Abstract
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 (ΩA), pH, and substrate-to-inhibitor ratio (bicarbonate to free hydrogen ions [HCO3−]/[H+]) projected to decline significantly. By 2100, under SSP3-7.0, ΩA 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 ΩA novelty in northern areas and higher pH and substrate-to-inhibitor ratio novelty in southern regions.
Key Points
- Three climate scenarios (SSP1-2.6, SSP2-4.5, and SSP3-7.0) show distinct implications for ocean acidification in the main Hawaiian Islands
- Aragonite saturation, pH, and substrate (bicarbonate) to inhibitor (free hydrogen) ratio lead to distinct spatial patterns
- Future conditions under SSP3-7.0 are projected to exceed historical variability with strong spatial differences along Hawaiian coasts
Plain Language Summary
Our oceans are acidifying as they absorb carbon dioxide from the atmosphere. This change threatens coral reefs, which are vital ecosystems supporting marine life and providing coastal protection. In this study, we used advanced computer models to project how ocean chemistry around the main Hawaiian Islands might change over the 21st century under different scenarios based on how much carbon dioxide we continue to emit. Our results show that ocean acidification is expected to increase significantly across all scenarios, but the extent and timing of these changes vary. In the high-emission scenario, ocean chemistry will become dramatically different from what we have seen historically, potentially posing challenges to coral reefs and their ability to adapt. Even in the low-emission scenario, some changes are inevitable, but they are less extreme and occur more gradually. We introduced a concept called “novelty” to measure how future ocean conditions might deviate from what coral reefs have experienced in recent history, and discovered that different parts of the Hawaiian Islands may experience acidification differently. This research helps us understand the future challenges facing Hawaiian coral reefs and provides information for researchers, conservationists and policymakers for preserving these critical ecosystems for future generations.
Hošeková L., Friedrich T., Powell B. S. & Sabine C., 2025. Patterns of ocean acidification emergence in the Hawaiian Islands using dynamically downscaled projections. Journal of Geophysical Research: Oceans 130(6): e2024JC021903. doi: 10.1029/2024JC021903. Article.
