How climate change is impacting the Great Lakes

The federal government recently released a National Climate Assessment report that broke down how our changing climate is expected to impact the planet region by region.

Jenna Jorns is one of the writers of the Midwest chapter of the landmark report. She is based at the University of Michigan’s School for Environment and Sustainability and serves as program manager for the Great Lakes Integrated Sciences and Assessments (GLISA) Center.

The Great Lakes affect our weather patterns in Michigan on a daily basis. They directly influence our temperatures, precipitation, and cloud clover throughout all four seasons. We are already starting to see a decline in ice cover and an increase in surface temperatures as a result of our warming climate. This trend is expected to continue and the changes could be very disruptive to the lake’s normal process.

The Great Lakes are ” thermally stratified” which means that their surface waters are much warmer than the deeper, cooler waters below. These layers typically mix in the spring and fall, in a process that brings oxygen-rich surface water down while pulling nutrient-rich deeper water toward the surface.This process, called “destratification,” is very important for a lake to maintain a healthy ecosystem.

Michigan Sea Grant describes the process of lake stratification as follows, “the warming of the surface of the water by the sun causes water density variations and initiates thermal stratification (Figure 1). Cooler, denser water settles to the bottom of the lake forming the hypolimnion. A layer of warmer water, called the epilimnion, floats on top. A thin middle layer called the metalimnion (or thermocline) separates the top and bottom layers and is characterized by a rapid change in water temperature. This separation often is strong enough to resist mixing of the layers by the wind.

The most extreme thermal stratification occurs within lakes during the warm summer months. During fall turnover, the epilimnion cools, sinks and falls below the thermocline, resulting in mixing. Thermal stratification of a lake depends on the lake’s depth, shape and size. Some small, shallow lakes may not experience seasonal thermal stratification because the wind mixes the entire lake. Other lakes, such as Lake Erie, have a combination of geographic location and water depth that regularly produces thermal stratification.”

Figure 1: Water Temperature and Lake Thermal Stratification. (Photo credit: Michigan Sea Grant)

Jorns explained that “as climate change warms the surface layer, it changes the frequency and the rate that those two layers mix. We’re seeing that that mixing is happening sooner, earlier in the year, and the projections show the mixing in the fall may actually not happen even at all. And we have seen that in Lake Superior.”

Jorns stated she hopes this is a “wake up call” to the citizens of the Great Lakes region given how important the lake’s are to the stability of our climate.

For information on how climate change could affect native and invasive species in the Great Lakes listen to Stateside’s conversation with Jenna Jorns.


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