Normally frigid Lake Superior has warmed up faster than usual this summer due to a winter with little ice and a record-warm spring, according to researchers.
Surface temperatures are about 20 degrees higher than normal for this time of year and could be on their way to record highs, researchers at the University of Minnesota Duluth's Large Lakes Observatory said.
That's good news for people who want to swim in the typically bone-chilling waters, but the long-term implications aren't clear.
In the spring, the sun warms the water fairly uniformly as deep and shallow water mix. Once it reaches about 39 degrees, however, the behavior of the water changes and warmer water starts to form a layer floating on the colder water below. The process, known as lake turnover, usually happens in mid-July on Lake Superior, but this year it happened in early to mid-June instead.
UMD researcher Jay Austin said data from three buoys in the lake show that the warm-up is on par with 1998, the fastest since records began being kept in 1979.
"We would normally just be getting to turnover, to 39 degrees, about this time in July," Austin said. "But it happened so early this year that we're already at 59 degrees (at a western Lake Superior buoy near the Apostle Islands). That's 20 degrees warmer than we should be right now."
The surface temperature of Lake Superior could reach record highs by mid-August, the typical peak for water temperature, they said.
In 1998, a warm year, the lake peaked at 68 degrees, "and we're well on our way to that, or higher," Austin said. That compares with a high of just 54 degrees in August 1996, a cool year.
Climate change is responsible for the warming effect, the researchers said.
"Lake Superior is responding to global climate shifts as clearly as anywhere on Earth," they said.
Scientists are just starting explore what effect the warmer water might have, but it could mean a more fertile lake with more organisms that thrive in warmer conditions. And it could cause "cascading biological effects to fish and other species that we can surmise but haven't confirmed as yet," said Steve Colman, director of the Large Lakes Observatory. For example, lake trout may have to move deeper or further offshore.
A 2007 study by Austin and Colman found that Lake Superior's summer water temperatures were warming twice as fast as air temperatures over the past 30 years, based on buoy data. They considered it one of the most pronounced temperature increases on the planet.
They say there's a self-perpetuating correlation: The warmer the air and water, the less ice forms. The less ice, the warmer the water gets. Then less ice forms next winter.
"There's a climate momentum going on out there," Colman said. "The traditional thought was that there really wasn't any carry-over from one year to the next with this kind of system. But it looks like there is."
The warmer water also means warmer breezes for people on shore. Now, even when winds are off the big lake, they're more refreshingly cool than chillingly cold. Alex Lamers, meteorologist with the National Weather Service in Duluth, said that's meant narrower air temperature differences in the past from near the lake to farther inland.
Chris Chandler, 39, of Duluth, said Thursday he has been coming to the beach on Duluth's Park Point all his life, and that the lake is "definitely warmer earlier."
In past years, he said, there might be a day here and there when the water felt warm, but otherwise it would be cold. "We'd come here, but we'd be blue," he said.
This year, "it's cold at first, and once you're in it for a minute it's very nice."
Austin said UMD researchers don't track water temperatures in the other four Great Lakes.
(Copyright 2010 by The Associated Press. All Rights Reserved.)