Coral reefs are some of the most biodiverse ecosystems in the world. They are also particularly sensitive to environmental change, and subject to numerous stressors, including ocean acidification, the process by which atmospheric carbon dioxide dissolves in seawater to form carbonic acid. Acidification makes it more difficult for corals to make aragonite, the form of calcium carbonate that they use to build their skeletons.

We found that in preindustrial times, 99.9% of the approximately 6000 reefs analysed were located in regions with aragonite saturations of greater than 3.5. By contrast, under a business-as-usual carbon-dioxide emissions scenario, every coral reef considered will be surrounded by water with aragonite saturations of less than 3.0 by the end of the 21st century. So there will be no water left in the ocean with the chemical characteristics that have supported coral reef growth in the past. The result clearly illustrates the risks that acidification presents to coral-reef ecosystems if we continue our current emissions trajectory.

Unlike certain outputs of complex Earth system models, there is high agreement among models when it comes to ocean biogeochemistry around coral reefs under rising carbon-dioxide emissions. There is strong uncertainty, however, about the aragonite saturation threshold for coral-reef sustainability. With deep emissions cuts, the fraction of reefs that will remain sustainable from a chemical point of view greatly depends on what the threshold of aragonite saturation around reefs actually is.

We believe that only important emission abatement efforts will allow for a significant fraction of reefs to remain in ocean waters with aragonite saturation states of 3.0 or more. And that only the most aggressive policies, possibly including direct capture of carbon dioxide from the atmosphere, will allow reef-amenable chemistry conditions to be preserved around a majority of reefs. In the absence of such drastic reductions in emissions, we will soon go outside the bounds of the chemistry that surrounded all coral reefs before the industrial revolution.

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