Shrinking Lake Superior Also Heating Up

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"That lake is much, much more driven by what's happening with precipitation and evaporation, because that's where its input is coming from," he said.

Minnesota and Wisconsin, for instance, have been declared moderate to severe drought areas for the past 18 months.

And recent storms out of the Gulf of Mexico have been skipping the upper lakes and hitting lakes Erie and Ontario.

The Heat Is On

When Jay Austin arrived in Minnesota two years ago, he was surprised to find "remarkably few" studies on Lake Superior's temperature. So Austin, an assistant professor at the Large Lakes Observatory and the University of Minnesota, set out to do one.

His result was striking: As the climate over the Great Lakes region warms, Lake Superior is heating up at nearly twice the climatic pace.

One buoy in the lake recorded a surface temperature up to 75 degrees Fahrenheit (24 degrees Celsius) in summer, unprecedented for a lake that's notoriously cold year-round.

The findings were published earlier this year in the journal Geophysical Research Letters.

Rising summertime air temperatures are only part of the story, however.

Milder winters mean less ice on the lake surface, and less ice means less sunlight is reflected—and more is absorbed.

So summer on the lake, defined as the time when a cap of warm water sits on top of cooler water, comes up to 12 days earlier. That means the lake has more time to warm up in response to the warmer air.

"Humans rely on the lake level because it fundamentally changes how we interact with the lake," said Austin, who also teaches at the University of Minnesota, Duluth.

But for most of the lake's ecosystems, water level isn't as important as temperature.

"Everything in nature depends on temperature," he said.

Austin wants to do future research to determine the role of ice in both increased temperatures and lower lake levels.

So far, he's seen one clear trend: Less ice and higher temperatures both lead to faster evaporation.

Beneath The Surface

Noel Urban, an associate professor of environmental engineering at Michigan Technological University, studies the lake's more subtle workings.

He originally set out to research whether the lake might help absorb some carbon dioxide emissions, a greenhouse gas that contributes to global warming. (What is global warming?)

Instead, he found that Lake Superior produces more carbon dioxide than it absorbs.

It's unclear whether the increased carbon output is a result of rising temperatures or other climatic factors, and that's exactly what Urban and his colleagues aim to learn with new computer models in development.

"We would expect the increasing temperature to cause the lake to be more of a carbon source than a sink," he said.

That's because warmer temperatures provide a boost to both carbon dioxide-producing bacteria and oxygen-making algae, though bacteria tend to respond faster in heat.

A carbon sink is a reservoir that traps carbon dioxide, such as forests and oceans.

Austin's research also adds a wrinkle, because reduced ice cover may let fierce winter winds stir up more sediment or release more heat and cause sediment to settle out.

In the first scenario, carbon dioxide levels might drop—in the other, levels could rise even higher.

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