As the world's largest lake warms, it is kicking up stronger winds. The extra-blustery weather on Lake Superior is causing faster currents, and possibly changing the ecology of the lake and the distribution of local air pollution.
So says a new study of Lake Superior — the world's largest lake by surface area, containing 10 percent of the unfrozen freshwater on the planet — published this week in Nature Geoscience.
"We're certainly moving away from a mental frame of mind that the lake is huge and has lots of inertia and is relatively insensitive to global changes to one where we're seeing all kinds of changes going on in the lake." said Robert Sterner of the University of Minnesota, Twin Cities, who was not a part of the study.
"Models are doing a credible job with predicting temperature, but a lot of variables are really tough to predict," he added. "Wind is potentially a really important factor in determining how the Earth will shape in the future."
Recent work by study co-author Jay Austin of the University of Minnesota, Duluth showed that Lake Superior's surface waters were warming 15 percent faster than the air above them and twice as fast as the nearby land.
The accelerated warming resulted from reduced ice cover on the lake, Austin found. Ice acts as a mirror, reflecting the sun's energy. With less ice, the relatively darker water absorbs more energy, creating an accelerating cycle of warming.
The researchers wanted to find out how the observed temperature rise was affecting other things, like the winds.
5 percent wind increase per decade
Using measurements from satellites and buoys placed around the lake, the team determined that average wind speeds on the lake had increased 5 percent per decade since 1985.
Winds are determined by temperature differences between the air and the water below. Typically, in summertime, the water is colder than the air above it, which creates stable atmospheric conditions and low winds, because the denser, cold air near the surface of the water is "content" beneath the less dense, warmer air above.
"As the lake is now getting warmer relatively quickly, that difference in temperature is getting closer to zero," said study lead author Ankur Desai of the University of Wisconsin, Madison. "That makes it less stable. As a consequence, the mean wind speed is increasing."
The gustier conditions caused a 10 percent increase per decade in the current speed on the lake, the researchers found.
Another possible consequence of the winds is a change in the depth of the warm, well-mixed layer of water that forms on top of the lake each summer, atop a layer that stays just below 40 degrees Fahrenheit year-round.
"With stronger winds you would expect the depth of mixing to increase," Desai said, which would spur the growth of organisms at the base of the lake's food chain. "When we look at the model, we see no change," Desai said. Measurements of the levels of phytoplankton in the lake have even been falling.
It appears that while stronger winds would increase the depth of the mixed layer by churning the water, the warming trend leads to a shallower layer, because the surface waters reach the temperature at which they stop mixing with the water below earlier in the season, before the heat has penetrated as deeply.
'Consequences' for marine life
Either way is likely to mean changes for the lake's organisms, Sterner said, many of which live just below the boundary between the warm upper layer and the colder deeper water.
"If we maintain the same depth of stratification, but we have a warmer, more turbulent upper layer, that will have its own set of consequences," he said. "I can't begin to tell you what this means for the organisms living at the transition, but it stands to reason that a change in the physical habitat will have effects on the food web."
"There may also be changes in how pollution is transported," Desai added. In particular, if a similar trend holds on Lake Michigan, pollution from Chicago which is blown north into Door County, Wis. along the lakeshore, could be affected.
"You could bring up more pollution. Or, you could be spreading it more. I'm not sure which would win," Desai said.