Recent work has demonstrated that marine heatwaves (MHWs) have become longer and more frequent over recent decades and these trends are likely to accelerate in the future. Here we examined historical trends in MHW characteristics and identified regions where MHW changes are collocated with areas of high diversity value, species living at the upper level of their thermal tolerances, and regions concurrently impacted by non-climatic anthropogenic stressors.

In a global meta-analysis of 116 research papers, we examined responses of organisms, populations and communities to eight distinct MHW events. All events were associated with negative ecological impacts. Sessile species like corals were most heavily impacted while some mobile species experienced no deleterious impacts. Some fish responses to MHWs were positive, largely related to the invasion of tropical species into temperate regions.

A species-level analysis demonstrated that populations living closer to the warm limit of their species distributions are more negatively impacted by MHWs, as they are more likely to experience conditions that exceed their thermal tolerances.

Finally, an examination of three globally important habitat-formers found that increases in the frequency of MHW days was significantly associated with increases in coral bleaching, decreases in seagrass density and deceases kelp biomass, explaining between 30 to 40% of the ecological performance of these taxa.

Our study shows that MHWs cause widespread ecological impacts (i.e. across multiple taxa, regions and processes) and can drive step-wise shifts in ecosystem structure and functioning. Given that they are predicted to intensify with anthropogenic climate change, MHWs are likely to be prominent agents of ecological change in the coming decades.