The Great Lakes are getting hot

The Great Lakes are beckoning swimmers with higher-than-average temperatures. But not all species are warming to the trend.

Those of us who cottage on there typically consider ourselves cold-water warriors for our ability to swim in the frigid water. There have been years where it’s close to Labour Day before the lake feels more like a tepid bath than a recently melted snowbank.

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This year, however, we are without cold waters to test our mettle. It was only the second week of July before many of us noticed that the water felt decidedly temperate, even warm. And no, say the experts, we aren’t imagining it.

Surface water temperatures for the four smaller lakes (Erie, Ontario, Huron, and Michigan) are on average 3.3˚C to 5.5˚C warmer this time of year than usual. Superior is also warmer, but as a bigger and deeper lake, its temperature hasn’t increased as much. 

“The increase is very significant because it takes a long time for those Lakes’ surface water to warm up,” says Andrea Vander Woude, manager of the National Oceanic and Atmospheric Administration’s Great Lakes CoastWatch program and a satellite oceanographer. 

Lake Huron’s average temperature was at the high end of this range on July 9, reaching 22.3˚C, the warmest on record for early summer. Lake Ontario’s average was a scorching 25˚C the day later, on July 10. Lake Erie topped them all at almost 26.5˚C on July 10. 

But while fair-weather swimmers might rejoice, the high temperatures can negatively impact marine life, including cold-water species of fish that become squeezed between warmer surface temperatures and deeper water that lacks sufficient oxygen. Algae growth is also boosted by warm water temperatures. 

Blame (or credit, depending on your outlook) a high-pressure system that moved into the Great Lakes region in early July and sat there, warming the lakes, before being replaced by a low-pressure system with storms and cooler temperatures mid-month. 

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And this isn’t as warm as the lakes are expected to get, says Vander Woude, who notes that, historically, water in the Great Lakes reaches its high temperatures in August.

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