Marine heatwaves are defined as prolonged periods where the sea is anomalously warm (Oliver et al., 2021). Since Pearce et al. (2011) first used the term, there has been a rapid growth in our understanding of their causes and how they impact ecosystems. In recent decades, rising temperatures have caused marine heatwaves to become more and more frequent around the world (Oliver et al., 2019, 2018a, b; Marin et al., 2021; Scannell et al., 2016; Darmaraki et al., 2019; Yao et al., 2023). They have multiple causes, ranging from the occurrence of atmospheric heatwaves to changes in wind speeds and variations in ocean mixing (sen Gupta et al., 2020; Amaya et al., 2020).

Marine heatwaves have largely negative impacts on marine ecosystems (Smith et al., 2023). Negative impacts on seaweeds (Thomsen et al., 2019; Gurgel et al., 2020; Arafeh-Dalmau et al., 2019; Filbee-Dexter et al., 2020), seagrasses (Arias-Ortiz et al., 2018; Strydom et al., 2020), seabirds (Jones et al., 2018; Piatt et al., 2020), coral reefs (Leggat et al., 2019; Le Nohaïc et al., 2017), crustaceans (Chandrapavan et al., 2019), fish (Wild et al., 2019; Roberts et al., 2019), and plankton (Brodeur et al., 2019; Nielsen et al., 2021), among others, have been recorded. However, the impacts of marine heatwaves are complex and vary across ecosystems (Smale et al., 2017), and it remains unclear to what extent ecosystems can adapt to the continued rise in their frequency (Pershing et al., 2018).

The impacts of marine heatwaves on the sea surface and seafloor are likely to be distinct. Pelagic species and communities are typically advected or can swim across large regions and can therefore acclimate or reorganize in response to climate change relatively quickly. In contrast, benthic and demersal species tend to have limited mobility, such that tracking optimal thermal habitats, e.g. though the deepening of assemblages, is seen to be more challenging. It is thus more likely that negative impacts will be observed (e.g. Queirós et al., 2021, 2023), especially in species with slow growth rates, which slow the ability to respond to disturbances (Ceccherelli et al., 2007). Effective management of these species and habitats thus necessitates that projections of future marine heatwaves at both the ocean surface and the seafloor are considered. While most of the marine-heatwave assessments to date have looked at sea surface temperature data, recent studies suggest that sub-surface heatwaves can be equally relevant (He et al., 2024; Sun et al., 2023a).

A large body of work has used global climate models to project how future marine heatwaves are expected to increase this century (Azarian et al., 2023; Cheng et al., 2023; Oliver et al., 2019; Plech and Soares, 2019; Qiu et al., 2021; Rosselló et al., 2023; Sun et al., 2023b; Xue et al., 2023). However, global models are substantially limited in their ability to project marine heatwaves at both the sea surface and seafloor in shelf seas. Global models typically have a coarse spatial resolution, which can result in poor representation of ocean shelf processes, and the representation of critical processes, such as riverine inputs which affect key processes in shelf environments, is poor (Holt et al., 2017; Kay et al., 2023). In combination, this results in poor representation of processes such as seasonal thermal stratification of the water column, resulting in modelling projections which do not provide sufficiently credible representation of coastal processes. Finally, marine-heatwave calculations require daily temperature data (Hobday et al., 2016). These are available for the sea surface for many global climate models (Wilson et al., 2024); however, daily near-bottom temperatures are not available in those models, including the models used in the global climate change modelling standard, the Coupled Model Intercomparison Project Phases 5 and 6 (CMIP5 and 6). This leads to an unavailability of reliable ocean modelling projections from which to derive marine-heatwave estimations to inform marine science, policy, and industry development operating at the international, national, and sub-national levels.

The limitations of global climate models on continental shelves can largely be resolved using dynamical downscaling (e.g. Drenkard et al., 2021), whereby a regional model with a higher horizontal and vertical resolution is used to project future changes in ocean physics and biogeochemistry (e.g. Butenschön et al., 2016). Through this approach, a regional model is forced at the boundaries, i.e. at the air–surface boundary and the geographic limits of the regional model domain, by the outputs of a global model, offering a higher resolution of processes within the region of interest whilst being consistent with the broader processes simulated in the global model. Here, we apply this approach to understand future marine heatwaves on the northwestern European shelf.

The northwestern European shelf is an important marine ecosystem that provides ecological, cultural, and economic services to many countries (Culhane et al., 2020). It is one of the world's most heavily fished regions (Halpern et al., 2008) and has the world's largest and fastest-growing offshore wind industry. Understanding how climate change will impact the region is thus critical to enable effective spatial and fishery management into the future (Queirós et al., 2021). Due to its shallow bathymetry and the complex circulation system (Holt et al., 2010), the drivers of marine heatwaves in this region are likely to differ significantly from those in more studied oceanic systems (Jacobs et al., 2024). Importantly, marine heatwaves are likely to be due to the combined impacts of atmospheric (Berthou et al., 2024) and regional changes (Mohamed et al., 2023), necessitating the use of high-resolution regional modelling to understand their present-day drivers and future evolution.

