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.

The Great Barrier Reef (GBR) is a globally significant coral reef system supporting productive and diverse ecosystems. The GBR is under increasing threat from climate change and local anthropogenic stressors, with its general condition degrading over recent decades. In response to this, a number of techniques have been proposed to offset or ameliorate environmental changes. In this study, we use a coupled hydrodynamic-biogeochemical model of the GBR and surrounding ocean to simulate artificial ocean alkalinisation (AOA) as a means to reverse the impact of global ocean acidification on GBR reefs. Our results demonstrate that a continuous release of 90 000 t of alkalinity every 3 d over one year along the entire length of the GBR, following the Gladstone-Weipa bulk carrier route, increases the mean aragonite saturation state ( Ω a r ) across the GBR’s 3860 reefs by 0.05. This change offsets just over 4 years (∼4.2) of ocean acidification under the present rate of anthropogenic carbon emissions. The injection raises Ω a r in the 250 reefs closest to the route by ⩾ 0.15 , reversing further projected Ocean Acidification. Following cessation of alkalinity injection Ω a r returns to the value of the waters in the absence of AOA over a 6 month period, primarily due to transport of additional alkalinity into the Coral Sea. Significantly, our study provides for the first time a model of AOA applied along existing shipping infrastructure that has been used to investigate shelf scale impacts. Thus, amelioration of decades of OA on the GBR is feasible using existing infrastructure, but is likely to be extremely expensive, include as yet unquantified risks, and would need to be undertaken continuously until such time, probably centuries in the future, when atmospheric CO₂ concentrations have returned to today’s values.

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