In the summer of 2008, a seven year old female polar bear (with a radio collar) left the coast near Barrow, Alaska and began an epic journey to reach the sea ice—she swam continuously for 9 days, covering a distance of 450 miles, without rest.
When she reached the pack ice, she traveled another 1,118 miles, walking on the ice and swimming until she reached the coast again, this time near the Alaska-Canada border.
The arduous journey came at a tremendous cost—the bear had lost 22% of her body mass.
And she had lost her yearling cub. Biologists can't say for certain, but it is likely the cub drowned.
There is widespread agreement among scientists that the arctic climate is warming due to greenhouse emissions—at a rate more than double that of temperate areas farther south.
One result is an unprecedented shrinking of arctic sea ice, and this threatens the future existence of polar bears in all but the most extreme northern regions of Canada.
Sea ice and the life it sustains are critical to polar bear survival. But why is the ice so important? And why can't polar bears live on land or in the water if the sea ice disappears for all or part of the year?
To fully appreciate the predicament of polar bears requires an understanding of their specialized adaptations to the sea ice and the ecology of arctic marine environments.
Polar bears are masters of life on the ice. From the white fur that disguises them from prey, to the broad furry paws for swimming and for maneuvering on sea ice, to their crafty ways of hunting seals—polar bears are uniquely suited to this complex and challenging environment.
Polar bears evolved from the brown bear—a land mammal—about 250,000 years ago. Since that time, polar bears have adapted to become a highly specialized predator on the frozen sea. They are officially classified as marine mammals.
Like other marine mammals—such as whales, seals, and sea lions—polar bears have a thick layer of blubber for protection from the cold. To maintain this insulation they need a very high fat diet. Their main prey species—ringed seals and bearded seals—are rich sources of both protein and fat.
The only place where polar bears can hunt seals is on the ice. Although these bears are strong swimmers, they are no match for lightning swift seals in the water. A polar bear has brilliantly clever strategies to overcome this disadvantage. In winter the bear waits motionless beside a seal's breathing hole, which is a narrow vertical tunnel through the ice. Often many hours pass before the seal comes up for air and the bear kills it with a powerful blow of its paw. In spring and summer, the bear carefully stalks close to seals resting on the ice, moving only when the animals are asleep, until the bear gets close enough for a final deadly sprint for the kill.
The sea ice might seem like a barren, frozen wilderness encircled by equally uninviting and frigid water, but in fact, it is a complex environment that teems with life.
Large marine mammals, such as beluga and bowhead whales, frequent the areas where the ice meets the sea because they are so rich with food.
At the bottom of the food chain, microscopic plants like green algae live beneath the ice, reaching peak abundance in the endless days of summer. The plants become food for tiny animals, or zooplankton, which are eaten in turn by swarms of larger creatures called krill. Squid, shrimp, and herring live on krill, as do the prolific schools of arctic cod—another marvel of arctic adaptation with blood that acts like antifreeze.
Ringed seals prey on arctic cod, along with other fish, as well as shrimp, squid, and krill. Bearded seals mainly live on clams and crabs on the sea floor, but they also feed on worms and fish.
The ocean is especially rich near the far northern coasts of Alaska and Canada, where for much of the year sea ice covers the shallow waters overlying the continental shelf. This is the habitat for bearded seals, which dive to the bottom for most of their food. In deep waters outside the continental shelf, the sea floor is beyond their reach.
Bearded seals must also have sea ice, because this is where they give birth and nurse their young; and it is crucially important that the seals have access to ice overlying the shallow waters of the continental shelf.
As the climate becomes warmer, arctic pack ice is retreating farther and farther from the coast in the spring, summer, and fall. This creates a severe dilemma for bearded seals. Increasingly today, the shallow areas where food is available no longer have ice where bearded seal pups can rest and nurse. And far out at sea, where ice still exists, the water is much too deep for mother seals to reach their food. The two conditions needed for bearded seals to survive as a species no longer reliably exist in the same place
Ringed seals face the same problem, but in a different way. During the springtime, they hollow out caves in snowdrifts on the ice, and then give birth to their pups inside these snug shelters where they are protected from predators and cold temperatures. But as the climate warms and snow melts earlier, it's either impossible to make snow caves or they melt quickly and leave the pups exposed.
For these reasons, biologists predict that ringed and bearded seal populations will decline and possibly become endangered. This also means that polar bears will find less and less of the prey on which their lives depend.
Polar bears are excellent swimmers. They can easily cross open ponds, cracks, even leads many miles wide, formed when ice floes drift apart. In such places the water is relatively calm, but as the polar ice shrinks there are great open stretches, sometimes hundreds of miles across.
