Wait, no ice? Were you asleep last winter? I’m talking about Lake Superior — the largest body of freshwater in the world, a lake so big that if you could somehow grab a corner of its shoreline and drag it as far south as you could go, you’d find yourself, exhausted, somewhere around Miami.

It’s a body of water so large that if you bought the biggest cookstove ever imagined by anyone and poured Superior’s water into a pot so big it would empty the Iron Range of all remaining taconite to make it, you’d need so much energy to bring that water to a boil it would blow our nation’s annual energy budget many times over. According to Microsoft’s AI engine, the required BTUs would approximately equal “43 times the total energy consumption of the United States in 2020.” (Of course, AI is known for occasional fits of fancy, and don’t forget that if you were watching that pot, it might never get there.) Back on Earth, people intimate with Superior have noted that the lake is, indeed, warming.

View the documentary

What: The documentary film, “A Sea Change for Superior: The Warming of the World’s Largest Lake.”

Premiere: It premieres at 7 p.m., Thursday, Nov. 30, at the College of St. Scholastica. The premiere is free; registration is required at cgee-hamline.org/ Sea Change Premiere.

Broadcast debut: The documentary’s broadcast debut is at 7 p.m. on Dec. 4 on PBS North.

 In producing the documentary, “A Sea Change for Superior: The Warming of the World’s Largest Lake,” we spent time with cold-water marathon swimmers, whose idea of fun includes full-body ice-cream headaches, and they’ve noticed it. Last August, six of them undertook a record-setting relay swim — without wetsuits — of 46 miles, from Split Rock Lighthouse to Duluth, in water that remained above 60 degrees Fahrenheit.

Mark and Katya Gordon, owners of Amicus Adventure Sailing in Knife River, now advise customers to bring swimsuits. A comfortably cool dip has become a regular feature of a summer’s day sail, a ridiculous notion just a dozen years ago.

Commercial fisherman Stephen Dahl finds his herring nets increasingly twisted by the crazy currents that occur when surface waters climb into the 50s and 60s while temps remain about 40 degrees Fahrenheit deep down. He says those warm surface temperatures are lasting into late October now.

Chiming in, Jay Austin, a physicist with the Large Lakes Observatory at the University of Minnesota Duluth, notes that, “Since about 1980, we’ve seen a dramatic increase (in lake temperature) compared to what happened the previous 40 years.” He says the lake is 4 to 5 degrees warmer now than it was 40 years ago.

“Winter conditions play a huge role in determining what happens the following summer,” Austin says. “We are having much milder winters with less ice, and winter conditions wind up rolling forward into what happens the next summer.”

Last winter delivered enough snow to pretty much blot out everything else, yet Lake Superior was almost completely ice free.

So, what might this year bring? What does the warming of Superior mean for the natural systems of a lake that is famously clean, clear, and cold? Should we be concerned? All good questions.

The state of the lake brings to mind a dear friend who last spring started experiencing some unusual symptoms: fluctuating fevers, night sweats, fatigue. The docs went to work to figure out what was happening, and among the possible diagnoses were two forms of cancer: Hodgkin’s lymphoma and non-Hodgkin’s lymphoma.

The first is treatable, the second you really don’t want to get.

Unprecedented algae blooms; extreme storm events like the deluges of 2012, 2016, and 2018; and increasing impacts of invasive species are symptoms that scientists associate with rising Lake Superior temperatures and our warming climate. They are the kinds of symptoms you would want to pay attention to if the health of the world’s greatest lake is important to you.

John Shepard is assistant director of Hamline University’s Center for Global Environmental Education in St. Paul. He is also the producer of the documentary, “A Sea Change for Superior: The Warming of the World’s Largest Lake.”