The exposure of the Great Barrier Reef to ocean acidification

The Great Barrier Reef (GBR) is founded on reef-building corals. Corals build their exoskeleton with aragonite, but ocean acidification is lowering the aragonite saturation state of seawater (Ω_a). The downscaling of ocean acidification projections from global to GBR scales requires the set of regional drivers controlling Ω_a to be resolved. Here we use a regional coupled circulation–biogeochemical model and observations to estimate the Ω_a experienced by the 3,581 reefs of the GBR, and to apportion the contributions of the hydrological cycle, regional hydrodynamics and metabolism on Ω_a variability. We find more detail, and a greater range (1.43), than previously compiled coarse maps of Ω_a of the region (0.4), or in observations (1.0). Most of the variability in Ω_a is due to processes upstream of the reef in question. As a result, future decline in Ω_a is likely to be steeper on the GBR than currently projected by the IPCC assessment report.