Sam Oliver/UW-Madison Center for Limnology
More frequent algae blooms are one consequence of warmer lake temperatures.
There were headlines this summer and fall when Great Lakes water temperatures reached record-level highs from over the past few decades.
Lake Michigan is above its long-term average by 3.81 degrees Fahrenheit. That may not sound like much, but, as the Detroit Free Press puts it, “Think about how long, and how much energy, it takes to warm that volume of water that many degrees.”
Similarly, Madison’s lakes are also warming, according to data accumulated over a century.
“Lake Mendota, at least 100 years ago, was frozen for an average of four months out of the year,” notes Dane County Executive Joe Parisi. “Now that average is three months out of the year.”
Because they are large, lakes tend to have consistent temperatures. And so temperature variations can be easier to spot in lakes.
“Because lakes have this big thermal mass, we can measure temperature change in lakes and typically see this clear warming pattern that would be quite a bit more variable in other systems — for example, in measuring the temperature above an agricultural field or elsewhere,” says Jordan Read, scientist at the Madison-area office of water information, U.S. Geological Survey.
Consistent Great Lakes temperature data go back only to 1979. We’re a lot better off locally. One of the benefits of having a research university on its shores is that Mendota serves as a living laboratory.
“Lake Mendota is often referred as one of — if not the — most well-studied lakes historically, on earth,” notes Read. “So we have temperature records that date back to at least the early 1900s, and I think there might even be a few from the late 1800s.”
These records show that the warming up of our lakes came relatively recently. “What we’ve seen with Mendota, and a lot more lakes with those longer-term records, is a pretty rapid rate of warming in the time period from about 1985 to present day,” says Read.
The Great Lakes are warming at rates that are slightly higher than those of inland lakes. “There are a number of different ideas as to why that difference exists,” he says. “One of them is the very big difference in ice cover on those large lakes.”
For example, Lakes Superior and Michigan some years can have lots of ice cover, and other years relatively little. “What that does is impact how the spring and then summer temperatures are going to set up,” he says. On inland lakes that freeze over every year, such as Mendota, variability of temperature is less drastic.
“Lake Mendota was included in a global study that we put together that looked at rates of warming across almost 300 lakes spread out over the globe,” says Read. The 2015 study included the Great Lakes but had a number of inland Wisconsin lakes as well. The study found that warming rates were on the order of .3 degrees centigrade per decade — or 1.5 to 2 degrees Fahrenheit over the past 30 years.
And the consequences? Warmer temperatures typically are associated with a greater frequency of algae blooms, which are bad for lakeshore property owners. Temperature also affects the growth and reproduction of fish.
“With warming in lakes, there are winners and losers,” says Read. “For example, if you’re really into largemouth bass, you would see warming lakes potentially as a good thing.”
Looking toward the future, Parisi notes, “this has a lot of implications for runoff into the lakes, for pollution and for flooding, and what we do about that.”