Observers in airplanes are now spotting polar bears in the water far from any land or ice, sometimes swimming in very rough seas. Drowned bears have been documented in recent years, and thoroughly exhausted animals have come ashore after swimming for many days. At some point, it may impossible for even the strongest bears to swim between the pack ice and the arctic coasts.
As the sea ice diminishes, increasing numbers of polar bears are taking refuge on land, although they find very little to eat. Warmer temperatures mean the ice melts earlier in the spring and forms later in the fall, so the bears must endure longer periods without access to seals and other prey.
Warming temperatures also mean the sea ice becomes thin earlier in spring and then breaks into millions of floes or pans. Bears can rest and drift on these ice fragments, but they have great difficulty hunting seals under these conditions.
For thousands of years, polar bears have wandered across a thick, solid continent of ice, and as that continent shrinks or even disappears, these remarkable animals face a hungry and uncertain world.
Although polar bears are fundamentally animals of the frozen sea, in certain areas—including parts of Alaska's North Slope—some females move onshore to excavate wintering dens in deep snowdrifts. This is where their cubs are born and nursed until spring, when the family travels back out onto the ice.
The number of females wintering on land has increased in recent years, apparently because warmer temperatures are making the ice thinner and more susceptible to fracturing.
Why can't polar bears spend their entire lives on land if the sea ice melts completely? The answer lies in the specialized adaptations they have made to the sea ice environment.
We can learn about this through studies of polar bears along the coast of Hudson Bay in Canada. These animals are forced to spend months on shore during the ice-free summer, resulting in reduced body size (Hudson Bay bears are now 20% smaller), decreasing reproductive rates, and lower survival.
Although they occasionally scavenge the remains of dead animals and eat small amounts of seaweed, only rarely are polar bears able to kill prey such as caribou or arctic hares. And even when this happens, their metabolism requires much more fat than these foods can provide.
Polar bears also lack the powerful shoulder muscles and long claws that grizzly bears use to dig for plants and excavate ground squirrel burrows.
Polar bears are incredible hunters on the ice, using their intelligence and stealth to hunt wary seals. Their ivory color is perfect camouflage for stalking prey on the monotone white of ice and snow. But they are extremely conspicuous on land, making them easy to spot by skittish prey and by their only predator, humans.
The shrinking sea ice has caused other problems for polar bears on land…starving bears are increasingly coming into villages, where they may be killed either for food or safety.
As the ice vanishes, walrus herds are being forced to haul out and rest on remote beaches. When polar bears try to hunt these animals, walrus pups may be trampled to death by the resulting stampedes, and the angry adult walruses sometimes kill bears with their tusks.
Near one village on the Alaskan arctic coast, hungry polar bears swim ashore every fall to feed on the remains of whales taken in traditional subsistence hunts. People and bears have reached a nervous co-existence, but this situation is dangerous for the bears and risky for the villagers.
There is also competition for the whale remains between polar bears and grizzlies that come from the tundra. Interestingly, the polar bears usually abandon their scavenging whenever a grizzly shows up to feed. The biggest North Slope grizzly is less than half the size of an adult polar bear, but polar bears are apparently unwilling to argue with their far more aggressive relatives.
Since the late 1970's, the National Snow and Ice Data Center has used satellite imagery to track the extent of sea ice coverage on the Arctic Ocean and around the coasts of Alaska, Canada, Greenland, northern Europe, and Russia.
Because ice moves and changes throughout the year, and from year to year, an especially important measurement is the maximum winter extent—when the ice reaches its largest coverage for the year. This usually happens in March, just before the spring thaw begins.
The maximum winter sea ice extent for 2013, which occurred on March 15, was one of the lowest on record.
Each summer, some melting occurs, and by early fall, the sea ice has diminishd to the smallest area for the year. When satellite images were first taken in the 1970's, summer sea ice covered about 50 percent of the Arctic Ocean. In 2012, the summer ice had shrunk to a new record low, covering only 24 percent.
If these dramatic declines continue, the ice edge will eventually be so far offshore that polar bears could no longer swim to land as a refuge during the warmer months. Conversely, bears that happen to be on the land could not swim back to the ice.
Some scientists now believe that by the year 2020, the sea ice will completely disappear from the Arctic Ocean for at least part of the year. When this happens, it is likely that polar bears will also disappear from nearly all of their present range, except for the northernmost reaches of the Canadian high arctic. There, among remote and isolated islands, the remaining polar bears will spend the summer months fasting on shore, moving back onto the ice when it freezes again each winter.
US Fish and Wildlife Service
North Pacific Research Board
National Science Foundation
Polar Bears International
National Snow and Ice Data Center
Alaska Wildlife Notebook Series
Alaska Department of Fish and Game