Multi-centennial ocean biogeochemical responses to extended Shared Socioeconomic Pathways

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 CO 2 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 p CO 2 disequilibrium exhibits strong scenario dependence. Under the low-emission scenario SSP1–2.6, oceanic CO 2 undersaturation gradually weakens and approaches near equilibrium by 2100. By contrast, under the high-emission scenario SSP5–8.5, rapid increase in atmospheric p CO 2 results in increasing undersaturation of CO 2 in the ocean over the 21st century. Thereafter, surface-ocean CO 2 supersaturation progressively expand into the equatorial and subtropical oceans between the mid–23rd and late 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. Under SSP5–8.5, oceanic CO 2 uptake continues to increase until the late 21st century. Thereafter, it gradually weakens as surface-ocean p CO 2 approaches atmospheric levels, whereas the Southern Ocean remains a persistent carbon sink throughout the simulation. Across all scenarios, early oceanic p CO 2 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 CO 2 buffering capacity, driven by cumulative CO 2 uptake, increases the sensitivity of surface-ocean p CO 2 to additional carbon uptake. The buffering capacity approaches its effective minimum levels by the late 22nd century. In parallel, enhanced upper-ocean stratification and a slowdown of the deep overturning circulation reduce the upward transport of high alkalinity waters to the surface. This results in a decline in surface-ocean alkalinity that further enhances the influence of alkalinity on surface-ocean p CO 2 during this period. Marine biological productivity and export vary significantly as a function of atmospheric CO 2 trajectory. It increases under low-emission scenarios but declines under sustained high-emission scenarios. The transition between the two behaviors occurs when pCO2atm reaches approximately 3 times the preindustrial level and is closely associated with the weakening of the Atlantic Meridional Overturning Circulation and the resulting nutrient redistribution. 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 CO 2 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 p CO 2 , highlighting the long memory and limited reversibility of oceanic ecosystem stress.