Does the Evolution of Indian Ocean Acidification Follow CMIP6 Projections Under a High‐Emission Scenario?

Abstract

The Indian Ocean (IO) surface acidification is governed by pronounced regional variability that is inadequately resolved in coarse‐resolution CMIP6 models. Here, we employ eddy‐resolving, dynamically downscaled outputs from three CMIP6 Earth System Models under the high‐emissions SSP5‐8.5 scenario to project future changes in IO acidification, with the model ensemble selected to account for uncertainty in future projections. Eddy‐resolving simulations improve representation of mesoscale‐driven surface acidification variability by ∼5%–10% relative to CMIP6 models, particularly in coastal regions, while reducing inter‐model spatio‐temporal heterogeneity. The analysis indicates a relatively weaker mean increase in [] compared with CMIP6 projections. The downscaled models yield a dynamically consistent, improved representation of the seasonal cycle of [] compared to CMIP6 models, particularly in the northern IO. This facilitates a more robust assessment of future anthropogenically driven changes in [] amplitude, with the monthly composite amplitude projected to increase by ∼63%–83% during the far‐future period (FF; 2071–2100) across the IO. However, in the Arabian Sea, the increase is ∼37% smaller than in CMIP6, whereas other regions exhibit largely comparable changes. This amplification is primarily driven by enhanced sensitivity of [] to surface temperature and dissolved inorganic carbon under warming and increased uptake. Consistent with these changes, acidification extremes intensify (∼26%–45%) and become more frequent (∼3–5 times higher) in FF. Notably, in coastal regions after 2070, anthropogenic acidification signals emerge later in the downscaled projections than in CMIP6. These results highlight the need for high‐resolution projections to improve estimates of future IO acidification and its associated risks.