AMBRIDGE 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.

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