Reversing ocean acidification along the Great Barrier Reef using alkalinity injection
Mathieu Mongin, Mark E. Baird, Andrew Lenton, Craig Neill, John Akl
Abstract
Abstract 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 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi mathvariant="normal">Ω</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">a</mml:mi> <mml:mi mathvariant="normal">r</mml:mi> </mml:mrow> </mml:msub> </mml:math> ) 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi mathvariant="normal">Ω</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">a</mml:mi> <mml:mi mathvariant="normal">r</mml:mi> </mml:mrow> </mml:msub> </mml:math> in the 250 reefs closest to the route by <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mo>⩾</mml:mo> <mml:mn>0.15</mml:mn> </mml:mrow> </mml:math> , reversing further projected Ocean Acidification. Following cessation of alkalinity injection <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi mathvariant="normal">Ω</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">a</mml:mi> <mml:mi mathvariant="normal">r</mml:mi> </mml:mrow> </mml:msub> </mml:math> 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 CO 2 concentrations have returned to today’s values.