Meg Jones

Milwaukee Journal Sentinel

Lake Mendota has frozen over, then opened, then refrozen this winter, a phenomenon that's alarming scientists who say within two or three decades there may be no ice at all.

Mendota hasn't closed twice since the winter of 2001-'02 when the Madison lake that creates one side of the city's famous isthmus was frozen for only 21 days. The only other time it has happened was 1984-'85.

This winter it officially closed on Dec. 15, reopened just six days later and then closed again on Jan. 10. Though the University of Wisconsin's Center for Limnology, whose building is located on Lake Mendota near the Memorial Union, tweeted that there was open water on Jan. 16, the lake wasn't officially considered open.

"It's unusual for the lake to freeze and open up, freeze and open up," said John Magnuson, UW-Madison limnology professor emeritus and director emeritus.

Lakes are, in a way, akin to canaries in coal mines — an advance warning that winters are getting warmer, which means less ice and snow. Lake ice is a sensitive measure of global warming.

"People will ask 'what's so bad about the lake not freezing?' People haven't looked very hard at what happens in an ice-covered winter compared to a non-ice-covered winter," said Magnuson, who came to Wisconsin in 1969 to work at the Center for Limnology, which studies freshwater lakes.

"There's an awful lot we don't know about what we're losing," Magnuson said.

Magnuson and other experts predict that, within the next two or three decades, Lake Mendota won't ice over at all.

Detailed statistics of Lake Mendota date back to the 1850s. Temperatures and weather conditions fluctuate each year, but the overall trend is fewer days of ice on Mendota.

In the 18 winters so far this century, not including this winter, Lake Mendota has been closed by ice an average of 85½ days. The previous 18 winters, Mendota was closed an average of 88 days.

The State Climatology Office determines when lakes Mendota, Monona and Wingra open and close, which Assistant Wisconsin State Climatologist Ed Hopkins admits is an inexact science. In general, it's when observers determine a lake is at least 50 percent covered by ice from specific vantage points, including the 13th floor of the Atmospheric and Space Sciences building on West Dayton Street where the State Climatology Office is located.

And the old rule of whether someone can row a boat between Picnic Point and Maple Bluff still applies for determining if Lake Mendota is iced over or not, Hopkins said.

Hopkins saw open water again on Jan. 16, but when it closed up within 24 hours, he decided it didn't qualify as the whole lake being open.

Lake Mendota has an average depth of 42 feet with a maximum depth of around 83 feet. Shallow lakes in general freeze quicker or more often than deep lakes because the deeper the lake, the more heat that must be lost before it can freeze.

Lake Mendota was a major source of commercial ice throughout the 1800s and early 1900s when ice was sliced out of the lake in the winter, packed with sawdust and sold throughout the Midwest. Ice fishing was a good source of protein during the winter, noted Magnuson, and a frozen Lake Mendota was a well-traveled highway because it was quicker than traversing muddy and frozen roads in horse-drawn vehicles.

Back then, Lake Mendota was frozen for more than 100 days each year and sometimes didn't reopen until early May. In 10 of the winters this century, Mendota has reopened in March. The latest the lake opened since 2000 was April 12, 2014, the winter a polar vortex hit Wisconsin and Mendota was closed for 117 days.

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"Mendota is one of the lakes we've used to point out in Wisconsin where we're losing winter as we knew it," said Magnuson.

"It's very clear in any scientific community, it's climate change, it's caused by greenhouse gases," Magnuson said. "One of the things that's very sensitive to warming is ice. Ice is a harbinger of what climate change will bring."