Historical exposure of Australia’s bioregions to marine heatwaves and cold spells with implications for management and climate-preparedness

Extreme temperature events pose a growing threat to marine ecosystems with cascading impacts on coastal communities and economies. Evaluating the historical exposure of marine regions to periods of temperature extremes provides critical context for understanding cumulated stress on ecosystems and prioritising areas for adaptive management. Using 43 years (1982–2024) of high-resolution (0.05° × 0.05° horizontal grid cell) daily sea surface temperature (SST) data, this study quantified marine heatwaves (MHWs) and marine cold spells (MCSs) across Australia’s 60 mesoscale marine bioregions by assessing event frequency, intensity and duration. To facilitate comparison among regions, we applied MHW and MCS exposure indices (MHW-EI/MCS-EI) that integrate multiple event characteristics. All MHW metrics displayed statistically significant positive trends, with over 95% of bioregions experiencing longer MHWs. The MHW-EI identified highly exposed regions in Western Australia, southeast Tasmania, and northeast Queensland, shaped by distinct global climatic and regional oceanographic features. In contrast, MCSs frequency and duration declined in 55% of bioregions, while exposure index analyses showed that several regions, such as King Sound, Cambridge-Bonaparte, Pilbara (nearshore), and Batemans Shelf, have been highly exposed to both warming and cooling extremes. Highly exposed bioregions showed an expansion of ‘Strong’ to ‘Extreme’ MHWs between 2015 and 2024, while the spatial extent of MCSs contracted. Together, these findings demonstrate a shift toward warming-dominated coastal climates, with intensifying heat extremes and spatially constrained cold events. The analysis approach is applicable to other geographic regions globally to compare regional thermal exposure, thereby supporting proactive climate-smart management across marine systems.