In the northwestern European shelf, a suite of dynamically downscaled projections exist and have been used to consider the impacts of climate change on sea level rise (Hermans et al., 2020), changes in oxygen concentration (Wakelin et al., 2020; Galli et al., 2024), changes in circulation patterns (Holt et al., 2018), and the impacts of climate change on primary production (Holt et al., 2016). These have been used further to consider the impacts on higher trophic levels and the human uses of the marine environment, such as fisheries, aquaculture, and conservation, as well as on the full marine ecosystem (Palmer et al., 2021; Queirós et al., 2021, 2023; du Pontavice et al., 2023; Townhill et al., 2023).

The key aim of this study is to understand how future marine heatwaves will change in northwestern Europe under a high-emission greenhouse gas scenario. We separately assess the future evolution of marine heatwaves at the surface and the seafloor and, furthermore, disaggregate changes seasonally. This assessment will provide a more nuanced and complete understanding of future marine heatwaves in shelf seas.

In line with expectations, all models project an increase in sea temperatures across the northwestern European shelf by the end of the 21st century (Fig. ). Reflecting the fact that climate models have broad climate sensitivities (Scafetta, 2022), projected temperature increases vary across the ensemble. The lowest projected increase in annual average surface temperature on the shelf between 1990–2009 and 2080–2099 is 1.34 °C in GFDL-ESM2M, and the highest is 4.14 °C in the MIROC-ESM model, with an average of 2.55 °C across the five-member ensemble. Notably, across all models, there is a higher level of warming in the seasonally stratified surface waters, which is notable in the northern North Sea (Fig. ), compared with the seafloor. This reflects how increased thermal stratification (Holt et al., 2022a) will result in less warming translating to deeper waters in the future compared with today. Across the ensemble, surface warming during summer is projected to be 0.88 °C higher than at the seafloor, while differences in warming in winter are negligible (Fig. S1 in the Supplement).

This large level of ocean warming translates to a large and sometimes rapid increase in annual marine-heatwave frequency across the northwestern European shelf across model runs (Figs. –, Table ). In line with warming levels, the MIROC-ESM run projects the highest levels of heatwave frequency, with these occurring almost year-round by the end of the 21st century across much of the shelf at the sea surface. Similarly, the GFDL-ESM2M model projection shows the lowest future frequency of marine heatwaves, but it still projects that surface heatwaves will occur for approximately 63 % of the year at the end of the 21st century on average across the shelf. The shelf average annual frequency of marine heatwaves at the surface in the mid-century ranges from 29.5 % to 90.5 %, and that at the end of the century ranges from 63.3 % to 99.0 %. Similarly, the annual frequency of marine heatwaves at the seafloor in the mid-century ranges from 39.7 % to 96.4 %, and that at the end of the century ranges from 77.2 % to 99.7 %.

While the pattern of increased heatwave occurrences is evident at both the sea surface and seafloor, the projections show that the increase is expected to be larger at the seafloor. This is reflected by mapped changes (Fig. ) and time series of average changes in heatwaves (Fig. ). Overall, the model projections show a clear pattern where there are minimal differences in terms of projected heatwaves at the sea surface and seafloor during winter when stratification is negligible, while there are pronounced differences in summer when waters are stratified (Figs.  and ). We illustrated these differences by identifying the first 20-year period when, on average, surface waters are under heatwave conditions for at least 50 % of the summer. This highlights the magnitude of differences when temperatures will exceed upper thermal thresholds for species. Figure  shows that, during these 20-year periods, summer heatwaves in deep waters are much more frequent than at the surface. The most extreme difference is the projection from IPSL-CM5A-MR, which shows that, in 2060–2080, on average, the sea surface will be under heatwave conditions for 50.4 % of the summer, but the seafloor will be under heatwave conditions for 89.9 % of the summer. Furthermore, these differences are most pronounced in seasonally stratified waters, while permanently mixed waters, such as the southern North Sea, see smaller differences between the surface and seafloor.

This difference between heatwaves at the surface and seafloor runs counter to the fact that warming is projected to be higher at the surface than at the seafloor. Instead, it should be viewed as consequence of the lower natural inter-annual variability of temperature at the seafloor. This is shown in Fig. , which shows how much warmer, on average, heatwave temperature thresholds are than the average temperature in the models and observational data. In seasonally stratified waters, the 90th-percentile threshold is much lower relative to average conditions at the seafloor than at the surface. Consequently, it takes lower warming levels to translate into more frequent heatwaves. Critically, we also find that the ability of the models to represent inter-annual variability in surface temperature varies, as measured by comparing the temperature anomaly of the 90th-percentile threshold with an observational figure (Fig. ). Specific geographic features of individual projections may therefore be partly due to biases in the representation of inter-annual variability. For example, in the northern North Sea, the GFDL-ESM2M has lower inter-annual variability in summer than what is observed, which partly translates into higher future surface heatwaves than would otherwise be expected. These differences highlight the importance of an ensemble approach to projecting heatwaves.

