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.

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.

Still, as this season shows, short-term weather patterns can trump multi-year trends. Winter arrived early and with a vengeance and refuses to loosen its grip.

“That arctic vortex came down, and the ice just kept going,” said George Leshkevich, a physical scientist with the federal lab.

The deep freeze is more than a novelty. By limiting evaporation, it may help replenish lake water levels – a process that began last year after a record-breaking slump dating to the late 1990s.

Also getting relief are cities along the lakes that have been pummelled with lake-effect snow, which happens when cold air masses suck up moisture from open waters and dump it over land.

Buffalo, New York, America’s snowiest big city, got nearly 110 centimetres of snow in January, but this month just 33 centimetres have fallen, a decline resulting largely from the freeze-over of Lake Erie even though Lake Ontario has remained largely open, said forecaster Jon Hitchcock of the National Weather Service.

Heavy ice can also protect fish eggs from predators, and it has delighted photographers, ice anglers and daredevil snowmobilers.

At Apostle Islands National Lakeshore in Wisconsin, the rock-solid cover has allowed around 35,000 visitors to trudge miles over Lake Superior to explore caves featuring dazzling ice formations. It’s the first time in five years the lake surface has been firm enough to allow passage.

With no letup in the cold, the ice hasn’t experienced the usual thaw-and-freeze cycle, so nature’s artistry is even more delicate and beautiful, with needle-like hoarfrost crystals sprinkled across sheets that dangle from cave ceilings like giant chandeliers.

“Seeing them like this is almost a once-in-a-lifetime experience,” Superintendent Bob Krumenaker said.

There’s even an (apparently) tongue-in-cheek Facebook page inviting people to join a convoy of snowmobiles, cars and other vehicles on a nearly 130-kilometre trek across Lake Michigan. Never mind that its waters remain partly open and experts warn the ice can be dangerously unstable.

“If it freezes, and you miss this chance, when will it happen again?” the page says. “Feel free to invite more folks!”

For Coast Guard icebreaker teams, it’s all business. They’ve logged four times more hours this season than the average for the same period in recent years, said Kyle Niemi, spokesman for the agency’s Cleveland district headquarters.

The 73-metre-long Mackinaw began its duties December 16 – several weeks earlier than usual – and worked nonstop until February 8, when traffic slowed enough to allow a break.

“As you can imagine, the crew’s tired,” Commander Michael Davanzo said this week during a tour of the ship in its home port of Cheboygan.

The Mackinaw, commissioned in 2006 to replace an older vessel with the same name, is designed specifically for duty on the Great Lakes.

Davanzo hopes for rain and warmer temperatures that would melt some ice before the locks reopen in late March, when the Mackinaw will venture onto Lake Superior and clear paths for iron ore and coal haulers.

“But if the weather stays like this,” he said, “we could be breaking ice all the way to the middle of May.”