As Earth’s temperatures shift, seasons are changing — underwater, too

A new report analyzing 30 years’ of data collected hundreds of feet below the surface of Lake Michigan documents what scientists have suspected — the seasons are changing differently, even at the very depths of Earth’s fresh water lakes.

Eric J. Anderson is a physical scientist with the National Oceanic and Atmospheric  Administration’s Great Lakes Environmental Research Laboratory, and lead author of the study published in Nature Communications. 

“We found that this long-term data set not only confirms that Lake Michigan’s deep waters are warming, but also shows that winter is vanishing from them,” Anderson says.

“As climate change has gradually delayed the onset of cooler autumn weather over the past three decades,” he says, “the deep waters of Lake Michigan have reflected this change by showing shorter winter seasons.”

NOAA GLERL scientists deploy a thermistor chain for long-term temperature monitoring in southern Lake Michigan, 1999. Photo by NOAA GLERL

This key finding may be the harbinger of dramatic changes in the lake’s foreseeable future, including permanent changes in the way the lake’s layers mix — which in turn may alter the entire web of life its waters support.

Why do we care what happens down there?

Most of Earth’s fresh surface water is consolidated in just a few of its largest lakes and the Great Lakes make up the largest surface freshwater system in the world. 

As such, they have an enormous impact on the more than 34 million people who live within their collective basin, Anderson says.

So of course they have been the subject of extensive study via water sampling, buoys, and satellites as scientists try to determine how those impacts may shift as the earth’s climate changes. 

Will we face drought and water resource challenges in the future, or need to shore up coasts to preserve coastal communities? Can we identify and manage risks to fishing stock and lake ecosystems? As conditions change what will the effect be on commercial shipping? 

Documenting changes in the lakes may be crucial to helping address whatever the impacts of changes may be, Anderson says. 

But while scientists have compiled much information about the response of lake surface-water temperatures to climate change, their understanding of how deep-water temperatures in large lakes are responding has been limited. 

The Lake Michigan study presents a first-of-its-kind analysis of 3-hourly and hourly subsurface water temperature data over 30 years at depths of up to 360 feet.

The data shows temperatures inching upward over that time, and also provides precise measurements of the timing of annual “fall overturn,” the point of minimum temperature, and the duration of the winter cooling period in Lake Michigan — all of which give a clearer understanding of the lake’s underwater “seasons.”

The unique study goes deep

On land, Michigan’s changing seasons are not subtle. Winter ice and snows melt in the spring, buds burst into bloom, days lengthen and spring turns to long hot summer days. Then the days begin to shorten, nights get cooler, the leaves turn from green to brilliant orange and fall to the ground, where they will be covered by winter snow again. Surface waters freeze.

Though we can't see it here on land, the seasons change deep underwater, too.

In deeper lakes and in the ocean, water temperature conditions vary by depth and form layers, Anderson explains. From late spring through early fall, some lakes in temperate climates separate into three distinct thermal layers, initiated by warming of the surface of the water by the sun. Cooler, denser water settles to the bottom of the lake; a layer of warmer water floats on top, and a thin middle layer separates the top and bottom layers. 

In Lake Michigan, the water mixes from top to bottom twice a year; the standard pattern for Lake Michigan’s deep waters goes like this: 

Summer: Deep water temperatures remain fairly constant around 39 degrees Fahrenheit. 

Late fall:  Deep water temperatures spike by a degree or two as “warmer” waters mix from above. 

Winter: Deep water cools over the winter period down to temps of 34 - 37 degrees F, depending on the severity of the winter, before warming up again to 39 degrees F the following summer. 

“This temperature fluctuation helps us mark the “turnover” or complete-mixing timing in the lake, which is another way to mark the seasonal changes as seen by the deep waters,” Anderson says. “There is a lot of variability from year to year, but the trend is that winter temperatures are warming on the surface and also warming to a lesser extent in the deep waters.”  

Data tells the story

In 1990 NOAA Great Lakes Environmental Research Laboratory chose Lake Michigan to study in order to track climate signals in the lake. The location is convenient for set-up and maintenance of the equipment that such a study uses. The NOAA GLERL Lake Michigan Field Station is reasonably close by and just off the shore is the NOAA National Data Buoy Center surface buoy Station 45007 that gives surface conditions in the ice-free months. 

To collect the deep water temperature readings, scientists sunk chains equipped with special temperature monitors to take regular readings at various depths, day in and day out.

It was the charting of these readings, over time, that has allowed patterns to emerge, Anderson says.

“Monitoring programs like this one are not easy to maintain,” he says, “which is partly why this is unique.”

It’s still just the start

“Thirty years is generally the lower limit on analyzing long-term climate trends. But we can put these trends in context with other changes observed in the atmosphere or on the land/lake/ocean surface, which have much longer records,” Anderson says.

“The water temperature trends over this 30-year period coincide with similar trends in rising air temperatures and other meteorological changes.”
The study thus far confirms some of what scientists are finding in other studies, Anderson says.

“Importantly, it provides the measurement-based evidence. It also gives us new insight into how surface warming is affecting subsurface dynamics. 
What we learn in this location is useful for other large lakes, but we need observations in additional locations for analysis of climate trends for lakes around the world,” Anderson says. 

“We are trying to expand our network in the Great Lakes. This study highlights why high-frequency data like these are critical to documenting change in our largest lakes and providing communities with information needed to make decisions and plan adaptations strategies.”

What impact will these changes have?

These studies help people understand what already has happened and what is projected to happen, Anderson says, information that can improve our resilience to extreme weather and climate events.

 For instance, an increase in a lake’s overall water temperature can lead to changes that could eventually disrupt the structure of the lake’s entire food web — a change that could have negative impacts on fisheries and recreation, the report says. 

According to the Fourth National Climate Assessment report the Great Lakes are already at risk from rising temperatures, changes in seasonal stratification of lake temperatures, and increased summer evaporation rates. Added to that list are stresses from pollution, nutrient inputs that promote harmful algal blooms, and invasive species.

Canary in the coal mine

“Without high-frequency long-term monitoring of subsurface waters of the world’s deep lakes,” Anderson says, “we will be blind to the impacts of climate change on most of Earth’s fresh surface water.”

Anderson says the Lake Michigan study, “is one piece in an entire climate and trend narrative, but one that we haven’t had until now.”  

To read the study:
Link to NOAA Research, Climate-driven shifts in deep Lake Michigan water temperatures signal the loss of winter

Nature Communications: “Seasonal overturn and stratification changes drive deep-water warming in one of Earth’s largest lakes.”

Read more articles by Rosemary Parker.

Rosemary Parker has worked as a writer and editor for more than 40 years, most of that time in Southwest Michigan.