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Wisconsin's winters are getting warmer, and it's altering our agriculture, economy, health, and way of life. On the heels of Wisconsin's warmest winter ever, Thin Ice explores the impacts.

Great Lakes ice cover is shrinking. What does that mean for shipping?

on an overcast winter day, little chunks of ice melt on a beach
Jen Day/NOAA
Lake Michigan, north of Muskegon, on February 11, 2024. Typically, in mid-February, this stretch of the beach would be covered in thick ice.

For millennia, the Great Lakes-St. Lawrence Seaway has been a highway for transporting goods, from beaver furs to iron ore. It connects Duluth, Minnesota, in the west to the Atlantic Ocean in the east.

It’s always been shaped by the seasons. Winter brings ice, and shipping takes a break. Timing hinges on the Soo Locks at the Sault Ste. Marie river and lock system, where Lake Superior spills into Lake Huron.

“That is the only entry and exit from Lake Superior to the lower lakes,” said Daniel Rust, an associate professor of transportation and logistics at the University of Wisconsin-Superior. “By law, [the locks] are shut down from January 15 to March 25. That is concurrent with the maximum ice coverage on the upper Great Lakes.”

Climate change is shaking up that pattern. This winter saw record low ice cover across the Great Lakes, among the fastest-warming lakes in the world. It’s part of a long-term trend, where warm winter temperatures are slowing ice formation and shaving off around 5% of ice each decade since the 70s. The climate pattern El Niño exacerbated the effect.

Typically occurring between mid-February and early March, the average maximum ice cover is 53%. This year, the maximum — 16% — came during the mid-January blast of Arctic air and steadily shrank back since then. By mid-February, ice coverage plummeted to a historic low of 2.7%, and the lakes were virtually ice-free.

“We’ve crossed a threshold in which we are at a historic low for ice cover for the Great Lakes as a whole,” says Bryan Mroczka, a physical scientist at NOAA’s Great Lakes Environmental Research Laboratory, in a release. “We have never seen ice levels this low in mid-February on the lakes since our records began in 1973.”

a graph of Great Lakes Average ice cover between 1973-2023 shows current ice cover is far below the average
NOAA-Great Lakes Environmental Research Laboratory
The 2023-2024 ice cover season compared to other years.

Stretching out the shipping season

Less ice means ships can travel longer. Last December, the Port of Duluth-Superior posted its longest season ever for international shipping, with records broken for both the earliest-arriving and latest-departing ocean-going ships. (Ice cover is a factor, but so is the market; in this case, the early-arriving ship was carrying cement from Turkey, for which there was high demand at the time.) These trends have some wondering whether the shipping season could be extended, a potential “silver lining” of declining ice.

“Almost everybody realizes that, while climate change is not a good thing, but there are opportunities that perhaps will open up,” said Rust, who co-authored a study on the effects of an extended season. “How are we going to, as an industry, as a nation, address these? And what can we do to make the best of the situation?”

a red carrier ship arrives under a bridge
Duluth Seaway Port Authority
The Federal Dart was the earliest-arriving oceangoing vessel to arrive in the Port of Duluth-Superior on record. Arriving March 28, 2023, the Dart kicked off what would become the Port's longest international shipping season.

One consequence of seasonal shipping is that commodity companies keep buffer stock, extra supply to keep production going during the winter layup. With a longer season, they wouldn’t need as much buffer. So Rust crunched a lot of numbers to see what might happen if the season went about a month longer.

“The conclusion was that if you have a longer operating season, same amount of cargo, existing vessels could carry more cargo longer, and potentially not need as many ships,” he said.

That could pass savings — tens of millions of dollars — to companies. And consumers too, down the line.

Back in the 70s, the Army Corps of Engineers actually experimented with year-round shipping. They found that it could work on the upper Great Lakes, but there were many winter navigation issues.

a satellite image of the great lakes shows virtually no ice on the lakes
Satellite views of the Great Lakes on Feb. 24, 2024, show virtually no ice on the lakes.

A lot has changed since then. The Great Lakes have lost about 25% of their ice, and the ice season has shortened about 27 days. The ice that does form is thinning.

Another big difference? At Sault Ste. Marie, the 1200-feet-long Poe Lock is the only one that the biggest lakers can pass through. It’s serviced during the winter pause. Now, a second lock its size is under construction, a project expected to be complete in 2030. So, one could stay open while the other is under maintenance.

Many considerations to extended seasons

Shipping is one of the greenest ways to move cargo, so getting more goods on the water could be an important climate adaptation. The U.S. and Canada are looking to slash emissions on the Great Lakes-St. Lawrence Seaway even more, by establishing what’s known as a “green shipping corridor network.”

“One Great Lakes freighter can carry the equivalent of 700 rail cars or 2,800 trucks,” said Jayson Hron, director of communications for the Duluth Seaway Port Authority. “So in that sense, the more goods we can shift to waterborne transport, and the longer the navigation season, the better.”

Hron said a longer season would be attractive to the companies who want regular service.

“There’s certainly an interest from a commerce perspective,” he said. “But you know, there’s just a lot of considerations involved in it.”

The U.S. Coast Guard's Mackinaw breaks a path through ice in Lake Superior for freighters traveling from Duluth. April 3, 2014.
The U.S. Coast Guard's Mackinaw breaks a path through ice in Lake Superior for freighters traveling from Duluth on April 3, 2014.

Considerations like maintenance, ecological impacts and the U.S.-Canada agreements that govern season length. Without ice capping the lakes, the coast is also more vulnerable to strong winter storms that can drive erosion, flooding and damage shoreline infrastructure.

And ice is variable. Long-term, it’s declining, but that doesn’t mean we’re not going to see ice ever again.

“Even though we have climate change, we have severe swings of some severe ice winters, and some that are very low,” said Eric Peace, vice president of the Lake Carriers’ Association, which advocates for growing the aging fleet of icebreaker ships that keep commerce moving through the winter.

Reliability makes it possible for the fleet to navigate these swings. Peace added that the seaway, like any other highway, needs continuous investment to make green shipping a part of its future. Senator Tammy Baldwin has championed the importance of icebreaking, securing authorization of $350 million for a new Great Lakes icebreaker in 2022.

The costs of losing ice cover

The Lake Michigan coast, near Newport State Park, had hardly any ice the first weekend of March 2024.
Lina Tran
The Lake Michigan coast, near Newport State Park, had hardly any ice the first weekend of March 2024.

There are many costs to diminishing ice cover, which can influence lake water levels and lake-effect snowfall. Countless winter businesses rely on ice formation, while animals, from plankton to whitefish, need ice to protect themselves or shelter eggs. And what happens in winter doesn’t stay in winter; warmer winters have cascading effects in the summertime.

“It turns out that ice is a really, really strong predictor of what happens the following summer,” said Large Lakes Observatory physicist Jay Austin in a recent talk for the University of Minnesota’s Institute on the Environment. “The low ice years that we’re seeing right now tend to result in an earlier onset of summer conditions [and] warmer summer water temperatures.”

That’s linked to harmful algal blooms on Lake Superior’s south shore and the bacteria behind beach closures.

“I would say that the loss of ice is a giant price to pay for a very small payoff,” Austin said.

But reversing that trend means totally shifting our society’s reliance on fossil fuels.

Lina is a WUWM news reporter.
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