The world’s lakes are warming up — even frigid Lake Superior — scientists warn.

Dozens of researchers pooled decades’ worth of data from hundreds of lakes and concluded that the world’s lakes are warming even more rapidly than the oceans or the atmosphere. The warmer waters threaten fish populations, ecosystems and fresh water supplies around the globe.

Closer to home, University of Minnesota Duluth Professor Jay Austin says the thick sheets of ice that blanketed Lake Superior for the past two winters did nothing to change the fact that Superior, like the other Great Lakes, is growing ever warmer.

“Lake Superior is one of the more rapidly warming lakes” among the 235 lakes in the study, Austin said. A two-degree temperature shift can mean the difference between an iced-over Superior or an ice-free lake, he said. “Relatively small changes can lead to large changes in systems that define our region. Duluth would be a fundamentally different place if Lake Superior never formed ice.”

The study, which was funded by NASA and the National Science Foundation, found that lakes have been warming by more than half a degree per decade. That might not sound like much, but when lakes warm up, toxic clouds of algae can bloom, fish habitats can be disrupted and invasive species currently held at bay by Superior’s inhospitable cold might be able to make themselves at home.

The lake study is the first of its kind to use both satellite temperature data and long-term ground measurements. More than 60 researchers surveyed more than 200 lakes that hold more than half the planet’s freshwater supply, using data that stretched back at least 25 years. Their findings were announced Wednesday at the American Geophysical Union in Washington, D.C.

“These results suggest that large changes in our lakes are not only unavoidable, but are probably already happening,” the study’s lead author, Catherine O’Reilly, an associate professor of geology at Illinois State University, said in a statement. O’Reilly’s research found that as lakes warm, their productivity declines.

A ‘climate antenna’

The world’s lakes are warming faster than the oceans or the atmosphere, Austin said. Unlike air temperatures, which can fluctuate wildly from day to day or even hourly, lake temperatures are stable, making them ideal systems for measuring climate change. It takes a significant shift to change the temperature of a lake — much as it takes as much energy to heat a pot of water on the stove as it does to heat an entire room.

“Obviously, Lake Superior is going to stay cold for a very long time,” Austin said. “But these lakes provide a sort of ‘climate antenna’ that allows us to look at these global trends.”

The current rate of lake warming — an average of 0.61 degrees per decade — carries the risk of a 20 percent increase in algae blooms, which deplete oxygen in the water and can be toxic to fish.

The lakes seem to be warming faster in northern climates like Minnesota, where lakes are losing their ice cover earlier.

In 2007, Austin and his colleagues found that the average summer water temperature on Lake Superior had risen more than 4 degrees since 1979.

“We have documented, since 1970, a significant reduction in the ice on Lake Superior,” despite the past two winters, when the ice was so thick that tourists could trek across the lake to gawk at the Apostle Islands ice caves along the Wisconsin shore. “It sounds a little bit hollow, after the last two winters when we had quite a bit of ice. … I’m not suggesting that we won’t see ice on Lake Superior again, but we are going to see more years like 2012 when we had no ice.”