiso has found that the amount of annual ice in the Arctic is decreasing by 2 to 3 percent each decade. The loss of perennial ice is much more dramatic. Comiso says, "The thing I think is remarkable from our studies is the fact that the perennial ice is decreasing at the rate of more than 9 percent per decade."

"If you look at this trend and try to project it into the future, the perennial ice would disappear within this century," Comiso says. "Some models project a disappearance by 2050. Some are more conservative—they go as far as the end of the century. There is quite a bit of disagreement in the models. However, they all predict declines, using the scenario of ever-increasing greenhouse gases."

And what would happen to polar bears if there were no sea ice? Polar bears evolved from brown bears into a specialized pagophilic (ice-loving) species. Their adaptations suit their ice-based lifestyle—white fur for camouflage, black skin to soak up heat from the sun, huge paws to act as snowshoes, and a lot of fat for insulation. These adaptations evolved over thousands of years. "I don't think polar bears will evolve back into terrestrial bears fast enough if the various predictions of these models come to pass and we do lose sea ice or lose it in places forever," says Lunn. "The future doesn't look good for a species that depends upon it such as polar bears."

There are an estimated 21,500 to 25,000 polar bears in the world today, living in about 20 relatively discrete populations in the Arctic. There is considerable variability in the natural history and ecology of each population. For example, bears that live in areas of perennial ice can hunt the entire year instead of fasting through the summer like the western Hudson Bay bears. Most of the 20 populations have not been closely studied; only four are considered to have good population estimates. That means there are not a lot of data about how climate change may be affecting the majority of polar bears.

The Hudson Bay bears have been studied for longer and in more depth than any other population. "The Canadian Wildlife Service has been doing research on that population since the late 1960s," Lunn says. "The value of the work is that it is such a long-term database that we are able to look at the past and the present and make comparisons between what things were like 20 or 25 years ago and what they are like now." Because there is considerable variability in the timing of breakup from year to year, meaningful patterns emerge only when decades of data are analyzed.

Map of polar bear territory
This map shows the territories of the Beaufort Sea and Chukchi Sea polar bears. (USFWS)

Another polar bear population that has been fairly closely studied is the Beaufort Sea population, which ranges on the north shore of Alaska near the city of Barrow east into Canada. This population is showing changes in distribution that appear to be linked to environmental conditions. Scott Schliebe, polar bear project leader for the U.S. Fish & Wildlife Service in Anchorage, has been studying polar bears for more than 20 years. He says of the Beaufort Sea bears, "We're finding more bears on the shore during the fall open-water and freeze-up period, and they seem to be staying there longer. In the 1980s and early 1990s, it was rare to find bears on the coast and we didn't find them in aggregations of substantial numbers like today." Schliebe and his colleagues considered a number of possibilities for the change: This population of polar bears could be growing, the distribution of seals in the area could be changing, the bears could be coming ashore to scavenge bowhead whale carcasses left behind by indigenous hunters, or retreating sea ice could be forcing the bears to move ashore. In an effort to tease out the most likely cause, scientists conducted five years of bear surveys combined with five years of ice surveys. Each year they calculated the average distance of the ice from the shore. "We found there was a relationship: The farther the ice is away from the shore, the greater the number of bears on shore, and that relationship was statistically significant. During all years, bowhead whale carcasses were a constant so we believe that ice conditions have a pretty good probability of being a contributor to this distribution change. We're quite concerned about it."

The Alaska Beaufort Sea population is adjacent to, and sometimes overlaps, the Chukchi Sea population, which ranges from Barrow west to Russia. There is anecdotal evidence that bears from the Chukchi Sea population have also been appearing on land more frequently than in the past. "Most research emphasis has been in the Beaufort Sea because the bears are accessible," Schliebe says. "Bears in the Chukchi Sea are less accessible, they occupy a larger area, and a good bit of the area is in Russia. The United States signed a treaty with Russia in October 2000 for the joint conservation, management, and study of this population, and we are waiting for the U.S. Congress to pass legislation to enable us to implement the treaty—including a program of joint research. We're really hopeful that once Congress passes the implementing legislation we can start active management programs and work with our Russian colleagues and overcome data deficiency issues."

While most of the world's polar bears remain little-studied by scientists, those that are being studied show signs of being affected by climate change. It's hard to avoid the conclusion that as sea ice disappears, polar bears are at risk.

Lunn says that what's happening with the polar bears in western Hudson Bay right now is not happening to all the polar bears in the world. But, he adds, "if the various climate change and sea ice models are correct and we do lose a lot of sea ice over time, I think the things we're seeing in western Hudson Bay now we'll probably be seeing down the road in some of these other populations."

—Mary-Russell Roberson is a writer living in Durham, North Carolina. Her daughter's favorite animal is the polar bear.

Sidebar: Baffin Bay Narwhals, Out in the Cold

ZooGoer 34(4) 2005. Copyright 2005 Friends of the National Zoo.
All rights reserved.



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