二酸化炭素排出を抑えても、海洋への影響は数世紀にわたって続く ― 2500 年までの予測で明らかになった深海の炭素蓄積と海洋生態系へのストレス―

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. By 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, surface-ocean CO₂ 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 CO₂ uptake continues to increase until the late 21st century. Thereafter, it gradually weakens as surface-ocean pCO₂ approaches atmospheric levels, whereas the Southern Ocean remains a persistent carbon sink throughout the simulation. 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-ocean pCO₂ 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 pCO₂ during this period. Marine biological productivity and export vary significantly as a function of atmospheric CO₂ trajectory. It increases under low-emission scenarios but declines under sustained high-emission scenarios. The transition between the two behaviors occurs when pCO^atm₂ 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 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.