Multi-centennial ocean biogeochemical responses to extended Shared Socioeconomic Pathways

Abstract. Despite their profound consequences for the global carbon cycle, multi-centennial ocean biogeochemical responses to anthropogenic greenhouse gas forcing remain poorly constrained. Here we investigate long-term oceanic responses to atmospheric CO₂ forcing using the Model for Interdisciplinary Research on Climate, Earth System version 2 for Long-term simulations (MIROC-ES2L) Earth system model driven by extended Shared Socio-economic Pathway (SSP) scenarios through to the year 2500. Ocean–atmosphere pCO₂ disequilibrium exhibits strong scenario dependence. Under the low-emission scenario SSP1–2.6, oceanic CO₂ undersaturation gradually weakens and approaches near equilibrium by 2100. In contrast, under the high-emission scenario SSP5–8.5, rapid increase in atmospheric pCO₂ results in increasing undersaturation of CO₂ in the ocean over the 21st century. Thereafter, the ocean becomes increasingly supersaturated, with CO₂ supersaturation expanding into the equatorial and subtropical oceans between the mid-23rd and the 25th century. The Southern Ocean remains persistently undersaturated throughout the simulations, consistent with sustained influence from deep waters that are weakly affected by anthropogenic perturbations. Across all scenarios, early oceanic pCO₂ changes are primarily driven by increases in dissolved inorganic carbon, whereas alkalinity becomes an increasingly important control from the mid-22nd century onward under SSP5–8.5. A progressive decline in the oceanic CO₂ buffering capacity, driven by cumulative CO₂ uptake, increases the sensitivity of surface pCO₂ to additional carbon, with the buffering capacity approaching its effective minimum levels by the late 22nd century. In parallel, a reduction in surface alkalinity further enhances the influence of alkalinity on surface pCO₂ during this period. Notably, thermal stress and nutrient limitation persist in regions such as the Arctic Ocean until the late 25th century even under SSP1–2.6, indicating a centennial-scale lagged response of marine ecosystems to atmospheric CO₂ forcing. In the surface ocean, marine ecosystem stress (integrating thermal and biogeochemical stressors) continues to intensify through the late 22nd century under SSP5–8.5, despite the stabilization of atmospheric pCO₂, highlighting the long memory and limited reversibility of oceanic ecosystem stress.