These differences in inter-annual variability have further consequences. Summer temperatures vary more than during the rest of the year (Fig. S2). As a consequence, we find that heatwaves increase in frequency more slowly during summer than during the rest of the year (Figs.  and , Table ). This is most pronounced at the surface, where, across the ensemble, sea surface heatwaves occur, on average, 40.2 % of the time during summer and 52.8 % of the time annually in 2040–2059 and 73.1 % of the time during summer and 85.6 % of the time annually in 2080–2098. The impact of these differences is even more pronounced when the severity of heatwaves is considered (Fig. ), where extreme sea surface heatwaves are much less likely in summer than they are on average. Notably, in four of the model projections, extreme surface heatwaves are less than 50 % as frequent in summer than during the rest of the year at the end of the century. This indicates that marine heatwaves need to be categorized at a seasonal level to accurately assess their impact.

Up until now, most research projecting future marine heatwaves in the world's oceans has focused on the sea surface, and changes are typically not disaggregated seasonally. We have shown that this may lead to an incomplete picture in shelf seas. On the northwestern European shelf, future seafloor heatwaves are expected to grow at a significantly faster rate than at the sea surface, while summer heatwaves are expected to be less frequent than during the rest of the year. This has nuanced implications for marine ecosystems.

Our study highlights the complex relationship between rising temperatures and marine heatwaves and how focusing on temperature change alone may mask many nuances. We find that, across the northwestern European shelf, seafloor temperatures are projected to rise at a lower rate than at the surface due to increasing thermal stratification (Holt et al., 2022a). However, a lower level of warming on the seafloor may not translate to lower levels of future marine heatwaves. In fact, due to the lower variability of seafloor temperatures, we find that lower levels of warming are required to shift them to heatwave conditions compared to in surface waters and, therefore, that it is more likely that seafloor habitats will experience more frequent heatwave events than the surface ocean in the region. This indicates that assessing future heatwaves purely based on surface waters is likely to significantly underestimate future heatwave impacts on many benthic communities in shelf waters. This finding has important implications for seafloor communities and processes in the region, especially for species where population dynamics and lower genetic plasticity resulting from millions of years of more stable thermal conditions may limit the ability of communities to adapt (Somero, 2010; Hiddink et al., 2014).

A critical finding in this study is that summer heatwaves in northwestern Europe are projected to be relatively less frequent and potentially significantly less severe than during the rest of the year. This is not because summer warming is expected to be less severe per se but rather because of how heatwave calculations are affected by present-day temperature variability. This indicates that assessing the impacts of marine heatwaves purely based on annual changes may overestimate their impacts based on the assumption that marine heatwaves primarily have a significant negative impact in summer. To date, assessments of the ecological impacts of marine heatwaves have largely focused on warm summer months (Smith et al., 2023) when temperatures reach their highest; the data regarding the potential ecological heatwaves in cooler months are still much sparser (e.g. Shanks et al., 2020). It is therefore critical that research establish whether heatwave impacts largely reside in summer months when temperatures are highest.

Increasing attention has been placed on defining marine heatwaves in ways meaningful to ecosystem management, with some arguing that a shifting baseline should supplement the fixed-baseline approach (Amaya et al., 2023a, b), with the latter being carried out here. A shifting-baseline approach is likely to be of significant value for pelagic ecosystems, which, as suggested by marine ecosystem models, can reorganize very rapidly in response to climate change. Pelagic ecosystems are likely to be adapted to a climate resembling a shifting baseline. However, it is unclear if the shifting-baseline approach is sensible for all ecological systems and, in particular, benthic systems. Seabed benthic species may have a poor ability to track more suitable habitats in response to extreme weather events due to organisms having low dispersal potential or being adapted to historically stabler, deeper water environments (Somero, 2010). This may partially explain why seabed invertebrates in the North Sea have been found to have a poor ability to shift range distributions in response to seabed temperature changes (Hiddink et al., 2014).

Identifying regions that are more resilient to climate change is an important task in providing scientific projections of relevance to marine management (Queirós et al., 2021). Notably, our multi-model ensemble shows that some seafloor regions, such as that of northwestern Ireland and the area west of the Norwegian Trench in the North Sea, are less impacted by future marine heatwaves than most of the northwestern European shelf. This finding indicates that some regions could be reasonably resilient to future heatwaves.

Our study highlights the importance of looking at heatwave impacts on waters deeper than the sea surface. The general conclusion of the study that seafloor heatwaves are likely to be greater in future than at the surface is likely to be transferable to most shelf seas due to the common existence of seasonal stratification. However, future research should consider the magnitude of the difference in other regions. Furthermore, our conclusion sits within the context of a growing body of research indicating that the sub-surface heatwaves have grown more rapidly than those at the surface (Sun et al., 2023a; He et al., 2024).

This study provides the most comprehensive assessment to date of future heatwaves on the northwestern European shelf. However, a number of aspects require future attention. For ecosystems and species that can reorganize quickly in response to climate change, projections using a shifting baseline (Giménez et al., 2024) may be more informative. Secondly, many species have critical temperatures, which can either trigger key life cycle events (Wilson et al., 2024) or represent thermal limits above which there is rapid mortality or tissue damage (Savva et al., 2018). Projecting future heatwaves based on known thermal thresholds for key species is therefore the next step in understanding how marine ecosystems will change in the future.