The new norm: Climate change, marine heatwaves and the impact on aquatic systems

As we see out the last of summer in the northern hemisphere, it seems a poignant time to take stock of the climate emergency and the impacts it is having on both us and the natural world. In the United Kingdom, the summer of 2026 is provisionally the hottest on record, for now, with the season's mean temperature made 130 times more likely due to human-induced climate change according to the Met Office (Met Office, 2026b). Just the May and June heatwaves alone are thought to have caused 2877 excess deaths in the United Kingdom (UKHSA, 2026), a number that rises to 35,000 when looking at the figures until August in a broader European region (Henley, 2026). One particularly alarming phenomenon associated with the climate crisis is the influence it is having on fundamental oceanic processes. Around the United Kingdom, this has manifested itself in an increasing number of marine heatwave (MHW) events (Met Office, 2026a) with associated implications on marine ecosystems. At this year's FSBI symposium in Southampton, we heard from keynote speaker Bryce Stewart, who gave a particularly striking example of unprecedented common octopus (Octopus vulgaris) blooms off the southwest coast of the United Kingdom. Commercial landings of octopus in 2025 were almost 65 times greater than the annual landings from 2017 to 2024, with negative ramifications for brown crab (Cancer pagurus), European lobster (Homarus gammarus) and king scallop (Pecten maximus) fisheries (Stewart et al., 2026). MHWs on the other side of the Atlantic are also causing large shifts in marine ecosystems, and not just on the range expansion of more tropical species. In this issue, Desforges et al. (2026) combine acoustic telemetry with sea surface temperatures to understand how Arctic char (Salvelinus alpinus) are responding to MHWs. By comparing detections not in MHWs with those experiencing MHWs, they show that, physiologically, internal temperatures of the char were warmer (+0.64°C) in MHWs. Behaviourally, smaller fish were more likely to occupy estuaries and fjords compared to coastal or headland areas. Larger fish also migrated back to rivers earlier in the season during a year characterized by frequent and intense marine heatwaves. These fundamental changes to physiology, habitat use and life history show complexities of the impacts caused by MHWs, while also demonstrating that climatic impacts on freshwater and marine ecosystems do not happen in isolation to one another.