Subsurface heatwaves in lakes

nature climate change Article https://doi.org/10.1038/s41558-025-02314-0 Subsurface heatwaves in lakes Received: 14 May 2024 Accepted: 11 March 2025 Published online: 10 April 2025 Check for updates R. Iestyn Woolway 1 , Miraj B. Kayastha Haoran Shi 1 & Pengfei Xue 2,3,5 , Yan Tong 2,3 , Lian Feng 4 , 4 Lake heatwaves (extreme hot water events) can substantially disrupt aquatic ecosystems. Although surface heatwaves are well studied, their vertical structures within lakes remain largely unexplored. Here we analyse the characteristics of subsurface lake heatwaves (extreme hot events occurring below the surface) using a spatiotemporal modelling framework. Our findings reveal that subsurface heatwaves are frequent, often longer lasting but less intense than surface events. Deep-water heatwaves (bottom heatwaves) have increased in frequency (7.2 days decade−1), duration (2.1 days decade−1) and intensity (0.2 °C days decade−1) over the past 40 years. Moreover, vertically compounding heatwaves, where extreme heat occurs simultaneously at the surface and bottom, have risen by 3.3 days decade−1. By the end of the century, changes in heatwave patterns, particularly under high emissions, are projected to intensify. These findings highlight the need for subsurface monitoring to fully understand and predict the ecological impacts of lake heatwaves. Lake heatwaves, prolonged periods of anomalously warm water events, have recently become distinguishable features of lake temperature variability. Studies have demonstrated that climate change has driven a notable increase in the frequency, duration and intensity of these hot extremes1. Model projections suggest that, during the twenty-first century, lake heatwaves are likely to intensify, become longer lasting and their occurrence frequency is expected to increase1–3. The rise of unprecedented temperatures during a lake heatwave can benefit some aquatic species by expanding their thermal habitat4, but can be detrimental for others, particularly to those that live in regions close to their thermal limit5–7. Quantifying changes in lake heatwaves is thus critically important to anticipate the likely impact of climatic warming on lakes. Previous studies investigating the impacts of climate change on lake heatwaves have focused on surface conditions1,2. Most notably, lake heatwaves have been defined, like marine heatwaves8–11, as periods in which surface water temperatures increase above a seasonally varying 90th percentile threshold1. This one-dimensional (temporal) approach for describing a lake heatwave is important, not only for understanding how extreme lake surface conditions respond to climate change, but also for anticipating the likely impact of these extreme heat events on aquatic organisms that cannot or will not move. However, mobile aquatic species can respond to environmental disruptions, such as extreme surface temperatures, by relocating to favourable habitats12–14. In stratifying systems, bottom waters are often cooler than the lake surface and, if other environmental factors are favourable, many aquatic species could migrate to these deeper layers to escape surface thermal stress15. Moreover, the thermal response of lakes to climate change can differ considerably between surface and bottom waters, with the latter often, although not always16,17, experiencing a somewhat muted climatic response18–20. In turn, cooler water at depth could provide a potential thermal refuge for aquatic species as surface heatwaves become more common and intense1,14. However, unlike in marine systems21–24, the vertical dimension of lake heatwaves has not yet been considered, thus limiting our understanding of how lake environments below the water surface are responding to a more extreme world. This study aims to fill this knowledge gap by investigating the depth to which thermal anomalies associated with lake surface heatwaves penetrate, as well as exploring how this has changed over the historic period (1980–2022) and will probably change in the future School of Ocean Sciences, Bangor University, Menai Bridge, UK. 2Great Lakes Research Center, Michigan Technological University, Houghton, MI, USA. Department of Civil, Environmental and Geospatial Engineering, Michigan Technological University, Houghton, MI, USA. 4School of Environmental Science and Engineering, Southern University of Science and Technology, Shenzhen, China. 5Environmental Science Division, Argonne National Laboratory, Lemont, IL, USA. e-mail: ; 1 3 Nature Climate Change | Volume 15 | May 2025 | 554–559 554 Article (2080–2099). This study also introduces (1) the concept of subsurface heatwaves, defined as periods in which depth-specific water temperatures reach extreme levels and (2) the presence of a vertical thermal escape, instances where aquatic species could theoretically move to deeper water to escape the thermal stress of lake surface heatwaves. By focussing on key metrics used to describe the severity of lake heatwaves—their average duration, average intensity and cumulative intensity—this study evaluates how these extreme events have changed. A multimodel approach is followed to investigate changes in lake surface and subsurface heatwaves. To quantify these changes, simulated water temperature profiles available from ISIMIP2b25 were investigated, as well as lake temperatures simulated from a suite of independently developed one-dimensional lake models. Moreover, to capture changes in some of the largest lakes of the world, and to investigate within-lake variations in heatwaves, the outputs from a three-dimensional (3D) model of the Laurentian Great Lakes of North America26 was investigated. A vertical thermal escape from lake surface heatwaves Over the past four decades, lake surface heatwaves have occurred widely and exhibited a marked increase (Fig. 1 and Supplementary Figs. 1–4). These extreme hot events have occurred more frequently (7.8 ± 0.5 annual days decade−1) and experienced an increase in their average duration (2.1 ± 0.1 days decade−1). They have also become stronger with an increase in their average (0.4 ± 0.02 °C decade−1) and cumulative (5.8 ± 0.4 °C days decade−1) intensity (Fig. 1a–d and Supplementary Fig. 1). These lake surface heatwave metrics have undergone even greater change in some individual lakes (Supplementary Fig. 3 and Supplementary Tables 1–3), with prominent alterations in the Laurentian Great Lakes (Supplementary Fig. 4). Lake surface heatwaves in the Great Lakes have occurred more frequently (7.5 ± 1.03 annual days decade−1), become longer lasting (1.4 ± 0.36 days decade−1) and experienced an increase in their average (0.01 ± 0.03 °C decade−1) and cumulative (3.7 ± 1.05 °C days decade−1) intensities. Our study identified the presence of a vertical thermal escape in lakes, described as instances where motile aquatic species could theoretically move to deeper water to escape surface thermal stress. Several lake surface (...truncated)