Prince of Wales Strait, Northwest Territories—A century ago, traversing the Northwest Passage was a grueling effort that often took years. Explorers frequently perished as their boats sunk or got trapped in the abundant pack ice. Today, these waters are mostly ice-free during the summer, and ships can sail through with ease.

The Arctic is warming twice as fast as the rest of the planet, while much of the Antarctic is warming three times as fast. As a result, the amount of Arctic sea ice has shrunk dramatically, and last year’s extent was the lowest ever recorded. In the last 30 years, the annual minimum size of the polar ice cap has shrunk by more than half, receding by 1.5 million square miles, an area approaching the size of India, France, and Italy combined. This melting is occurring more rapidly than predicted and continues to accelerate. Strikingly, between 2011 and 2012,...

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