Aragonite saturations surrounding coral reefs as projected for RCP 8.5

<p><strong>Figure 1.</strong> Aragonite saturations surrounding coral reefs as projected for RCP 8.5. Maps show coral reef locations (in gray) and Ωa in areas where it can be computed from the CMIP5 ensemble and bias corrected with the GLODAP dataset. Histograms show those reefs binned by aragonite saturation of surrounding waters. Panels show ensemble median results for ensemble-member 10-year mean Ωa at: (a) the beginning of the ensemble simulations in the 1860s (287 ppm xCO<sub>2</sub>); (b) around present day (400 ppm); and for future conditions at (c) 550 ppm (mid-century for RCP 8.5 and end-of-century for RCP 4.5), and (d) 900 ppm (end-of-century under RCP 8.5). Contour lines show Ωa<sub>crit</sub> thresholds of 3, 3.25 and 3.5; panel (d) has no contours because all ocean waters have Ωa < 3. Histogram error bars show ensemble quartile values and the maximum and minimum range.</p> <p><strong>Abstract</strong></p> <p>Coral reefs are among the most biodiverse ecosystems in the world. Today they are threatened by numerous stressors, including warming ocean waters and coastal pollution. Here we focus on the implications of ocean acidification for the open ocean chemistry surrounding coral reefs, as estimated from earth system models participating in the Coupled Model Intercomparison Project, Phase 5 (CMIP5). We project risks to reefs in the context of three potential aragonite saturation (Ωa) thresholds. We find that in preindustrial times, 99.9% of reefs adjacent to open ocean in the CMIP5 ensemble were located in regions with Ωa > 3.5. Under a business-as-usual scenario (RCP 8.5), every coral reef considered will be surrounded by water with Ωa < 3 by the end of the 21st century and the reefs' long-term fate is independent of their specific saturation threshold. However, under scenarios with significant CO<sub>2</sub> emissions abatement, the Ωa threshold for reefs is critical to projecting their fate. Our results indicate that to maintain a majority of reefs surrounded by waters with Ωa > 3.5 to the end of the century, very aggressive reductions in emissions are required. The spread of Ωa projections across models in the CMIP5 ensemble is narrow, justifying a high level of confidence in these results.</p>