Ocean Compound Extreme Events Under Emission Reduction and Negative CO2 Pathways

Extreme events like marine heatwaves are among the most severe impacts of climate change, profoundly affecting marine ecosystems, biogeochemical cycles, and the human communities that depend on ocean resources. When these events occur simultaneously or in close sequence with other extremes, like low pH and low oxygen extreme events, they form compound events whose impacts can intensify nonlinearly. Yet, our knowledge of these events remains limited, particularly under scenarios of both rising and declining CO₂ emissions. Using the Max Planck Institute Earth System Model driven by CO₂ emissions under the emission-driven SSP5-3.4 overshoot scenario, this study explores how ocean compound extreme events evolve along a pathway marked by initial rapid emissions increases followed by steep reductions, ultimately reaching net-negative emissions.

The emission-driven simulations incorporate an interactive carbon cycle, which in this setup allows for a prognostic computation of atmospheric CO₂ and hence enables an investigation of how the global carbon cycle and climate respond dynamically to changing emissions. Previous studies have shown that under negative emissions, the ocean may transition from a sink to a source of CO₂. However, it remains unclear how this shift could influence ocean compound extreme events, potentially altering their frequency, intensity, and duration. This is especially relevant for both surface and subsurface extremes, where responses to emission changes may vary considerably.

By focusing on the SSP5-3.4 overshoot scenario, this study provides a novel perspective on the implications of emission reductions and negative emissions for marine extreme events. Linking physical and biogeochemical extremes offers a broader understanding of compound events and their interactions with the global climate system. The findings of our research will further provide guidance for future climate adaptation and mitigation strategies that consider the ocean’s critical role in a changing climate.