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.