The unprecedented marine heatwave that swept through the northern European seas in June 2023 has now been rigorously analysed by a collaborative team of scientists from the University of Exeter, the Met Office, and Cefas. This extreme event, characterized by a remarkable 2.9°C rise above the long-term June average sea surface temperature, persisted for an extraordinary 16 consecutive days across the shallow waters bordering the UK, including the North Sea and the Celtic Sea. While on the surface this temperature anomaly appears extraordinary and previously unseen in observational records, the team’s extensive model simulations reveal that such marine heatwaves, once considered rare, are becoming increasingly frequent and should be anticipated in present-day climatic conditions due to ongoing anthropogenic influences.
Marine heatwaves are episodes where sea surface temperatures soar about a standard threshold for an extended period, causing multifaceted impacts on marine ecosystems. In this case, the June 2023 heatwave had dramatic effects on the timing and intensity of phytoplankton blooms—a critical component of the marine food web responsible for driving primary productivity and oxygen generation. Disruptions in phytoplankton can cascade through the marine food chain, impacting fish populations, marine mammals, and human industries such as fisheries and aquaculture. Additionally, prolonged thermal stress during such heatwaves can elevate concentrations of harmful bacteria, amplifying risks to human health through contaminated seafood and degraded water quality.
Emerging from the research is a stark indictment of how climate change is not a distant future threat but a present-day reality reshaping marine environments. The team’s climate model simulations, built upon an ensemble approach to robustly capture uncertainty and variability, illustrate that the probability of experiencing heatwaves like June 2023 has risen dramatically within the last three decades. Specifically, in the Celtic Sea off Ireland’s south coast, the annual likelihood for such an event ascended from a modest 3.8% in 1993 to an alarming 13.8% currently. In the central North Sea, this probability shifted from a mere 0.7% to nearly 10%. These quantitative shifts help frame how steady global warming triggers an exponential rise in extreme ocean temperature anomalies.
Importantly, the research team underscores marine heatwaves’ role in terrestrial weather phenomena. Warmer seas act as vast heat reservoirs, intensifying the thermal energy exchange between ocean and atmosphere. This oceanic heat directly contributes to escalating land temperatures around adjacent coastal regions, such as the British Isles. Furthermore, the capacity of warmer air to carry increased moisture fosters enhanced precipitation cycles, which was observed in tandem with the marine heatwave—resulting in both record-breaking temperatures and unprecedented rainfall during the affected period. This synthesis of ocean-atmosphere interaction exemplifies the interlinked nature of Earth’s climate components responding to localized marine overheating.
The unprecedented heatwave event catalyzed a surge in public and scientific awareness regarding the intensity and immediacy of marine heatwave hazards in European shelf seas. These findings carry profound implications for marine management, conservation strategies, and coastal planning, as heightened frequency of extreme thermal stress can undermine biodiversity and ecosystem resilience. Species adapted to more temperate conditions face elevated physiological stress, which can trigger shifts in species distributions, altered reproductive cycles, and increase vulnerability to disease outbreaks—a triad of stressors that collectively jeopardizes marine ecosystem stability.
The methodological backbone of the study involved leveraging a large suite of climate model simulations—spanning historical data and projected scenarios—to ascertain the evolving risk profile of extreme marine temperature events. The approach incorporated future climate trajectories and accounted for internal variability, enabling researchers to parse signs of climate-change-driven trends from natural oceanographic fluctuations. This advanced modeling effort demonstrated that the recent prevalence of marine heatwaves is consistent with steady anthropogenic warming patterns rather than being anomalous outliers, marking a significant advancement in attributing marine extremes to climate change drivers.
Given the widespread ecological and socio-economic consequences of marine heatwaves, the study calls for intensified interdisciplinary research to decipher the long-term impacts across European North-West shelf seas. Current knowledge gaps persist regarding how recurrent extreme ocean temperatures influence ecosystem services such as fisheries productivity, carbon cycling, and habitat integrity. Furthermore, the interactions between warming, acidification, and deoxygenation precipitated by climate change compound the complexities marine organisms must withstand. Addressing these knowledge gaps will require coordinated monitoring programs, enhanced ocean observing systems, and the integration of biological, chemical, and physical data streams.
The June 2023 marine heatwave has emerged not only as an isolated anomaly but as a portent of the broader climatological shifts reshaping Earth’s marine environment. The study provides a critical framework for anticipating future occurrences and underscores the necessity for proactive adaptation measures within marine policy and coastal community planning. As marine heatwaves become increasingly embedded in the climate baseline, understanding their dynamics and consequences assumes paramount importance for safeguarding both natural ecosystems and human livelihoods dependent on ocean health.
Dr. Jamie Atkins, who spearheaded the study during his doctoral research at the University of Exeter, emphasizes a vital nuance: while the extreme nature of the heatwave gained intense media attention, its occurrence aligns with expectations for a world already warmed beyond pre-industrial levels. This underlines a paradigm shift in climate risk communication—from viewing such extremes as rare aberrations to recognizing them as emergent norms necessitating responsive strategies. Professor Adam Scaife, a co-author and Head of Long Range Forecasting at the Met Office, further highlights the exponential increase in extreme climatic events fueled by incremental warming, reflecting the nonlinear sensitivities embedded in Earth’s climate system.
While the study’s focus was geographically centered on the Celtic and North Seas, its findings resonate globally, revealing mechanistic links between marine thermal extremes and atmospheric feedbacks. The ramifications stretch beyond European waters, providing a template for assessing marine heatwave risks in other continental shelf regions vulnerable to rapid climatic shifts. By melding observational data with comprehensive climate modeling, the study exemplifies the rigorous scientific inquiry necessary for decoding the evolving face of oceanic extremes amid a warming planet.
This research project was supported by funding from the Natural Environment Research Council (NERC) via the GW4+ Doctoral Training Partnership, underscoring the importance of sustained investment in scientific training and environmental research. The full study titled “Recent European marine heatwaves are unprecedented but not unexpected” is detailed in the journal Communications Earth & Environment and offers a crucial foundation for steering future investigations aimed at enhancing resilience to ongoing climatic transformations in marine domains.
Subject of Research: Ocean temperature anomalies, marine heatwaves, and their relationship with climate change in northern European seas.
Article Title: Recent European marine heatwaves are unprecedented but not unexpected
News Publication Date: 7-Oct-2025
Web References:
- DOI: http://dx.doi.org/10.1038/s43247-025-02802-3
- NERC GW4+ Doctoral Training Partnership: https://www.nercgw4plus.ac.uk/
Keywords: Ocean temperature, Climate change, Oceanography, Heat waves
