March 2024 eNewsletter

40 Years of Warming in the Laurentian Great Lakes: Using a Hydrodynamic Model to “Fill the Gaps” in a Sparse Observational Data Record

Observational studies in the Laurentian Great Lakes show dramatic changes in lake surface conditions over the last several decades. Warming surface temperatures and dramatic ice cover declines exacerbate a range of risks to the Great Lakes, including changes in the range and distribution of some species, increases in invasive species and harmful algal blooms, and declines in beach health. However, understanding changes in water dynamics below the surface is more complicated and challenging. Whereas lake surface water conditions can be observed remotely using satellites, measurements at depth need to be collected manually using instrumented buoys and moorings, which can be expensive and dangerous, especially during winter. Very little subsurface data is available for long-term trend analysis, with only a single mooring in existence ever recording more than a decade of measurements in the entire Great Lakes basin.

A spatial map showing the modeled rates of lake bottom temperature change in degrees C per decade from 1979 to present.

“This is a serious problem for natural resource management,” said CIGLR Assistant Research Scientist in Hydrodynamics David Cannon, PhD. “Subsurface temperatures and thermal structure act as a critical control on many ecosystem processes, with implications for dissolved oxygen, fisheries habitat, water quality, and even contaminant transport. Understanding how subsurface conditions are changing is crucial for managing the lakes under continued climate warming. Direct observations are the gold standard for climate change research, but we just do not have the observations we need to understand how the Great Lakes are changing, especially over large spatial scales. You can’t go back in time to invest in better long-term monitoring, but you can try to simulate changes using hydrodynamic models.”

In collaboration with Ayumi Fujisaki-Manome, PhD (CIGLR), Jia Wang, PhD (NOAA GLERL), and James Kessler (NOAA GLERL), Dr. Cannon helped develop a hydrodynamic model to hindcast thermal structure in the Laurentian Great Lakes. In doing so, the team was able to estimate historical lake-wide temperatures and ice cover going back to 1979, essentially “filling the gap” in the observational record.

Model results suggest that temperatures below the lake surface have increased significantly over the last 40 years, with all five Great Lakes warming at 50 meters depth by between 0.2 and 0.4 degrees Celsius per decade. “A 0.25 degree Celsius increase in temperature every 10 years might not seem like much,” said Cannon, “but it can really add up. The deepest waters in the Great Lakes are almost 1 degree Celsius warmer now than they were 43 years ago.”

This study also detected significant decreases in ice cover (between -1% and -10% per decade) and ice thickness (between -1 cm and -4 cm per decade) across the region, along with a general lengthening of the stratified summer season (between 4 and 14 days per decade) when surface waters are warmer than 4 degrees Celsius. “Together, these seemingly small changes in lake temperature and ice cover can have a large impact on lake ecosystems,” said Cannon. “For example, increasing bottom-water temperatures can drive fish out of their preferred habitats, effectively restructuring food-web dynamics in the lakes. This is especially concerning for historically productive fisheries like Green Bay in Lake Michigan and Saginaw Bay in Lake Huron, where model simulations showed near-bottom warming in excess of 1.4 degrees Celsius since 1979. One of our concerns is that increasing temperatures at the lake bottom may affect fish spawning habitats or the quality and viability of fish eggs. We’re hoping that these data might be useful for lake managers as they set fishing regulations and plan habitat restoration initiatives moving forward.”

The Great Lakes have changed considerably over the last several decades, but the climatic processes driving these changes are still unclear. “Our research was designed to investigate the root cause of warming in the region,” said Cannon. “When working with climate data, it’s really difficult to separate global warming driven by human activities from multi-decadal climate variability, which occurs naturally. The longer lake records produced by our simulations provide a great way to start separating natural and human-induced trends. In fact, preliminary analysis suggests that as much as 30% of the surface warming trends described in this study may be linked to naturally occurring climate oscillations, like the Atlantic Multi-decadal Oscillation. Does this mean global warming is not important? Absolutely not. It means that global warming does not tell the whole story. Long-term climate oscillations play an important role in regulating temperatures and ice cover in the Great Lakes, and they should not be ignored as we move forward with climate projections. Next step, modeling the future!”