Marathon swimmers attempt 24 hours in Lake Superior

DULUTH — The surface temperature of Lake Superior is expected to hover in the mid-60s Tuesday — well above the limit for a group of cold-water swimming enthusiasts plunging in near Split Rock Lighthouse for a 24-hour, 48-mile relay to Canal Park's rock beach.

Sans wetsuits, the group, including Plymouth's Karen Zemlin, is swimming one of the coldest lakes in the world to spread the message that it's heating up.

Surface waters are about 5 degrees warmer than they were four decades ago, said John Shepard of Hamline University's Center for Global Environmental Education, which is co-producing a documentary with PBS North that will include footage of the swim.

"This is a significant change for the way people have thought about and understood Lake Superior," he said.

Zemlin, 55, holds the record for swimming the English Channel faster than any other woman over age 50. She trained in Lake Superior before that feat, she said, and is ready to complete the longest observed and documented swim in the lake without a wetsuit, something that would have been too cold to attempt decades ago. The team of six will each take at least four one-hour shifts, swimming through the night.

The group has studied temperature trends and the phenomenon of "upwelling;" when wind causes cold water to be pulled from deeper parts of the lake closer to shore. If that happens, the swimmers, accompanied by kayaks and dive, Zodiac and sail boats, will need to move father out onto the lake, lengthening the duration of the attempt.

An upwelling earlier this month led to a 30-degree temperature drop near Split Rock, Zemlin said, although such conditions aren't expected Tuesday, with a typical temperature expected for this time of year.

If it happens, she said, "I'm going to say in my head, 'it's okay to be cold.' And I am going to try to just stick it out until we get to warmer water."

But if the water becomes too frigid, swimmers will don wetsuits to finish the relay, willing to give up their record attempt in pursuit of their loftier goal.

The winter's ice cover is related to a warming lake, and while extremely high ice cover years like 2014 still occur, Lake Superior is seeing a growing number of below-average years.

Not only is Lake Superior the largest (and coldest and deepest) of the Great Lakes, it also has the largest surface area of any freshwater lake in the world.

This past winter ice cover reached just 20%, with ice nearly non-existent on the open lake, said Jay Austin, a researcher with the University of Minnesota Duluth's Large Lakes Observatory. He also manages UMD's buoys that measure the lake's temperature near Knife River's McQuade Harbor.

While the lake's rising temperature might make it easier for swimming, it has wide-ranging implications, Austin said.

"We're turning the lake into a more hospitable environment for algal blooms, and also invasive species," he said, something the lake has in past decades avoided to a large extent because of its cold temperatures.

As a swimmer, Zemlin worries about those invasive species and toxic algal blooms, along with unpredictable storms, all stemming from climate change.

"I have to be aware that the lake I love to train in might be changing," she said.

The documentary, "A Sea Change for Lake Superior," explores climate change and its effects on the lake. It will air later this year.