暖水阻止美国北极海冰的形成

Climate change has brought a difficult new reality for the U.S. Arctic. Open water -- rather than sea ice – has become normal for the Chukchi Sea in the month of November.

Researchers are studying how this "new normal" may affect coastal communities in northern Alaska.

The researchers are from the University of Washington. The group is taking its 80-meter-long ship several places and will record observations. One of those places is Utqiaġvik, the northernmost community in America.

Jim Thomson is an ocean scientist with the team. He told The Associated Press that the researchers are trying to understand changes to the fall season in the Arctic.

Each day since mid-October, sea ice in the Chukchi Sea has been the lowest on record, said Rick Thoman. He is a climate expert at the University of Alaska Fairbanks' International Arctic Research Center.

For example, on November 7, the National Snow and Ice Data Center recorded sea ice at about one-sixth of the usual amount for that date from 1981 to 2010, Thoman said.

Less ice is a problem for people living on the coast. Communities north and south of the Bering Strait depend on coastal ice to act as a natural sea wall. The ice protects land from erosion caused by winter storms.

Sea ice also provides a place from which to catch seafood in Nome, a transportation center between villages in Kotzebue Sound. It also serves as a work area on which to kill walrus near the town of Gambell.

Sea ice is one of the most important physical elements of the Chukchi and Bering seas. The cold, salty water underneath ice creates columns that separate Arctic animals from valuable fish catches such as Pacific cod and walleye pollock. When sea ice melts, it creates conditions important for the growth of small organisms at the bottom of the food chain.

Sea ice also provides the main living space for polar bears. Female bears use ice as a place to give birth. And walrus mothers use sea ice as a resting place. They follow the ice edge south as it moves into the Bering Sea.

The formation of sea ice requires the ocean temperature to be about -1.8 Celsius, the freezing point of saltwater. Historically, ice has formed in the northernmost waters. It gets moved by currents and wind into the southern Chukchi and Bering seas, where it cools the water. This helps even more ice to form, explained Andy Mahoney. He is a sea ice physicist at the University of Alaska Fairbanks' Geophysical Institute.

Mahoney said, "Even at the end of summer you couldn't get enough heat into the ocean to raise the water temperature" much above freezing. "So it didn't take much cooling to cool the ocean down to the freezing point."

But high summer temperatures have warmed the water column in the Bering and Chukchi seas. Water temperatures from the surface to the ocean bottom remain above normal. This leads to a delay in ice formation.

"We've got a cold atmosphere. We've got a strong wind. You'd think we'd be forming ice, but there's just too much heat left in the ocean," Mahoney said.

The water may be warm enough to melt ice moving south from northern areas.

"I haven't seen any direct observations where ice has been transported into the Chukchi Sea and then melted," Mahoney said. But he says water temperature maps that he has seen are still much above zero degrees Celsius. And even if you bring ice from somewhere else, it will soon begin melting because of the water temperature, he adds.

Thomson and other scientists will look at how the changes could affect coastlines, which are already eroding. Less ice and more open water mean a big threat. Ice keeps down the size of waves. Open water increases the distance over which wave-causing winds can blow.

"We know from other projects and other work that the waves are definitely on the increase in the Arctic," Thomson said.

That means even more erosion and greater chances of winter flooding in villages. It also means increased danger to hunters in small boats, who will have to travel longer distances to find seals and walruses.

I'm Jill Robbins. And I'm Alice Bryant.

The Associated Press reported this story. Alice Bryant adapted it for Learning English. Ashley Thompson was the editor.