Ocean Compound Extreme Events Under Emission Reduction and Negative CO₂ Pathways

Extreme events in the ocean, including marine heatwaves, low pH, and low oxygen events, 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 leading to compound extremes, their impacts can intensify nonlinearly. Yet, we know very little about these events, especially under scenarios of both rising and declining CO₂ emissions. Using the Max Planck Institute Earth System Model with the emission-driven  SSP5-3.4 overshoot scenario, this study explores how 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 an examination of how CO₂ fluxes between the ocean and the atmosphere respond dynamically to changing emissions. Previous studies have shown that under negative emissions, the ocean may transition from a CO₂ sink to a source. However, it remains unclear how this shift could influence  marine extremes, 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 at the implications of emission reductions and negative emissions for marine extreme events. These insights are crucial for understanding the potential risks associated with compound oceanic extremes and their impacts on the ocean’s climate-mitigating functions. 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.