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1. Subsurface heatwaves in lakes

   https://doi.org/10.1038/s41558-025-02314-0  

   Woolway, R Iestyn; Kayastha, Miraj B; Tong, Yan; Feng, Lian; Shi, Haoran; Xue, Pengfei (May 2025, Nature Climate Change)

   Abstract 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. 

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2. Spatiotemporal Evolution of Marine Heatwaves Globally

   https://doi.org/10.1175/JTECH-D-23-0126.1  

   Scannell, H A; Cai, C; Thompson, L; Whitt, D B; Gagne, D J; Abernathey, R P (December 2024, Journal of Atmospheric and Oceanic Technology)

   Abstract The spatiotemporal evolution of marine heatwaves (MHWs) is explored using a tracking algorithm called Ocetrac that provides the objective characterization of MHW spatiotemporal evolution. Candidate MHW grid points are defined in detrended gridded sea temperature data using a seasonally varying temperature threshold. Identified MHW points are collected into spatially distinct objects using edge detection with weak sensitivity to edge detection and size percentile threshold criteria at each time step. Ocetrac then uses 3D connectivity to determine if these objects are part of the same event, but Ocetrac only defines the full MHW event after all time steps have been processed, limiting its use in predictability studies. Here, Ocetrac is applied to monthly satellite sea surface temperature data from September 1981 through January 2021. The resulting MHWs are characterized by their intensity, duration, and total area covered. The global analysis shows that MHWs in the Gulf of Maine and Mediterranean Sea are spatially isolated, while major MHWs in the Pacific and Indian Oceans are connected in space and time. The largest and most long-lasting MHW using this method lasts for 60 months from November 2013 to October 2018, encompassing previously identified MHW events including those in the northeast Pacific (2014–15), the Tasman Sea (2015–16, 2017–18), and the Great Barrier Reef (2016). Significance StatementThis study introduces Ocetrac, a method to track the spatiotemporal evolution of marine heatwaves (MHWs). It is applied to satellite sea surface temperature data from 1981 to 2021. The method objectively identifies and tracks MHWs in space and time while allowing for splitting and merging. The resulting MHWs are characterized by intensity, duration, and total area covered. Marine heatwaves can have significant ecological consequences, including biodiversity loss and mortality, geographical shifts, and range reductions in marine species and community structure changes when physiological thresholds are exceeded. This results in both ecological and economic impacts. Ocetrac provides a method of tracking the space and time evolution of MHWs that can provide a visualization that demonstrates the global impact of these events. 

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3. Megacities are causal pacemakers of extreme heatwaves

   https://doi.org/10.1038/s42949-024-00148-x  

   Yang, Xueli; Wang, Zhi-Hua; Wang, Chenghao; Lai, Ying-Cheng (February 2024, npj Urban Sustainability)

   Abstract Global climate change has been shown to cause longer, more intense, and frequent heatwaves, of which anthropogenic stressors concentrated in urban areas are a critical contributor. In this study, we investigate the causal interactions during heatwaves across 520 urban sites in the U.S. combining complex network and causal analysis. The presence of regional mediators is manifest in the constructed causal networks, together with long-range teleconnections. More importantly, megacities, such as New York City and Chicago, are causally connected with most of other cities and mediate the structure of urban networks during heatwaves. We also identified a significantly positive correlation between the causality strength and the total populations in megacities. These findings corroborate the contribution of human activities e.g., anthropogenic emissions of greenhouse gases or waste heat, to urban heatwaves. The emergence of teleconnections and supernodes are informative for the prediction and adaptation to heatwaves under global climate change. 

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4. Mechanisms by which marine heatwaves affect seabirds

   https://doi.org/10.3354/meps14625  

   Piatt, JF; Arimitsu, ML; Thompson, SA; Suryan, R; Wilson, RP; Elliott, KH; Sydeman, WJ (June 2024, Marine Ecology Progress Series)

   Marine heatwaves (MHWs) are characterized by periods of extreme warming of local to basin-scale marine habitat. Effects of MHWs on some seabirds (e.g. mass die-offs) are well documented, but mechanisms by which seabirds respond to MHWs remain poorly understood. Following from a symposium at the 3^rdWorld Seabird Conference, this Theme Section presents recent research to address this knowledge gap. Studies included here spanned one or more MHW event, at spatial scales from individual seabird colonies to large marine ecosystems in subtropical, temperate, and polar oceans, and over timespans from months to decades. The findings summarized herein indicate that MHWs can affect seabirds directly by creating physiological heat stress that affects behavior or survival, or indirectly by disrupting seabird food webs, largely by altering metabolic rates in ectothermic prey species, leading to effects on their associated predators and prey. Four main mechanisms by which MHWs affect seabirds are (1) habitat modification, (2) physiological forcing, (3) behavioral responses, and (4) ecological processes or species interactions. Most seabird species have experienced limited effects from MHWs to date, owing to ecological and behavioral adaptations that buffer MHW effects. However, the intensity and frequency of MHWs is increasing due to global warming, and more seabird species may have difficulty coping with future heatwave events. Also, MHW impacts can persist for years after a MHW ends, so consequences of recent or future MHWs could continue to unfold over time for many long-lived seabird species. 

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5. More Than Marine Heatwaves: A New Regime of Heat, Acidity, and Low Oxygen Compound Extreme Events in the Gulf of Alaska

   https://doi.org/10.1029/2023AV001039  

   Hauri, Claudine; Pagès, Rémi; Hedstrom, Katherine; Doney, Scott C.; Dupont, Sam; Ferriss, Bridget; Stuecker, Malte F. (February 2024, AGU Advances)

   Abstract Recent marine heatwaves in the Gulf of Alaska have had devastating impacts on species from various trophic levels. Due to climate change, total heat exposure in the upper ocean has become longer, more intense, more frequent, and more likely to happen at the same time as other environmental extremes. The combination of multiple environmental extremes can exacerbate the response of sensitive marine organisms. Our hindcast simulation provides the first indication that more than 20% of the bottom water of the Gulf of Alaska continental shelf was exposed to quadruple heat, positive hydrogen ion concentration [H⁺], negative aragonite saturation state (Ω_arag), and negative oxygen concentration [O₂] compound extreme events during the 2018–2020 marine heat wave. Natural intrusion of deep and acidified water combined with the marine heat wave triggered the first occurrence of these events in 2019. During the 2013–2016 marine heat wave, surface waters were already exposed to widespread marine heat and positive [H⁺] compound extreme events due to the temperature effect on the [H⁺]. We introduce a new Gulf of Alaska Downwelling Index (GOADI) with short‐term predictive skill, which can serve as indicator of past and near‐future positive [H⁺], negative Ω_arag, and negative [O₂] compound extreme events near the shelf seafloor. Our results suggest that the marine heat waves may have not been the sole environmental stressor that led to the observed ecosystem impacts and warrant a closer look at existing in situ inorganic carbon and other environmental data in combination with biological observations and model output. 

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