In a groundbreaking advancement in our understanding of freshwater ecosystems, scientists have unveiled compelling evidence of subsurface heatwaves occurring in lakes across the globe. These hidden pulses of extreme warmth beneath the surface challenge long-standing assumptions that lake heatwaves are predominantly surface phenomena. Leveraging cutting-edge climate simulations and sophisticated lake models, the study illuminates the complex thermal dynamics at play beneath the water’s surface, offering new insights into how warming trends impact these critical environments. This revelation promises to reshape how researchers and policymakers approach freshwater management under climate change.
The research draws upon an unprecedented dataset generated through large-scale climate model simulations spanning over 16,000 lakes worldwide, ranging from the mid-latitudes to near polar regions. These simulations, part of the ISIMIP2b project’s lake sector, integrate bias-corrected climate projections to forecast lake temperature profiles from 1980 through 2099 under various greenhouse gas emission scenarios. By applying the SimStrat-UoG one-dimensional model to a globally representative suite of lakes, the study captures the nuances of vertical temperature variations across diverse climatic zones with striking detail.
Notably, the selection process for the lakes prioritized those typically shallower than 60 meters, aligning model constraints with the physical characteristics of the studied bodies. This threshold excludes deeper lakes where vertical mixing processes and heat distribution follow markedly different patterns. Moreover, the focus on lakes with at least two months of annual ice-free conditions ensures the relevance of heatwave dynamics to the biologically active seasons when aquatic organisms are most vulnerable to thermal extremes. The attention to depth-dependent resolution in temperature profiling—from fine 0.1-meter intervals near the surface to coarser resolutions at depth—further refines the fidelity of simulated data, allowing researchers to probe how heatwaves manifest and evolve vertically.
While the global-scale analysis offers a broad overview, the study’s most intriguing insights emerge from detailed investigations of 53 individual lakes, each examined through independent modeling efforts tailored to their unique features. For the Laurentian Great Lakes, whose vast extents and considerable depths pose challenges for simple modeling approaches, a state-of-the-art three-dimensional coupled lake-atmosphere model was deployed. This framework integrates atmospheric feedbacks and internal lake dynamics, thereby capturing the intricate processes governing thermal stratification, mixing, and ice cover over four decades of historical and projected climate scenarios.
In contrast, 42 smaller lakes predominantly in Europe and North America were simulated using an ensemble of one-dimensional models known for their robust representation of vertical temperature gradients. These models accommodate the diversity of bathymetric and thermal regimes found among lakes of differing size and climate, ensuring that heatwave metrics derived from simulations reflect real-world variability. To broaden the geographic and environmental scope, six additional lakes, including high-altitude lakes from the Tibetan Plateau, were simulated with the FLake model. This model excels in representing lakes in remote or extreme settings, accounting for factors such as snow and ice cover, and offering computational efficiency suitable for regional to global scales.
Central to the research is the quantification of lake heatwaves based on rigorous statistical thresholds. Following established methodology, heatwaves are identified when daily lake temperatures exceed the local, seasonally varying 90th percentile for a minimum of five consecutive days. Such criteria capture ecologically meaningful extremes rather than transient fluctuations. Importantly, the analysis distinguishes between heatwaves experienced at the lake surface and at various subsurface depths, revealing patterns of vertical propagation and refuge zone dynamics. The concept of thermal escape depth—defined as the depth below which water temperatures remain below the heatwave threshold—emerges as a critical parameter for understanding the habitat availability for aquatic organisms during these stressful events.
The study also reveals that heatwaves can compound vertically, with simultaneous extreme warming at both the surface and bottom waters. This phenomenon has profound implications for lake ecology, as it constrains species’ ability to find suitable thermal refuges within the water column. The global dataset assembled here serves as a valuable resource for examining these vertically compounding heatwaves across a diversity of conditions, promoting new perspectives on risk assessment and vulnerability mapping for freshwater ecosystems under climate change.
