Polar bear foraging at garbage dump
Incident on the sea ice
After killing a yearling cub for food in the Norwegian Arctic, an adult male drags the young bear’s body across the ice before stopping to eat it. As temperatures continue to rise and arctic sea ice continues to disappear, it will be increasingly difficult for polar bears to hunt seals. Scientists believe such tragic instances of polar bear cannibalism may become more common as a consequence.
Marooned on land due to lack of sea ice, this bear forages for food at a garbage dump. Although trash can temporarily fill an empty belly, it cannot provide nourishing long-term sustenance. As rising temperatures trap fasting bears on land for longer periods, they are increasingly searching for food in close proximity to people, and that poses risks to both humans and bears.
Twin polar bear cubs in a snow den
Adult male polar bear (Ursus maritimus) on sea ice, Arctic Ocean north of Svalbard
Over 30 years ago, Ian Stirling initiated long-term research on Canada’s Western Hudson Bay (WHB) population of polar bears to gain a detailed understanding of their biology and ecology. Since then, the decades-long work of Stirling and others has proven climate change is imperiling the bears.
From late fall through early summer, the WHB polar bears are on frozen Hudson Bay, hunting seals. By early July, the ice breaks up and the bears must go ashore. For several months, they are marooned on land and food-deprived because they cannot hunt seals until the bay freezes again in winter.
Pregnant polar bears in this population must endure even longer periods of food-deprivation. After mating in spring, they attempt to build up huge fat reserves by gorging on seals until the ice breaks up in July; then they must move onto land. Once ashore, they’re obliged to subsist only on their stored fat while travelling to their denning area, giving birth, nursing tiny cubs through the winter, and waiting until their offspring are strong enough to travel to the sea ice the following spring. By the time mother bears are able to resume hunting on the ice in mid-March, they have not eaten anything for at least eight months. That is an astonishing biological feat, and one which pushes the limits of what can be endured.
Research by Stirling and colleagues has established that during the past few decades, due to climate warming, the sea ice on Hudson Bay has been breaking up progressively earlier in the summer. Break-up now occurs at least four weeks earlier than it did a few decades ago. Consequently, the WHB bears have much less time on the ice to hunt seals and accumulate the fat necessary to survive the ice- free period. Furthermore, explains Stirling, in addition to coming ashore with meager quantities of stored fat, “the bears are now being forced to fast for even longer periods because freeze-up is coming progressively later in the fall as well.”
The WHB polar bear population is declining from nutritional stress. Weights of adults are decreasing, litters are smaller, fewer young bears are surviving, and the overall population size is shrinking. The earlier break- up is particularly problematic for pregnant bears, because maximizing intake of seal blubber in the spring is crucial for successful reproduction. If current climate trends persist and the ice-free period continues to lengthen, Stirling says, “In a few more decades, not many adult females will be capable of reproducing in western Hudson Bay and any cubs that might be born will have difficulty surviving.”
Polar bears in the Beaufort and Chukchi Seas of Alaska and Russia are also struggling to cope as the ice disappears. Steven C. Amstrup, who has studied polar bears in Alaska for 30 years, and is now Chief Scientist for Polar Bears International, says that historically the sea ice remained over the biologically-productive waters of the shallow continental shelf all year round, and the bears could hunt seals even in summer. But now the ice recedes hundreds of kilometers away each summer, forcing the bears either to swim ashore and fast until the ice returns in winter, or remain with the ice as it shrinks toward the central polar basin where food is scarce. Polar bear size and survival rates are already declining in the Beaufort Sea region, Amstrup explains, due to reduced feeding opportunities as the result of ice retreat.
Pregnant bears are facing especially serious challenges in the Beaufort and Chukchi Seas as the summer sea ice recedes far from maternity denning areas on land. When the ice withdraws northward in summer, a pregnant bear must decide when to go ashore. If she chooses to remain on the retreating ice temporarily, she can continue hunting seals longer. That will help her maximize the fat reserves she’ll need to survive a lengthy period of food-deprivation while she’s denning. However, remaining longer on the receding ice could mean a pregnant bear will then have to swim much farther to reach her terrestrial denning area, which could be so energy-intensive and difficult that she might lose her pregnancy or even drown in rough seas. Eventually, this situation will result in fewer successful pregnancies, fewer surviving cubs, and decreasing populations of polar bears in the Beaufort and Chukchi Seas. Scientists believe sea- ice retreat will soon have the same effects on bears in other populations as well, for similar reasons.
To make matters worse, deteriorating sea-ice conditions may also affect polar bears’ reproductive rates by directly impairing their ability to locate one another for breeding. Canadian bear biologist Andrew Derocher of the University of Alberta, a former Chair of the Polar Bear Specialist Group in the International Union for Conservation of Nature (IUCN), has studied this issue. His research indicates that when increasingly warm spring temperatures cause sea ice to be fractured extensively, with many small mobile floes constantly moving in expanses of churning water, it may be difficult or even impossible for males to find females for breeding by following the scent of their tracks. “The males are trying to follow a broken path – whirling ice floes are like the pieces of a jigsaw puzzle – if it gets bad enough, there are so many pieces missing that the puzzle doesn’t make any sense,” he explains. This issue will likely contribute to precipitous population declines as arctic temperatures continue to climb.
