DULUTH, Minn. — A frigid winter is pushing Lake Superior toward a complete ice-over for the first time since 1996, though there’s still a ways to go before you can skate from Duluth to the Soo Locks.

Lake Superior had at least some ice across an estimated 91 percent of its surface as of Thursday, according to the Great Lakes Environmental Research Laboratory.

That compares with the 40-year average annual Lake Superior ice coverage for February of just 30 percent.

George Leshkevich has been tracking Great Lakes ice for the National Oceanic and Atmospheric Administration Great Lakes Environmental Research Laboratory since 1973. He’s seen all kinds of winters over those 41 years, and all sorts of ice cover.

So far, this winter has had among the most rapid ice buildups of his tenure. That matches air temperatures that show this is the coldest winter since 1979.

The widespread ice in January and early February this year “wouldn’t have been anomalous back in the ’70s or with some of the winters in the mid-’90s. But it certainly has been a while since we’ve seen this much ice this early,” Leshkevich said.

Areas of open water were visible Thursday from Duluth, which is normal as the lake ice builds and shifts. Even in 1996, pockets of open water lingered after “virtual ice-over” was officially declared.

If this winter’s cold trend continues, Leshkevich said the lake will virtually freeze over, as it last did in 1996. But he’s not willing to go out on a limb and predict a total freeze-over.

“If it warms up later in February, or we get a lot of wind, then things can change pretty fast,” Leshkevich said. “It’s all dependent on the weather, and I don’t predict that.”

While the percentage of the lake that’s frozen continues to climb, satellite photos have revealed pockets of open water along the North Shore and the large, deep eastern basin of the lake.

“There’s a lot of deep water that serves as a heat pump in those areas. They are usually the last to freeze,” Leshkevich noted.

In making his daily ice estimates, in addition to satellite photos and eyewitness reports, Leshkevich also uses radar imagery from space that often gives a more accurate depiction of ice cover, including ice thickness.

The building ice has made for some interesting sightseeing along the lake, including ice jams near shore and the sea caves at Apostle Islands National Scenic Lakeshore, now accessible by ice for the first time in five years. And it’s going to make ice fishing more accessible to more people in more areas.

But the vast amount of ice on Lake Superior has ramifications beyond simple curiosity, Leshkevich notes:

— More and thicker ice now makes it more difficult to break up the ice after March 25, when shipping on the upper lakes will reopen. That could make the Coast Guard’s job of opening spring shipping lanes much harder.

— An ice-covered Lake Superior will immediately stop producing lake effect snowfall. Those snows need the warmth and moisture from the lake, pushed by cold winds, to work their magic on shore.

— A frozen lake means evaporation will slow to a stop. It’s been the increasingly open, mostly un-iced Great Lakes that scientists say have been contributing to lower water levels in recent years. Open water allows evaporation, which, across the lake’s huge surface areas, can add up. Ice will keep the water locked in place, and should help bolster a trend of rising water levels over the past year.

— Some fish that spawn in shallow water in fall need ice cover to protect their spawning beds from winter storms.

The least ice on Lake Superior since accurate records have been kept occurred in 2012, with just 8 percent of the lake covered by ice at the peak of that almost nonexistent winter.

The lake came close to freezing over in 2003 and 2009 but hasn’t hit a high enough ice concentration to be considered officially frozen since 1996. According to the Great Lakes Aquarium, 1979 and 1962 saw nearly complete freeze-overs of Lake Superior, with the lake about 95 percent ice covered in 1972.

On average, about 40 percent of the Great Lakes surface area freezes each winter at its peak. Already this year that number has hit 70 percent and continues to climb. Michigan has hit about 55 percent ice cover; Huron 83 percent; Erie 93 percent and Ontario 27 percent this week.

On average, ice coverage peaks in early March and then rapidly diminishes.

This winter’s vast ice on the Great Lakes is bucking a long-term trend of less and less ice as winters have, on average, become warmer.

The average amount of ice covering the Great Lakes declined by 71 percent between 1973 and 2011, with Lake Superior ice coverage down 79 percent, according to a report published in 2012 by the American Meteorological Society.

Researchers used Coast Guard reports and satellite photographs taken from 1973 to 2010 to determine the ice coverage of all the lakes, with Lake Ontario ice dropping 88 percent while ice in Lake St. Clair (between Lake Huron and Lake Erie) diminished by 37 percent.