Winters on the Great Lakes are getting shorter.
A recent study published in the Environmental Research Letters found the lakes are losing an average of 1.43 days of ice-cover and near freezing surface water temperatures each year.
That adds up to about two weeks every decade.
“That number wasn’t expected,” said Eric Anderson, the study’s lead author and an associate professor with the Colorado School of Mines. “Even though there’s some consistency with what we knew, I think it still came as a bit of a surprise.”
Adding to existing research
The study adds to a growing body of research documenting the impacts of climate change on the Great Lakes.
Data collected by the National Oceanic and Atmospheric Administration shows ice cover on the lakes has been declining by about 5% every decade since the 1970s, when more reliable satellite data first became available.
“We know globally, lakes are losing ice,” Anderson said. “But that's really just one narrow view of what winter is if we think about winter from a lake’s perspective.”
So, in addition to measuring ice cover, this study also measured areas of the lakes with temperatures below 2 degrees Celsius, or about 36 degrees Fahrenheit.
“We wanted to look at areas that maybe don't get ice most winters,” Anderson said. “Lake Ontario, for instance, rarely freezes to a significant degree. But that doesn't mean it's not losing what we might consider winter.”
How surface temperatures affect lake mixing
Anderson says it’s important to consider these near-freezing surface water temperatures because when water cools past 4 degrees Celsius (about 39 degrees Fahrenheit), its behavior changes.
It starts to become less dense and more buoyant — which is why ice floats.
“That can really impact how the wind puts energy into the lake,” Anderson said. “If the lake is the same temperature from top to bottom, the wind can really do a good job of mixing that water and circulating it. And so that pushes oxygen through the lake column and pushes nutrients around.”
But colder surface temperatures in the winter make it harder for wind to churn the water, limiting how much oxygen and nutrients move down through the lakes.
As the lakes lose winter days, they’re gaining more of these “mixed” days, and that has ramifications for algae blooms, the lakes’ food web and the region’s $7 billion fishing industry.
The study didn’t make any projections for the future, but Anderson says he thinks it’s likely the trend of shortening winters on the Great Lakes will continue.
“Projections are showing that temperatures in the wintertime and really around the year tend to be increasing,” he said. “So we expect these trends to continue. The question will be at what rate.”