Today, there are approximately 20,000 polar bears living in 19 relatively discrete populations. The IUCN Polar Bear Specialist Group – the world’s foremost scientific authority on polar bears – has determined that eight of those 19 populations are already declining due to rising temperatures. Moreover, the scientists emphasize that all polar bears throughout the Arctic will ultimately be endangered by loss of their sea-ice habitat if temperature rise continues unabated.
Some have suggested that polar bears will adapt to an ice- free Arctic by consuming land-based foods such as bird eggs, rodents, berries and vegetation, as well as fish – essentially shifting to the diet of omnivorous brown bears. But Derocher, who has studied both polar bears and brown bears for nearly 30 years, says that theory is completely misguided. “Anyone who suggests that polar bears can survive by consuming terrestrial foods simply does not understand polar bear physiology,” he says. Unlike brown bears, polar bears are functionally obligate carnivores and they are physiologically dependent on consuming the blubbery bodies of marine mammals. Other foods simply cannot meet their tremendous energy needs over the long run. Furthermore, Derocher explains, it took tens of thousands of years for polar bears to evolve into the uniquely well-adapted, completely ice- dependent arctic predators they are today; it is just not possible for them to evolve into land-based omnivores within a few decades.
And if, in addition to being endangered by global warming and vanishing sea ice, polar bears are also subjected to increasing human disturbance, killed by hunters and poachers, and exposed to dangerous contamination in their shrinking habitat, they will decline into oblivion even faster. Of particular concern is the active push for oil exploration, production and shipping in many crucial regions of polar bear habitat as the Arctic Ocean becomes increasingly ice-free. “It is obvious that an oil spill would be devastating to polar bears,” says Stirling. “The contamination will almost certainly kill them.” Whether by swimming in oil-covered seas or travelling on oil-soaked ice, they would become contaminated in the event of a spill, and the consequences would be deadly. Once polar bear fur is fouled with oil, Stirling explains, it loses its insulating properties. Shivering bears will then attempt to groom their fur by licking themselves clean, and will ingest the toxic oil. Kidney failure and death will almost invariably follow. Because resources to contain an arctic oil spill are not currently available, and no proven technology exists to clean up oil in icy arctic seas, the danger to polar bears from oil drilling and transport in their habitat is indisputable and significant.
If humans continue to burn fossil fuels and pump ever- increasing amounts of heat-trapping gases into the air, arctic temperatures will inevitably continue to rise, and sea ice will surely continue to shrivel. Will any polar bears survive anywhere in the wild by the end of this century if we simply carry on with business-as-usual? It appears extremely unlikely. “As the sea ice goes, so goes the polar bear,” says Amstrup. Research by Amstrup and colleagues has shown that if current trends persist, at least two-thirds of the world’s polar bears will vanish within the next 40 years, and the rest are likely to disappear by 2100.
However, in a recent study published in the December 2010 issue of the journal Nature, Amstrup and his collaborators concluded there is still hope for polar bears. Their research used mathematical modeling to test the effects of a scenario in which global greenhouse-gas emissions are quickly and significantly reduced, and atmospheric concentrations of heat-trapping pollutants are stabilized within the next decade. Based on that hypothetical scenario, the scientists determined that the decline of arctic sea ice is not unstoppable, and some sea-ice habitat sufficient to support some polar bears can be preserved. “There’s a widely held perception that nothing can be done to help polar bears and the arctic ecosystem,” says Amstrup. “Our new findings show this isn’t true. Saving polar bears is all about temperature and sea ice. By minimizing greenhouse- gas emissions and therefore temperature rise, we will retain more sea ice. The more sea-ice habitat we retain, the more polar bears will survive.”
But is it feasible to reduce global greenhouse-gas emissions rapidly enough and drastically enough to avert an arctic melt-down and the demise of polar bears? Joseph Romm, a Senior Fellow at the Center for American Progress, says the necessary technological capabilities do indeed exist, but he stresses that lack of political will is the overriding problem. Romm, a physicist and climate expert, was the Assistant Secretary of Energy for Energy Efficiency and Renewable Energy in U.S. President Bill Clinton’s administration and is the editor of the respected blog Climate Progress. He explains that to stabilize atmospheric greenhouse-gas concentrations at 2020 levels within a decade, which is the hopeful scenario in the study by Amstrup’s group, the industrialized world would have to cut greenhouse-gas emissions about 60% to 70% almost immediately, and then continue cutting even further. Achieving that goal would be tremendously challenging yet technologically feasible, says Romm. But the stark reality, he notes, is that currently the world appears unwilling to do what is necessary.