Lake Superior heats up

Jay Austin had uncovered a mystery. Call it the mystery of the overheated lake. Austin's story starts with him poring over decades of data, showing the climate around Lake Superior getting warmer.

"And what you see is sort of a gentle warming up until about 1940, very little warming between 1940 and 1980, and then things just go bananas from 1980 to the present," says Austin, a Duluth professor and a researcher with the University of Minnesota-Duluth's Large Lakes Observatory.

The air temperature part is no mystery. The climate has warmed measurably just like that in many places worldwide. Climatologists are calling it global warming.

"But what surprised us is that Lake Superior's water temperatures were warming around twice as fast as that," Austin says. "And it's, as far as I know, one of the largest changes in temperature of any natural system that's been observed over the last 25 years."

In that time, Austin says, the average surface temperatures have gone up more than 4 degrees farenheit. That may not seem like a lot, but, for a lake the size of Superior, it's a pretty rapid increase. And it's having dramatic effects.

"The date of what we call the spring overturn, has been getting earlier in the year," Austin says. "It's basically the start of the summer season in the lake. It's when you start to develop strong positive stratification: warm water sitting on top of cool water."

"What you see is sort of a gentle warming up until about 1940, very little warming between 1940 and 1980, and then things just go bananas from 1980 to the present."

In two decades, that spring turnover has moved up two weeks from early July to mid-June.

And, Austin says, the speed of the warm up is probably caused by ice - or specifically a lack of ice. Records show an ongoing decline in ice cover that can be documented since the 1980s.

"Ice is really special because it is very reflective," says Austin. "Normally ice would have snow on top of it, and is very good at reflecting sunlight back out into space. So if you take the ice away, the lake is much better at absorbing heat."

That, in turn, speeds up the spring turnover and gives surface water more time to warm up. At some point, Austin says, the lake will stop heating up quicker than the air around the lake, when ice stops forming. The researchers are focusing on the year 2040.

"Lake Superior will see very little ice," Austin says. "That's not to say it will never freeze over. It's just means, on average, we're going to see very, very little ice cover on (Lake) Superior, in about 35 to 40 years."

Warm water and little or no ice sounds great for the people who swim or sail Lake Superior.

But it may not be good for the plants and animals that make Lake Superior home.

There's a theory that a warming lake could hurt the lake's native whitefish, according to Steve Colman, who directs the Large Lakes Observatory.

"If there's less ice over time, and there appears to be, there's a chance for greater storminess in the sort of shallow water (bays) that the whitefish spawn in," Colman says. "So, this has been pointed out as one particular, potential major biological impact on one of the more charismatic species that lives in the Great Lakes."

Typically, warming speeds up growth for fish and the plants they feed on. But rapid change can wreak havoc, according to Bob Sterner, a University of Minnesota biologist.

"So though you might initially guess that a warmer lake would be better for the organisms in it, (that) everything from algae to fish might grow a little faster if the lake warmed a little," Sterner says. "Paradoxically, you may well see the lake essentially becoming even more desert-like in the sense that you've reduced the flow of nutrients into the system across that temperature gradient."

Throw in the potential effect on exotic and unwanted species, like zebra mussels or sea lamprey, and who knows what you'd get.

And the effects aren't limited to Lake Superior. The researchers suspect any lake losing ice cover is warming faster than the air. But they've only got the data for Lake Superior.

The research will be published soon by the American Geophysical Union. Next the Duluth scientists will try to prove their suspicion that decreased lake ice might be a major reason why Lake Superior's surface keeps dipping lower; another trend well documented since about 1980.

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