CAMBRIDGE BAY, Nunavut Territory - The ice packs that block navigation across the top of the world are thinning as Earth's climate warms, raising the likelihood that commercial shipping and military vessels will soon routinely ply these long-frozen seaways.

The rapid retreat of the ice is also opening the way to direct challenges to Canada's control over the waters of its vast northern realms - above all, its claim to the legendary Northwest Passage.

The dramatic changes in one of the world's most vulnerable environmental zones and critical strategic arenas are making reality of the ancient dream of an east-west sea lane that would shave thousands of miles from the ocean routes between the markets of Asia and those of Europe and eastern North America.

The result could be the biggest change in global shipping patterns since the construction of the Panama Canal, completed in 1914.

''Extraordinary international issues are quite literally riding on thin ice,'' said Rob Huebert, associate director of the Center for Military and Strategic Studies at Alberta's University of Calgary, a specialist on Arctic issues. ''At stake is control of a sea lane of potentially major importance.''

Disagreements over the right to transit the northern channels have for decades been a source of usually low-key but occasionally bitter diplomatic sparring between the United States and Canada. It was a debate, in any event, over an abstraction, since the channels were so clogged by pack ice even during summer months as to make passage impossible to all but mighty icebreakers and specially designed research vessels.

But soon Canada may find itself pitted against not only its powerful southern neighbor but also seagoing nations around the world - including Russia, Japan, and nearly every European country with ocean ports - over its claim to own the northern waterways.

It is a conflict that could test the limits of diplomacy. The commander of Canadian forces in the far north said in an interview that he anticipates his country may be required to use firepower to enforce its claims.

''National security, the safety of native coastal communities, and the protection of an extremely vulnerable ecology are at stake,'' said Colonel Pierre Leblanc, commander of Canada's northern military zone, embracing 1.5 million square miles of frozen barrens. ''When it comes to these, Canada can be a very tough country. We will defend our interests and our honor, with force, if necessary.''

Already, there are signs that major sea powers may be quietly probing the seas of the Canadian Arctic.

Last Aug. 16, Inuit fishermen stared in disbelief as a strange and threatening sea beast burst from the frigid waters of Cumberland Sound, near the hamlet of Pangnirtung. This was no whale.

First loomed the conning tower and then the riveted steel skin of a submarine moving at speed, seven-foot wave curling off the dark bow.

An alert was sent to Canadian Forces northern region headquarters in Yellowknife, Northwest Territories. As orders and queries bounced back and forth across the country, a squadron of CP-140 Aurora maritime patrol planes lifted from a base in Greenwood, Nova Scotia. But by the time the sub hunters had flown the 1,600-mile distance - and peppered the ice-flecked waves with sonar buoys - the quarry had vanished to the deep.

There would be more sub sightings over the next two weeks, but only by Inuit villagers untrained in vessel identification. From their descriptions of the warcraft, however, Canadian military analysts suspect the sub was French or American.

If the mysterious incursion did not pose a true military threat, it did represent a slap at Canadian sovereignty over its vast Arctic archipelago - a humiliation that has left the country's downsized military quietly seething.

Worse may be yet to come. Scientists and military specialists warn that far more direct, even hostile, challenges are likely to question Canada's claims to control the sea passages between the North Atlantic and some of Asia's most important ports.

The ice packs that cover much of the Canadian Arctic are receding and thinning at rates that scientists describe as alarming, reflecting a planetwide warming trend.

The phenomenon has been much ballyhooed by environmentalists, some of whom predict flooding of coastal cities and catastrophic changes in climate. Doomsday scenarios aside, the sea change is also being watched closely by seagoing powers, especially the United States, as well as by hard-headed international shipping executives from Tokyo to Rotterdam.

''The waters are opening, this is not science fiction,'' said Andre Maillet, head of Arctic icebreaking operations for the Canadian Coast Guard. ''Whether it's a few years away or a couple decades, the passage is going to become a vital commercial channel.

''The shipping companies already have their eye on the route,'' he said. ''And they are not going to wait.''

Indeed, in a bit of nautical history-making that attracted almost no notice, a Russian ocean tug, the Irbis, last fall hauled a huge floating dry dock through the icy labyrinth - traveling from Russia's far eastern Kamchatka Peninsula to Freeport, Bahamas - the first industrial transit of the Northwest Passage by a non-Canadian vessel.

The name Northwest Passage refers to several possible routes through the maze of sounds, gulfs, channels, and straits that transect an almost incomprehensibly vast and barely populated region, and Canada insists it is an internal waterway. It says that Ottawa alone has the authority to regulate shipping and, above all, to set the environmental rules for the zone.

