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CHEBOYGAN, Mich. — From the bridge of the Coast Guard cutter Mackinaw, northern Lake Huron looks like a vast, snow-covered field dotted with ice slabs as big as boulders — a battleground for the icebreaker’s 58-member crew during one of the roughest winters in memory.

It’s been so bitterly cold for so long in the Upper Midwest that the Great Lakes are almost completely covered with ice. The last time they came this close was in 1994, when 94 per cent of the lakes’ surface was frozen.

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As of Thursday, ice cover extended across 88 per cent, according to the federal government’s Great Lakes Environmental Research Laboratory in Ann Arbor.

Sections of the lakes, which hold nearly one-fifth of the freshwater on the world’s surface, harden almost every winter.

That freezing keeps the Coast Guard’s fleet of nine icebreakers busy clearing paths for vessels hauling essential cargo such as heating oil, salt and coal. But over the past four decades, the average ice cover has receded 70 per cent, scientists say, probably in part because of climate change.