Star Tribune:
“Across Minnesota, lakes are losing up to four days of ice every 10 years, according to the state climatology office. And it’s not just Minnesota: Rivers and lakes across the continent are tending to freeze later and thaw earlier. “You think of all the ways people interact with lake ice — skating, fishing derbies, iceboats,” said John Magnuson, an ecologist and limnologist at the University of Wisconsin, Madison. “And already, in some of these lakes you have about a month less to do it.”
... at the University of Wisconsin, the first school in the country to study the chemistry and makeup of inland lakes, scientists have been keeping that data for decades. The university has tracked ice coverage on Wisconsin lakes since the 1850s. In those days, ice harvesters needed to know when they could venture out on a lake to cut large frozen blocks to sell during summer; parents and school superintendents waited for ice roads that would connect Lake Superior’s Apostle Islands to the mainland.
The longer the data go back, the clearer the pattern becomes: Lakes have less ice now, said Magnuson, who is semiretired after decades with the school’s limnology office. Lake Mendota in Madison gets 30 days less ice coverage — a full month — than it did during the Civil War, the data show. And the thaws have been accelerating, Magnuson said. Six of Lake Mendota’s 10 earliest ice-outs have happened since the late 1990s.
Ice coverage is not just a function of temperatures in the spring and fall. It reflects two larger factors: the area’s average annual temperature and the depth of the lake. Every body of water is continually heating up through the summer. The deeper the lake, the more volume it has to collect and store that heat. The more warmth a lake stores, the longer it takes to freeze, Magnuson said.
For the first time, some of the deepest lakes in southern Wisconsin are starting to have years when they don’t freeze at all. As temperatures continue to climb, that will happen to more and more lakes in Minnesota as well, Magnuson said. The magic number seems to be about 47 degrees Fahrenheit. Once the average temperature for an entire year reaches that point, lakes that have historically frozen over every year start to see some years where they remain open.”
