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Severe cold snaps followed by snowstorms and then another blast of cold. For many, this Wisconsin winter can’t end soon enough. So when U.S. Rep. Mark Pocan, D-Madison, took to the floor of the House of Representatives on Jan. 16, 2014, a speech about global warming was the last thing many folks wanted to hear. Pocan, a member of the House’s Safe Climate Caucus, spent his one-minute speech ticking off what he said were clear signs of climate change. This claim caught our attention: Ice fishermen are already noticing fewer days they can be out on our ice covered lakes. It feels like Wisconsin has been frozen solid for months, and many folks wouldn’t be surprised to see ice fishermen tromping around on Memorial Day. But Pocan’s claim is about more than this winter. His statement is about long-term trends and what is driving them. Is he right about climate change and fewer days on ice-covered lakes? When we asked Pocan for backup for the claim, his staff pointed us to Climatewisconsin.org , a web site that highlights research and reports prepared by environmental groups. The site includes interactive features that focus on the two largest lakes in Madison -- Mendota and Monona. It includes the amount of time they have been covered by ice in past years. The records show significant year-to-year variability in the length of the ice-cover season, but there is a clear trend of fewer ice-cover days over time, the site says. Overall, the average number of days of ice cover on the Madison lakes has decreased by around 29-35 days over the past 150 years. Significantly, the longest ice seasons on record are all clustered in the first few years of the record, while most of the shortest seasons fall towards the end of the record. The site also includes this statement, which is pretty similar to that made by Pocan: With climate change models predicting warmer temperatures, we can expect to see a trend towards fewer days of ice-covered lakes as each year passes. In fact, with the aid of a remarkable dataset from Madison’s lakes, scientists infer that a change in ice duration has already taken place. Information posted on the site comes, at least in part, from the work of John J. Magnuson, Emeritus Professor of Zoology and Limnology, who has been at UW since the late 1960s and helped found the UW-Madison Center for Limnology in the early 1980s. Limnology is the study of freshwater lakes, and the UW center says it’s the birthplace of the field of study, dating back to the 1870s. Magnuson is an expert on the effects of climate change on freshwater lakes, and has published numerous academic papers on the effect of climate change on lakes and fish. In an interview, Magnuson said researchers around the world, including Canada, Finland, Sweden, Russia, Japan and Madison, have studied lake ice for years. Researchers note the day lakes freeze and the day the ice goes out. It’s a more official version of what is waged in a less academic way by lakefront bars that stage ice-out guessing contests. Madison researchers have records for Mendota and Monona that go back to 1855. The long term trend for Lake Mendota is that, on average since 1855, there are 29 fewer days that the lake is covered with ice, according to Magnuson. Six of the shortest seasons for Mendota have been in the years since 1980. Five have been since 1995, including the two shortest seasons ever -- 21 days in the winter of 2001-02 and 47 days in 1997-98. Lake Monona has seen on average of 35 fewer ice-covered days, and also has seen a cluster of shorter seasons in recent years. The breakup of the ice is occurring, on average, two weeks earlier. And the freezing of the lakes have come two weeks later, on average, Magnuson said. The result is that four months of ice is reduced to about three months. To be sure, Wisconsin has some 15,000 lakes, and we cannot check each one. This is highly variable with every water body having different characteristics, and is obviously very weather dependent as well, said Thomas Van Haren, of the state Department of Natural Resources Bureau of Law Enforcement. But it seems reasonable to focus on Lake Monona and Mendota given their size and because they are in Pocan’s district. So what about this throwback winter? Magnuson said it’s a statistical blip. Some years the ice stays around longer than others, but over time the trend has been toward shorter ice seasons. The variability is very high in terms of the length of the seasons, he said. Not only does lake ice respond to the warming climate, it also responds to local weather. Lake Mendota iced over Dec. 16, 2013 -- about a month earlier than the previous year and four days earlier than the median date. Lake Monona froze Dec. 10, 2013, compared with Dec. 31 the previous year and a median date of Dec. 15. National Weather Service records from the weather office in Sullivan indicate this was the 9th coldest winter on record, so far, in Madison, and the coldest in 35 years. The average temperature was 15.8 degrees below normal. This is the sort of winter that’s typical of 50 or 100 years ago, Magnuson said. Going global So what about the connection Pocan made between shorter ice fishing seasons and climate change? Magnuson said the shorter seasons are a symptom of global warming -- one consistent with many others, such as changed bird and butterfly migration patterns, plants that bloom earlier, and disappearing glaciers. We can’t take this ice data and say ‘therefore we say greenhouse gas is causing this,’ he said. That’s a question for the atmospheric physicists. Our data is consistent with what other people are finding, with the kind of things that atmospheric scientists are seeing from C02. He added: There’s abundant evidence that it’s getting warmer and that spring is coming sooner. Unfortunately, he’s talking about long-term trends. Not what is going to happen this year. Our rating It’s been a longer and colder winter than in recent years. But that doesn’t erase a trend that’s been well-established. The number of days that the lakes have ice on them -- making them safe for ice fishing -- has declined. We rate Pocan’s statement True.
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