Lake Michigan’s water levels have dropped below long-term averages for the first time in 10 years. Experts said factors like this fall’s warmer temperatures and low precipitation have affected lake levels — the same factors that make it tricky to predict this winter’s lake outlook.

In addition to seasonal variables, lake levels and ice formation will depend on how the cooling pattern from an emerging La Nina will play out in the Midwest.

“It’s a complicated picture,” said Lauren Fry, a scientist with the National Oceanic and Atmospheric Administration Great Lakes Environmental Research Laboratory.

Lakes Michigan and Huron, which are considered one lake because they are connected at the Straits of Mackinac, were about 2 inches below their historical monthly average in October, according to the U.S. Army Corps of Engineers. On all lakes, average water levels since the start of October range from 3 to 7 inches below what they were at the same time last year.

A few inches might not seem like a lot, but Fry said “it’s noteworthy” that October was the first time since 2014 that the monthly average was below the long-term average for any month. The lake’s all-time record low was in January 2013.

Great Lakes’ water levels are determined by the net basin supply. It’s the sum of precipitation and runoff, minus lake evaporation. When there’s more evaporation than inflow, lake levels drop.

Evaporation drives the annual declines during the fall, Fry said. Cold air mixing with the warm surface water temperatures on the lakes leads to energy transfer, resulting in evaporation.

That process has been enhanced this fall because warmer weather makes surface water temperatures higher. Combined with the season’s very little rainfall, evaporation only increases. Swaths of the Midwest are experiencing moderate to severe drought, according to the U.S. Drought Monitor.

“It’s just been really dry, and the warm conditions don’t help. And you get just less moisture in the basin as a whole,” Fry said.

Low lake levels can affect shoreline ecosystems, access to docks and shipping routes. Some research suggests that ships can lose up to $30,000 a trip because of the lighter loads they’re forced to carry in low water conditions.

The drought conditions are expected to last through the winter, according to NOAA’s winter outlook issued last month. This could translate to a late start to the ice season on the Great Lakes, said Bryan Mroczka, a scientist with the NOAA Great Lakes Environmental Research Laboratory who studies atmospheric conditions.

Similar to lake levels, ice formation on the Great Lakes depends on the amount of cold and warm air.

And the emerging La Nina will influence a lot of those patterns this year. Mroczka said it’s still too early to say what that means for temperatures.

In projections, Illinois and much of the Midwest sit squarely in an area that has an equal chance of experiencing either above or below average temperatures. However, there’s a high chance of more precipitation than normal later in the winter.

There aren’t clear indications of how strong this La Nina might be, Mroczka said. “We’re on that battleground this year.”

Ice is a natural part of the Great Lakes ecosystem. Some of the largest storms occur during the winter and fall, resulting in waves that naturally occurring ice can break up. Without it, there’s no protective barrier.

“The shoreline infrastructure, roads, piers, jetties, docks, whatever, are at the full mercy of those raging lakes,” Mroczka said. There’s also more erosion during the years with lower ice.

Recent trends point to more of these years, as warmer winters become more common due to climate change.

Last winter, Lake Michigan saw a maximum ice cover of 20%, which is about half the typical amount. Data from NOAA showed historic lows across all the Great Lakes, too.

Winters aren’t completely gone, Mroczka said. But we’ll likely see shorter bursts of cold air instead of the long durations that ice needs to form.

“(Lake) ice doesn’t form overnight, like it does on a pond in your backyard,” he said.

Water levels are also projected to increase under climate change. A 2022 paper estimated a 1.5-foot increase in the average levels of lakes Michigan and Huron by 2040 due to increased precipitation and runoff in the spring.

There might be an increase in average levels over the decades, but within those years there will always be seasonal fluctuations, said Miraj Kayastha, a doctoral candidate at Michigan Technological University and co-author of the 2022 paper.

It’s exemplified in the current downward trend of lake levels, which followed historic highs in 2019 and 2020, he said.

With any projection, there’s a degree of uncertainty as researchers work with the current data available.

But that’s not to say they’re inaccurate, or that they aren’t good gauges for where things are trending toward under climate change, Kayastha said.

“Our models are good, but future greenhouse gas emissions, you know, that is something that we don’t know,” he said.

Kayastha points to one certainty, though. “The only constant thing is change.”