The Walrus’ Fatal Reaction to Global Warming

The Pacific walrus is tightly bound with the sea ice where it lives most of its time. The walrus spends the winter in the more southerly Bering Sea. When the sea ice in the Bering begins breaking up and melting in late March or April, they migrate northward to their summer range in the Chukchi Sea.

In summer, the walrus makes use of the Chukchi Sea’s scattered sea ice as a platform for diving, feeding and resting. They eat clams and a wide variety of other invertebrates they find on the seafloor of the Chukchi’s vast and comparatively shallow continental shelf.

They prefer diving at depths of up to about 80 meters.

In recent years, however, the melting ice has been retreating far north toward the center of the Arctic Ocean where it is too deep for their diving abilities and where food is scarce.

The USGS map above at top shows the late summer extent of sea ice, when it is at its minimum. The map depicts the sea ice in white, the shallow shelf in light shades of blue, deeper water in dark shades of blue, and the deep ocean floor in black. The historic location of the edge of the sea ice in late summer is shown with the yellow line. The sea ice in summer has retreated to a point several hundred miles beyond the yellow line. In six of the last nine years, the Chukchi Sea continental shelf has been ice-free — with periods of no ice cover extending from 1 week to as long as 2.5 months.

Typically, walruses will choose to dive from the small, isolated patches of ice that remain (top photo at right), but when those have melted they abandon the sea altogether and haul out on shore where there is little safety from hungry polar bears and little access to food.

The appearance of big coastal haulouts on Russia’s Chukotka Peninsula began in the mid-1990s and has been the walrus’ chief response to global warming, and for many, it has been a fatal reaction.

When the sea ice retreats, many walruses are still out at sea. They must swim back to shore, but given the great distance, not all make it.

During summer 2007 the Chukchi Sea shelf contained little or no ice for about 80 days. The edge of the ice was about 700 miles from shore, and as many as 40,000 walruses were forced to swim for their lives. An unknown number of calves apparently died, according to a 2009 report produced jointly by the World Wildlife Fund; the All-Russian Research Institute for Nature Protection, Moscow; and the Russian “Polar Bear Patrol” project.

“On the way from edge of drift ice to the mainland coast, walruses swim hundreds of kilometers,” the report said. “This long swim through rough, ice-free seas to the coast is exhausting for the walruses, leading to the death of many young, old, and sick animals.” Once on shore, thousands of the easily spooked walrus suffered another kind of deadly fate. In the summer of 2007, 3,000 to 4,000 mostly young walruses were trampled to death in stampedes at the extremely-dense, land haulouts on the Chukotka coast, according to the Center for Biological Diversity. None of these deaths would have occurred without the massive melting of sea ice triggered by global warming that forced the walruses to haul out on shore.

The U.S. Minerals Management Service says that of all oil and gas development activities, helicopter trips can trigger walrus stampedes, potentially compounding the walrus’ stress.

“Helicopters are more likely to elicit responses than fixed-wing aircraft, and walruses are particularly sensitive to changes in engine noise and are more likely to stampede when aircraft turn or bank overhead,” the agency said. No one knows whether the massive deadly stampedes in 2007 were related to oil and gas activities, or some other cause. “Given their importance to Pacific walrus, urgent measures are required to study and protect haul-outs on the Arctic coast of Chukotka,” the World Wildlife Fund report said.

There are between 200,000 and 235,000 Pacific walruses, and thousands more of the Atlantic walrus. The Center for Biological Diversity has petitioned the U.S. government to list Pacific walrus as “threatened” under the Endangered Species Act. A final decision is pending.