Signal Surface aragonite saturation at 2.85, just below the 2.86 boundary

Summary

The ocean acidification control variable is the global mean aragonite saturation state of surface seawater, which reflects how readily corals and shellfish can form the carbonate mineral used in shells and skeletons, with values below 1 indicating corrosive water. Observation-based data put the 2024 global mean at about 2.85, marginally below the revised boundary of 2.86. The boundary equals 80 percent of the 1750 pre-industrial value of 3.57, estimated from observationally adjusted CMIP6 models, meaning a 20 percent decline, while the provisional high-risk line is 2.50. The 80 percent level was chosen to prevent widespread undersaturation at high latitudes while keeping low-latitude waters well oversaturated, limiting harm to calcifiers. This year four observation-based products, all extended through 2024, are shown together to strengthen the evidence. Drawing on studies of vulnerable species, the report suggests the safe level may lie closer to a 10 percent decline.