Comparison of Compound Marine Extremes Under Overshooting vs straight-stabilization Scenarios

Marine heatwaves and biogeochemical extremes such as deoxygenation and acidification are intensifying with climate change, becoming more frequent, persistent, and spatially extensive. Of particular concern are compound events, simultaneous extremes in multiple stressors, which can interact nonlinearly and trigger severe ecosystem disruptions. While their occurrence under long-term warming is increasingly documented, much less is known about their evolution in overshooting scenarios, where global temperatures temporarily exceed the Paris Agreement’s 1.5 °C target before declining through large-scale deployment of carbon dioxide removal (CDR). Such pathways raise critical questions about whether and when marine stress conditions can return to earlier states. Here we use simulations from the Horizon Europe project RESCUE (Response of the Earth System to overshoot, climate neutrality and negative emissions), which develops pairs (overshoot vs straight-stabilization) of novel socio-economic scenarios incorporating a broad portfolio of CDR strategies and arriving at the same cumulative carbon budget by the end of the century. We assess differences in the spatial patterns, frequency, intensity, and duration of compound events between an overshoot and its respective straight-stabilization trajectory. In addition, we evaluate ecosystem exposure to cumulative stress using indices for heat, hypoxia, and acidification, defined as exposure time below ecologically dangerous thresholds for marine organisms. Our analysis focuses on the persistence of these new extreme regimes and on when and if they can be reversed.