DULUTH – The Madeline Island ferry that crosses Lake Superior to Bayfield isn’t getting a break this season, with ice coverage too thin for the famed ice road that typically gives winter residents the freedom to leave the island as they please.

It’s the sixth time in the last 25 years there hasn’t been enough ice for a road, with an additional two years where the road was open only a few days — a phenomenon that never happened between 1965 — when record-keeping began — and 1997, ferry operators say. Reliable ice year after year is no longer taken for granted.

“You can’t count on it; you can’t predict it,” said Robin Trinko-Russell of the Madeline Island Ferry Line.

As the world warms, so does Lake Superior. The Great Lake is seeing a growing number of below-average years of ice cover. Only 4.8% of the lake was covered Tuesday, significantly lower than the average coverage of 40% on Feb. 14 and matching the record low from 1999.

Warmer temperatures are affecting the other Great Lakes as well. Sunday marked a record low gauging ice cover on all the Great Lakes, at just 7.5% that day. The deep and vast Lake Superior retains solar heat much longer than the shallower Great Lakes, so it takes longer to form ice. And it’s sensitive to even a few degrees of temperature change.

“A few degrees warmer or colder will determine whether we have a heavy ice year or low ice year,” said Jay Austin, a professor with the University of Minnesota Duluth’s Large Lakes Observatory. “It doesn’t matter how windy it is, whether the previous summer was warm or cold or how much precipitation [lands]. All that matters is air temperature, and air temperatures are getting warmer.”

Lake Superior is projected to peak at 55%, which is about average, said Jia Wang, an ice climatologist with the National Oceanic and Atmospheric Administration’s Great Lakes Environmental Research Laboratory. But it will probably be short-lived, he said, similar to last year. Ice cover on Lake Superior spiked to 80%, but dropped quickly. Austin thinks the lake may have already seen the most ice cover this winter, at 15% earlier this month.

The Great Lakes are in a “warm phase,” based on the long-term fluctuation of the Pacific Ocean, a global weather pattern called Pacific Decadal Oscillation, Wang said.

Even an Alberta clipper probably wouldn’t change expected ice cover, he said.

Before 1998, average Lake Superior ice cover was mostly moderate to high. Since then, more than a third of the lake’s winters have brought low ice concentration, said Austin, who is studying reasons behind the shift.

The lake has reached high ice cover several times in the last couple of decades, such as the polar vortex of 2014, when it was mostly covered in February and March. But based on peak ice cover, the trend is downward every decade for the last 50 years on all the Great Lakes.

In another 50 years, that could mean some winters on Lake Superior with no ice cover, said James Kessler, a scientist with the NOAA Great Lakes Laboratory.

And while that downward trend of ice cover relates to the maximum amount each year, he said, there is evidence that the duration of ice cover is also shrinking over time.

It might bode well for the multibillion-dollar Great Lakes commercial shipping industry, but less ice can hurt coastal economies that depend on winter outdoor tourism activities like snowmobiling and ice fishing.

With the potential for ice chunks to break off and strand anglers, or worse, “this is not a good year to ice fish,” Wang said.

A lack of ice can also lead to coastal erosion, with winter storms wreaking more havoc on shorelines without ice protection. Ecologically, a lack of ice cover can also be damaging to certain species of fish and microorganisms crucial to the lake’s food chain, and can lead to warmer summers with the potential to grow toxic algae blooms.

The global temperature rising a couple of degrees by the end of the century doesn’t seem like a lot, Austin said, but consider its effects on Lake Superior.

“Here is a major chunk of our landscape that will be very different in a warming world,” he said.

The span of water between Bayfield and Madeline Island, nestled in the Apostle Islands National Lakeshore, is somewhat protected from the open waters of Lake Superior. So it’s usually easier for thick ice to form and remain undisturbed, Trinko-Russell said, enabling ferry operators to take a break and giving islanders more freedom in travel.

But milder temperatures and more severe wind and storms work against an ice road.

“We are like the canary in the coal mine,” she said of climate change. “This is happening.”