Canada's primary fear is the environmental havoc that would be caused by a ruptured oil tanker or the wreck of an ordinary container ship. The few creatures that survive in the frozen barrens are perhaps more susceptible to pollution than in any other part of the world because the food chain is so thin.

But overriding even ecological concerns are worries about national sovereignty and whether Canada can enforce territorial claims to its empire of ice-capped islands. The country is already proving rich in oil, gas, diamonds, gold, and other mineral wealth.

''The Canadians see the north as part of their heritage,'' said Chris Sands, a Canadian affairs analyst with the Washington-based Center for Strategic and International Affairs. ''It's an extremely sensitive issue.''

So a country known for its soft voice on defense matters may find itself forced to take a very hard line on the Arctic.

''These are Canadian waters,'' said Leblanc, responsible for the defense of a military zone stretching from the Alaska border to the Davis Strait off Greenland, but inhabited by only 90,000 people.

Many seafaring powers, most stridently the United States, take a radically different view of the Northwest Passage, claiming that the channels threading through the Canadian Arctic form an international waterway and must be freely navigable to all commercial ships and naval fleets.

''It's our firm position that Canada has no more right to restrict the Northwest Passage than Malaysia has to restrict the Strait of Malacca,'' said one senior official in Washington, referring to the heavily trafficked sea lane between the Indian and Pacific oceans. ''These are world waters.''

To make its point, the United States has twice in recent decades enraged Canada by sending ships through the ice without permission - the experimental heavy-hulled oil tanker Manhattan in 1969 and the icebreaker Polar Sea in 1985.

Despite the fury provoked in Ottawa, the American voyages were basically symbolic exercises, since only specially designed ships could make the treacherous transit from the North Atlantic to the Beaufort Sea.

But now many analysts predict that within a decade or so, the ice floes will thin or recede so much for at least six months of the year that the fabled passage will be navigable by container vessels, tankers, and even luxury cruise ships with only modestly reinforced hulls.

''It's a significant shortcut from Europe to Japan and other parts of Asia,'' said Sands. ''For the US, it would make shipping of crude oil from Alaska to Boston, or anywhere on the eastern seaboard, economically feasible for the first time.''

Most keen on the route are owners of ships plying between Western Europe and Japan, South Korea, Siberia, and northeastern China. To make that journey via the Panama Canal, vessels must travel 12,600 nautical miles. The best route through the Canadian Arctic, by contrast, would cover a bit less than 8,000 nautical miles, which would mean savings in fuel, time, and crew salaries.

Moreover, the Arctic channels are opening at a time when shipping companies are fretting about possible political instablity, corruption, and technical breakdowns in Panama, now that the United States has relinquished control over the historic canal, which may prove too narrow to handle the next generation of supertankers and mammoth container ships.

Opening the passage would be the fruition of a goal sought for centuries by European explorers seeking a shorter sea route to the Far East. In 1576, Martin Frobisher made it to the tip of Baffin Island, in present-day Nunavut. Henry Hudson combed the great bay that bears his name in 1610 but was thwarted in his dream of discovering a new way to China. John Franklin and his crew died after resorting to cannibalism when his two ships were trapped in ice in 1847.

Finally, in 1905, a plucky Norwegian, Roald Amundsen, completed a two-year voyage in a refitted herring boat to become the first to successfully navigate the entire Northwest Passage. Like the moon landing later in the century, however, Amundsen's journey was an epic achievement with no real practical significance. Until very recently, the route he charted remained dangerous and essentially off-limits to all except adventurers, scientific researchers, and flag-waving exercises, such as those mounted by the United States.

But now, Alaska-based cruise ships are starting to ply sections of the passage, sometimes asking Canadian permission - sometimes not. In a weird episode last summer, a Chinese government vessel landed at the tiny fishing port of Tuktoyaktuk without notice. A few crew members disembarked, wandering about and snapping photos until the local constable of the Royal Canadian Mounted Police started asking questions. The Chinese then reboarded and steamed away.

Yet for all Canada's tough talk about defending Arctic sea routes and national sovereignty, the country has only a token military presence in the far north: 200 soldiers, sailors, and aviators belonging to the regular military, plus 1,500 Indian and Inuit belonging to irregular Ranger units.

''The military sees the dangers - environmental, national security, and the prospect of new smuggling routes - and is trying hard to think of ways to deal with the issues before the passage becomes trafficked, [and] it's too late,'' said Huebert, the University of Calgary political scientist.

''But in Canada, the military doesn't set national priorities,'' he said. ''And the politicians who do just aren't thinking about this. The far north is too far away, even to Canadians.''

© Copyright 2000 Globe Newspaper Company.

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