ICEBREAKER CHANNEL, Northwest Passage — The Amundsen’s engines growl low, as if in warning. The ship steals ahead; its powerful spotlights stab at fog thick with the lore of crushed ships and frozen voyagers. Ice floes gleam from the void like the eyes of animals in the night.

The Canadian Coast Guard icebreaker Amundsen weaves in slow motion through the ice pack, advancing through the legendary Northwest Passage well after the Arctic should be iced over and shuttered to ships for the winter.

The fearsome ice is weakened and failing, sapped by climate change. Ultimately, this night’s ghostly procession through Icebreaker Channel will be the worst the ship faces on its late-season voyage. Much of the trip, crossing North America from west to east through the Northwest Passage, will be in open water, with no ice in sight.

The Amundsen is here to challenge the ice that has long guarded the legendary Northwest Passage across the roof of the Earth, and to plumb the scientific mysteries of an Arctic thawing from global warming.

A relentless climb of temperature — 5 degrees in 30 years — is shrinking the Arctic ice and reawakening dreams of a 4,000-mile shortcut just shy of the North Pole, passing beside the Arctic’s beckoning oil and mineral riches.

“Shipping companies are going to think about this, and if they think it’s worth it, they are going to try it,” the captain of the Amundsen, Commander Alain Gariepy, 43, said. “The question is not if, but when.”

More ships will bring the risk — the certainty, some say — of accidents and oil spills smeared on the Arctic.

“This water is our hunting ground,” an Inuit born 52 years ago in a tent on the beach of Igloolik, Maria Kripanik, told researchers who visited from the ship as it passed her village. There, hunters still use harpoons to snag beluga whales. “I don’t know if the people here will like the idea of seeing ships all the time in our hunting ground,” she said.

Equally wary are the scientists packed aboard the Amundsen. They occupy the Coast Guard ship for three months each year to study climate change in the fragile North, where the effects of a warmer globe are being felt first. They began this summer in Quebec City and churned west to the Beaufort Sea. As fall came on frigid gusts, the ship turned east again toward the Northwest Passage.

The Arctic ice pack rarely tolerates intruders in late October. It splintered the wooden ships of early explorers who stayed, seized fast the steel vessels that followed, and mocked dreams of regular transit through any of the routes in the maze of straits and channels of the passage.

British explorer John Franklin, whose search for the Northwest Passage transfixed the Western world, perished on a frigid island near here in 1847. Searching for him, many others fell. Their diaries, sometimes found by their frozen bodies, are grim accounts of waiting for a brief break in the ice, as starvation, scurvy and madness claimed them one by one. The map of the Arctic is littered with their names.

The Amundsen cautiously approaches Baffin Island at Fury and Hecla Strait, a dangerously narrow half-mile-wide passage. No ship has gone through this late, the captain said. But the Amundsen sails through in clear water. “This was easier than expected,” Mr. Gariepy acknowledges. At the eastern mouth of the strait, the residents of Igloolik are surprised the ship is coming through the Northwest Passage in late October. They are not pleased at the weather. They count on a frozen strait to travel to Baffin Island to hunt caribou.

“We get tired of eating seal meat and walrus by this time,” Michael Immaroitok, 38, tells visitors from the ship who helicoptered over to Igloolik, a village of about 1,600. Fishing boats are pulled onto the shore; dogs are gnawing on the carcass of a whale.

When hunters bring in whale, or narwhal, villagers share, and the animal ends up boiled, pickled, chopped like salad and served raw. But the hunting has been disrupted by “weird, crazy weather in the last five years,” Mr. Immaroitok complains.

Some believe the worries are overblown. “I think the passage is going to be used, but I don’t think it’s going to be a navigation highway,” a professor of geography at Laval University, Frederick Lasserre, onboard the ship, said. Costs of operating in the North are high, the ice cover is never certain and shipping companies do not want to risk delays, he said. “In 20 years, there might be less first-year ice. But there might also be more icebergs breaking off the ice cap that would be navigational hazards.”