Navigating a New Era: The Transformation of the Great Lakes

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Two heatmap-style graphs illustrate monthly temperature extremes for Lake Superior between 1941 and 2021. The y-axis signifies months (1 to 12), while the x-axis displays years.

"Degree heating days (°C·days)" are represented in the heat waves illustration with a crimson color gradient, progressing from 0 (white) to 120 (dark crimson). Four distinct heat waves are noticeable prior to 1996, with five appearing after. Black lines mark "breakpoints" or significant trend shifts around 1970, 1986, 1996, and 2001.

Conversely, the cold spell illustration presents "Degree cooling days (°C·days)" using a cobalt color gradient. Darker shades of blue indicate more severe cold spells, with white symbolizing no anomaly and deep blue representing a -120 degree cooling day. Three pronounced cooling episodes were recorded in the 1940s and early 50s, but predominantly disappear until 1996.

The most severe heat waves and cold spells generally occur during the summer months for Lake Superior. — Red boxes represent the severity of heat waves, whereas blue boxes indicate the severity of cold spells on Lake Superior, which has experienced the fastest warming among the Great Lakes since 1940. Black lines denote “breakpoints” or considerable shifts in trends (similar graphs are available for all of the Great Lakes). Credit: Adapted with permission from Abdelhady, H.U., et al. Commun. Earth Environ., 2025, DOI: 10.1038/s43247-025-02341-x (Used under a CC BY 4.0 license)

Heat waves and cold spells are increasingly prevalent on the Great Lakes, as per U-M research, impacting the region’s climate, economy, and ecosystem

Heat waves and cold spells are integral to life around the Great Lakes. However, recent findings from the University of Michigan reveal that the situation today is fundamentally different compared to 30 years ago.

“The emergence of these extreme temperatures is on the rise,” stated Hazem Abdelhady, a postdoctoral researcher at the U-M School for Environment and Sustainability, commonly known as SEAS. “Most lakes have seen an increase of over 100% compared to pre-1998 levels.”

This timing is crucial as it aligns with the significant 1997-1998 El Niño, one of the strongest recorded events, he further explained.

To uncover this trend, Abdelhady and his team devised an advanced methodology for modeling the surface temperature of the Great Lakes, enabling them to investigate heat waves and cold spells dating back to 1940.

The surface temperature of the Great Lakes plays a vital role in weather patterns, a critical concern for local residents, visitors, and shipping companies in the region. Furthermore, the rise in extreme temperature phenomena could disrupt ecosystems and economies reliant on the lakes in less obvious ways, according to Abdelhady.

“Such events can drastically affect the fishing sector, which is a billion-dollar industry,” he noted.

According to the Great Lakes Fishery Commission, tribal, recreational, and commercial fishing in the Great Lakes collectively generates over $7 billion annually.

While fish can migrate to cooler or warmer waters to adapt to gradual temperature shifts, sudden fluctuations pose significant challenges, Abdelhady remarked. Fish eggs, in particular, are highly vulnerable to drastic temperature variations.

Prolonged hot and cold spells can also disrupt the natural mixing and stratification cycles of the lakes, impacting the health and water quality of the lakes that serve as vital resources for recreation and drinking water.

Having established these trends across the Great Lakes, researchers are now focused on projecting future extreme temperature events as the average temperatures of the lakes—and the planet—continue to rise. By examining these phenomena and their correlations with global climate events, like El Niños and La Niñas, they aim to enhance preparedness for their effects, Abdelhady explained.

“If we grasp these occurrences, we can begin strategizing on how to safeguard against them,” he elaborated.

The study was carried out through the Cooperative Institute for Great Lakes Research (CIGLR) and published in Communications Earth & Environment, part of the Nature journal series. The research received support from the National Science Foundation, its Global Centers program, and the National Oceanic and Atmospheric Administration (NOAA).

Two line graphs illustrate temperature variations on Lake Michigan from 1940 to 2020, with an emphasis on heat waves (top graph) and cold spells (bottom graph). The y-axis for both represents degree days, which is a combination of the duration and magnitude of a temperature anomaly. For heat waves, this is denoted through degree heating days, while for cold spells, it is absolute degree cooling (indicating that upward spikes signify a more severe cold spell).

In the heat wave graph, a red vertical line designates a break year in 1996. Two dashed lines represent median degree heating days, with values at 24.8 before the break and 58.2 afterward—a 134.8% increase. For cold spells, a red vertical line marks a break year in 1991, where the median cooling day value rose from 17.0 to 46.0, a 170.7% change.

Both graphs display fluctuating annual values with prominent spikes following their respective break points, indicating a rise in temperature extremes. — The frequency and severity of heat waves and cold spells in the Great Lakes, illustrated as spikes in these graphs, transitioned into a new pattern in the 1990s, per new research led by the University of Michigan. Utilizing a measurement known as degree days, which integrates the amplitude and duration of a temperature anomaly, researchers indicated that the median value more than doubled following a “phase shift,” depicted by a red line (similar diagrams are available for all of the Great Lakes). Credit: Adapted with permission from Abdelhady, H.U., et al. Commun. Earth Environ., 2025, DOI: 10.1038/s43247-025-02341-x (Used under a CC BY 4.0 license)

Emphasizing the grandeur of the lakes

A major challenge of this research was the sheer scale of the issue at hand. While researchers have created computer models that simulate processes in numerous lakes globally, the Great Lakes are exceptional.

For one, they comprise an interconnected network of five lakes. They also hold over a fifth of the world’s freshwater surface supply. Additionally, the length of their shorelines is comparable to the entirety of the U.S. Atlantic coast—including the Gulf states.

In many ways,
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The Great Lakes share more similarities with coastal oceans than with other lakes, remarked co-author Ayumi Fujisaki-Manome, an associate research scientist at SEAS and CIGLR.

“We cannot utilize the conventional, simpler models for the Great Lakes since they don’t perform adequately,” she stated.

To address this, Abdelhady applied modeling techniques typically used for studying coastal oceans and customized them for the Great Lakes. However, there was also a data obstacle to navigate in addition to the modeling issues.

According to Fujisaki-Manome, satellites have provided regular direct observations of the Great Lakes for about 45 years. Yet, when discussing climate trends and periods, researchers need to operate with extended time frames.

“The significant aspect of this study is that we successfully extended that historical period by nearly double,” Fujisaki-Manome stated.

By utilizing available observational data along with reliable inputs from global climate simulations, Abdelhady was able to model Great Lakes temperature data and confidently validate it back to 1940.

“That’s why we frequently utilize modeling. We aim to understand the past, predict the future, or explore a location we cannot physically access. For the Great Lakes, we encompass all three facets,” explained co-author Drew Gronewold, an associate professor at SEAS and a head at the Global Center for Climate Change and Transboundary Waters.

David Cannon, an assistant research scientist at CIGLR, and Jia Wang, a climatologist and oceanographer with NOAA’s Great Lakes Environmental Research Laboratory, also played a role in the study.

The research exemplifies how collaborations between academic institutions and government scientific agencies can generate a stream of knowledge that benefits the public and the wider research community, Gronewold noted.

The model developed by the team is now accessible for other research entities investigating the Great Lakes to explore their inquiries. For the U-M team, the subsequent steps involve employing the model to investigate spatial variations across smaller regions of the Great Lakes and using the model for future projections.

“I am quite interested in whether we can foresee the next significant change or tipping point,” Gronewold expressed. “We did not predict the last major shift. No one anticipated that in 1997, a warm-winter El Niño would alter everything.”

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Emphasizing the grandeur of the lakes

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