BARROW, Alaska—Rapidly melting ice on Alaska’s Arctic is opening up a new navigable ocean in the extreme north, allowing oil tankers, fishing vessels and even cruise ships to venture into a realm once trolled mostly by indigenous hunters.

The Coast Guard expects so much traffic that it opened two temporary stations on the nation’s northernmost waters, anticipating the day when an ocean the size of the contiguous United States could be ice-free for most of the summer.

“We have to prepare for the world coming to the Arctic,” said Rear Adm. Gene Brooks, commander of the Coast Guard’s Alaska district.

Scientists say global warming has melted the polar sea ice each summer to half the size it was in the 1960s, opening vast stretches of water. Last year, it thawed to its lowest level on record.

The rapid melting has raised speculation that Canada’s Northwest Passage linking the Atlantic and Pacific oceans could one day become a regular shipping lane. And there is a huge potential for natural resources in a region that may contain as much as 25 percent of the world’s undiscovered oil and gas.

But scientists caution that it could be centuries before the Arctic is completely ice-free all year round.

Still, conservative estimates indicate the Arctic Ocean will be ice-free in the summer within 20 years, although some scientists believe that will occur much sooner.

As it thaws, the receding ice has made ocean travel along Alaska’s northern coast increasingly alluring, but ships can still be trapped by ice.

Earlier in August, three oil industry vessels bound for Canada became stuck in ice about 60 miles north of Point Barrow. The Coast Guard sent the icebreaker Healy to help, but before it could arrive from 300 miles away, the wind shifted and pushed the ice apart, freeing the vessels.

“They were able to get away,” Brooks said. “The problem with this ice is it’s very unpredictable.”

Because of such risks, the Coast Guard established temporary bases this month in Barrow, the country’s highest-latitude town, and at the North Slope’s Prudhoe Bay, the nation’s largest oil field. The bases will operate for a few weeks while Guard officials evaluate the need for the agency’s services.

The Northwest Passage is also increasingly popular with tourists.

Chuck Cross has been leading excursions to the North Pole with his Bend, Ore.-based Polar Cruises since 1991, and he’s noticed a big change over the years.

“It’s amazing to me when I go to the pole how thin the ice is, huge open spots of water in some areas,” he said. “Before, you spent more time getting there and more time in the ice. We’d have helicopters looking for breaks in the water for us.”

The thaw has added urgency to the race among neighboring nations to claim a piece of the North Pole’s resources. The U.S. is compiling mapping data that could bolster any claims for drilling rights.

Many countries have launched scientific expeditions, hoping to take advantage of a provision in international law that allows nations to claim rights over their continental shelf beyond the normal boundary of 200 nautical miles, if the claim can be supported with geologic evidence.