Climate change & variability: where does 2011’s late freeze rank?

Credit: UW Communications

We here at the CFL have been eagerly awaiting word so we could anoint the winner of our annual Lake Mendota “Ice On” pool. Well, the state climatology office has made it official – our fair lake finally donned its winter mantle of ice on January 14th. (Congrats are in order to payroll and benefits specialist, Val Seidel, who correctly guessed when Lake Mendota would be “frozen from Picnic Point to Maple Bluff and 50% covered).
While this winter offered an unusually long wait for lake ice, it wasn’t record breaking. That distinction goes to 1931 when Madison residents had to wait until January 30th before they could entertain thoughts of ice fishing, snowshoeing or skiing on the Yahara chain’s largest lake. This year is tied with 1999 as the third-latest “ice on” date. A chart, kept by the North Temperate Lakes Long-Term Ecological Research site can be found here.
But this year’s late date is another data point supporting a long-term trend – Madison lakes are getting ice later, and they’re thawing out sooner. Consider this ice on data from the last 150 years.
Historic "Ice On" dates on Lake Mendota since 1853. Credit: Aaron Stephenson

As you can see on the graph, “ice on” dates are all over the place. For example, right after 1931’s record late freeze, in 1932 Lake Mendota froze over earlier than average on December 10th. Such variability, or “noise” on the graph, makes it hard to find patterns. Thankfully, Lake Mendota has a hundred and fifty years’ worth of records.
John Magnuson, director emeritus of the Center for Limnology and our resident ice expert, says the long history of record keeping may have something to do with a bygone industry.
“Each one of our lakes in the late 1800’s had an ice storage facility,” he remarked in a recent talk about this late-arriving winter. “Individuals would make a living going out and sawing chunks of ice out of our lakes, dragging them up and burying them in sawdust in these storage facilities.” Those chunks of ice were used in the summer to cool that revolutionary precursor to the refrigerator, the ice box.
“it was not made in an adjacent freezer,” Magnuson says, “That ice was made on a lake and people cut it. We harvested it. Madison even used to ship ice to New Orleans.”
These ice houses were a form of income and essential to storing perishable food like milk and other creamery products, so residents were interested economically on when the ice formed and how soon they could go out and cut.
Another reason for keeping ice records could’ve been for transportation, Magnuson says. In the horse and buggy days, it was far quicker to pull a sleigh across Lake Mendota than head around the shore when trying to get somewhere on the other side.
Of course there’s also always the “Leopold affect.” People often keep records of things like the first pasque flower bloom or the first robin’s arrival and, perhaps, these phenological observations were reason enough to start charting lake ice.
For whatever reason, we’re lucky to have the resulting data. With so many variables like El Nino events or sunspots or North Atlantic oscilations to consider, divining a trend amid the noise requires a long dataset. “There’s so much noise in this system,” says Magnuson, “that even if you use our fancy statistics, you can’t even see a trend until you get out about 50 years or so.”
With 150 yearts at our disposal, though, the signal is unmistakable. Lake Mendota is, on average, freezing over later and thawing out earlier.
Taken alone, Magnuson says, this is not much evidence of anything. But add Lake Mendota’s century and a half of data to other temperate lakes with similar seasonal changes, and the evidence is undeniable – climate change is changing our lakes. It might not explain all of the changes and it certainly can’t be blamed on the year to year variability, but the trend that emerges is one where early freezes happen later and the late freezes happen, well, even later.
“We often use the lake as a very simple example of what climate change is doing,” Magnuson says. “You don’t need a thermometer. All you need is a good pair of eyes and a high enough location. You need to be a curious person about the seasons around you and just write down the ice date every year”