Patterns of Ocean Acidification Emergence in the Hawaiian Islands Using Dynamically Downscaled Projections

This study presents the first dynamically downscaled projections of ocean acidification (OA) for the Main Hawaiian Islands using coupled Regional Ocean Modeling System and Carbon, Ocean Biogeochemistry, and Lower Trophics models integrated with Coupled Model Intercomparison Project Phase 6 (CMIP6) outputs from the Community Earth System Model 2. We analyze three Shared Socioeconomic Pathways (SSP1-2.6, SSP2-4.5, and SSP3-7.0) and introduce a climate novelty metric to assess the extent to which future OA conditions exceed historical variability by comparing the magnitude of projected changes to past variability. Our results indicate unprecedented levels of OA within the next three decades across all scenarios, with aragonite saturation state (${\Omega }_A$), pH, and substrate-to-inhibitor ratio (bicarbonate to free hydrogen ions [HCO₃⁻]/[H⁺]) projected to decline significantly. By 2100, under SSP3-7.0, ${\Omega }_A$ novelty could exceed reference variability by a factor of 12. Spatial analysis reveals heterogeneous OA impacts, with windward coastlines consistently exhibiting higher novelty levels. Importantly, we find contrasting spatial patterns of OA indices due to varying sensitivities to temperature and dissolved inorganic carbon, resulting in higher ${\Omega }_A$ novelty in northern areas and higher pH and substrate-to-inhibitor ratio novelty in southern regions.