As late summer waves crashed around him on the beach along Park Point, 10-year-old Gage Jones of Superior, Wis., splashed in the often-frigid Lake Superior water.
"It's not the warmest it's been, but I think it's pretty good. Enough to swim in," he said.
"This is the warmest I've ever felt it, actually," said Candace Jones as she kept an eye on Gage and another son on the seven-mile spit of sand that stretches into Lake Superior from downtown Duluth.
In late August the average surface water temperature for the entire lake hit 68 and a half degrees.
That might not seem very warm, but only 2010 had a warmer high temperature.
And the water temperature at Park Point and other areas close to shore has remained around 70 degrees for weeks.
"It's very warm," said Drew Gronewold, a hydrologist with the National Oceanic and Atmospheric Administration's Great Lakes Environmental Research Laboratory in Ann Arbor, Mich.
Surface water temperatures across the Great Lakes, he said, have soared. "And on and off, they've been pretty high over the past 10, 15 years."
In Lake Superior, researchers at the University of Minnesota Duluth have found that summer surface water temperatures have increased by 5 degrees over the past 30 years.
That's twice as fast as the air temperature has increased, and some of the most rapid change observed on the planet.
So what's causing such rapid change? It turns out there's a strong connection between the amount of ice on the lake the previous winter, and how warm the lake gets the following summer, said Jay Austin, a physicist with the Large Lakes Observatory at UMD.
"We use the term facetiously of course, but the lake has a memory. In that, summer water temperatures reflect what happened the previous winter," he said.
And it's in the winter that Minnesotans are really feeling the effects of climate change. The average winter temperature in the state has risen by about 1 degree per decade since the 1970s.
That might not seem like a huge change. But according to Austin, a relatively small temperature change has a huge impact on ice formation.
"The difference between one of those low-ice and high-ice years can be due to as little as 1 to 2 degrees Celsius in winter air temperature," he said. "So very small changes in winter air temperature can lead to large changes in the amount of ice that's formed."
And less ice in winter translates to warmer water the following summer.
Austin joked maybe he should work with Duluth tourism officials during warm winters.
"You know, prepare for a great swimming summer, because we can tell them that in like February or March," he said.
Austin also contributed to a study released late last year that compiled data from more than 200 of the largest lakes in the world, including Superior, that showed them warming an average of 0.61 degrees Fahrenheit each decade — with the most rapid warming occurring in lakes at northern latitudes.
But something else could be going on in addition to ice cover, Austin and others note.
Researchers noticed a dramatic shift in the Great Lakes region around 1998. Before that year, there were lots of high-ice years. But since, with the major exception of 2014, there have been hardly any.
"The speculation is that there was a decrease in cloud cover that was sustained after the late 1990s," explained NOAA's Drew Gronewold. "But we're still looking into that and some other factors as well."
Simply put, if there are fewer clouds, there's more sun warming Superior and the other Great Lakes.
So what's the upshot of warmer water temperatures?
Less ice cover could be a major boon for the Great Lakes shipping industry. Record ice cover in 2014 delayed the start of the shipping season and damaged ships trying to plow through thick ice sheets.
In Lake Superior, warmer surface water could also result in a more productive fishery for species like salmon and brook trout, said Cory Goldsworthy, Lake Superior area fisheries supervisor for the Minnesota Department of Natural Resources.
"If there was a warmer water layer near the surface for a longer period of time throughout the year," he said, "that would increase the amount of time those fish have to grow."
But it could also make the lake more hospitable to non-native species like alewife, he said, which could have a detrimental impact on native lake trout.
In the short term, what we do know is that warmer water makes the lake more hospitable for the human species.
But before stripping down to your swimsuit and running into Lake Superior, you should first know about a phenomenon known as "upwelling," when the wind blows the relatively thin layer of warm water off the surface, and cold water from below rises up to take its place.
"Suddenly, overnight the water can drop from 70 degrees — marginally comfortable swimming water" to 40 degrees, said UMD's Jay Austin. "Life-threateningly cold water, in a matter of a few hours. It's really remarkable how quickly these things move around."
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