Besides external thermal forcings, internal lake processes such as stratification and mixing critically modulate when and where subsurface heatwaves occur. Lakes that are thermally stratified display distinct layers—the warm epilimnion, the thermocline characterized by a sharp temperature gradient, and the cold hypolimnion beneath. The study uses well-established criteria for stratification, applying temperature differences greater than one degree Celsius between surface and bottom waters as a threshold. Stratification breaks down the uniformly warm column characteristic of mixed lakes, creating complex vertical temperature profiles where subsurface heatwaves might be decoupled from surface extremes. The analysis leverages specialized tools and physical criteria to measure mixed layer depths, revealing how the thermal architecture of a lake influences heatwave penetration.
To interrogate temporal relationships, the authors conducted event-based correlation analyses comparing the intensities of simultaneous surface and subsurface heatwaves across lakes. These Pearson’s correlation coefficients quantify synchronization, while accounting for short time lags. Such statistical examination elucidates whether subsurface heatwaves lag or co-occur with their surface counterparts, offering mechanistic clues about heat transmission through the water column and the potential for delayed thermal stress to benthic communities.
Underlying the diversity of lakes and modeling approaches is an emphasis on rigorous evaluation and validation. For instance, GLARM simulations of the Great Lakes integrate atmospheric reanalyses (ERA-Interim and ERA5) and downscaled climate projections, ensuring that historical conditions are realistically reproduced and future scenarios are grounded in robust physics. Similarly, the FLake model parameter sets were carefully calibrated using in situ observations, with error criteria established to constrain simulated temperatures across depths and seasons to within 2°C median absolute error. Such diligence increases confidence that modeled heatwave metrics genuinely reflect physical phenomena rather than model artifacts.
Beyond advancing fundamental understanding, the study’s insights carry urgent ecological and socio-economic ramifications. As lake temperatures warm not only at the surface but also at depth, thermal refuges that aquatic organisms historically have relied upon during hot spells may become increasingly rare or altogether absent. This vertical homogenization of extreme heat could exacerbate stress on fish, invertebrates, and microbial communities, disrupting trophic interactions, biogeochemical cycles, and ecosystem services such as water quality and fisheries productivity. Recognizing subsurface heatwaves as a pervasive yet often overlooked hazard thus compels a reevaluation of conservation and management strategies for freshwater resources worldwide.
Moreover, the geographic breadth of the dataset, spanning from temperate to Arctic and high-altitude lakes, showcases that subsurface heatwaves are not isolated occurrences but part of a global pattern. This universality underscores the pressing need to integrate vertical thermal dynamics into climate impact assessments and adaptive planning. The incorporation of diverse model types suited to different lake characteristics exemplifies innovative approaches to enhance spatial coverage without sacrificing physical realism. As computational capacity grows and observational networks expand, such integrated modeling frameworks may serve as critical tools for monitoring and forecasting climate-driven ecological risks in freshwater systems.
Looking ahead, the authors advocate for intensified observational efforts to capture subsurface temperature profiles with higher vertical and temporal resolution, facilitating model validation and refinement. Emerging technologies such as autonomous profiling floats and remote sensing of lake thermal structure hold promise for addressing current data gaps. Coupled with advances in ecological modeling, these developments could enable predictive assessments of species vulnerability and ecosystem tipping points linked to heatwave dynamics beneath the water surface. Ultimately, bridging models and observations will be paramount to anticipating and mitigating the cascading effects of climate change in inland waters.
In summary, this pioneering research sheds light on the hidden dimension of lake heatwaves that lurk beneath the surface. By unveiling the vertical complexity of warming events in freshwater ecosystems, it complements and augments existing knowledge focused predominantly on surface waters. The findings trigger a crucial paradigm shift, emphasizing that protecting aquatic life and water resources requires attention not only to surface thermal extremes but also to the less visible, yet ecologically consequential, subsurface heatwaves. As climate warming accelerates, comprehending and managing these submerged threats will be essential to safeguarding the health and function of lakes worldwide.
Subject of Research:
Lake thermal dynamics and subsurface heatwaves under climate change
Article Title:
Subsurface heatwaves in lakes
Article References:
Woolway, R.I., Kayastha, M.B., Tong, Y. et al. Subsurface heatwaves in lakes. Nat. Clim. Chang. (2025). https://doi.org/10.1038/s41558-025-02314-0
Image Credits:
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