Meridional overturning circulation conveys fast acidification to the deep Atlantic Ocean

There has been about a forty per cent reduction in the transport of carbonate ions to the deep North Atlantic Ocean since preindustrial times, severely endangering cold-water corals. Accelerated acidification of the ocean deep Increasing atmospheric carbon dioxide concentrations are increasing the acidity of the oceans and reducing carbonate-ion concentrations, making it difficult for corals to maintain their calcium carbonate skeletons. This paper reports a roughly 40 per cent reduction in the transport of carbonate ions to the deep North Atlantic ocean since preindustrial times, with implications for cold-water coral habitats. The authors suggest that a further doubling of the concentration of anthropogenic carbon dioxide in the atmosphere could reduce the transport of carbonate ions to corals below 1,000 metres to a quarter of its preindustrial levels, potentially posing a severe threat to these habitats. Since the Industrial Revolution, the North Atlantic Ocean has been accumulating anthropogenic carbon dioxide (CO 2 ) and experiencing ocean acidification 1 , that is, an increase in the concentration of hydrogen ions (a reduction in pH) and a reduction in the concentration of carbonate ions. The latter causes the ‘aragonite saturation horizon’—below which waters are undersaturated with respect to a particular calcium carbonate, aragonite—to move to shallower depths (to shoal), exposing corals to corrosive waters 2 , 3 . Here we use a database analysis to show that the present rate of supply of acidified waters to the deep Atlantic could cause the aragonite saturation horizon to shoal by 1,000–1,700 metres in the subpolar North Atlantic within the next three decades. We find that, during 1991–2016, a decrease in the concentration of carbonate ions in the Irminger Sea caused the aragonite saturation horizon to shoal by about 10–15 metres per year, and the volume of aragonite-saturated waters to reduce concomitantly. Our determination of the transport of the excess of carbonate over aragonite saturation ( xc [CO 3 2− ])—an indicator of the availability of aragonite to organisms—by the Atlantic meridional overturning circulation shows that the present-day transport of carbonate ions towards the deep ocean is about 44 per cent lower than it was in preindustrial times. We infer that a doubling of atmospheric anthropogenic CO 2 levels—which could occur within three decades according to a ‘business-as-usual scenario’ for climate change 4 —could reduce the tran