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Meridional overturning circulation conveys fast acidification to the deep Atlantic Ocean

Abstract

Since the Industrial Revolution, the North Atlantic Ocean has been accumulating anthropogenic carbon dioxide (CO2) and experiencing ocean acidification1, 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 waters2,3. 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[CO32−])—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 CO2 levels—which could occur within three decades according to a ‘business-as-usual scenario’ for climate change4—could reduce the transport of xc[CO32−] by 64–79 per cent of that in preindustrial times, which could severely endanger cold-water coral habitats. The Atlantic meridional overturning circulation would also export this acidified deep water southwards, spreading corrosive waters to the world ocean.

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Figure 1: Aragonite saturation and cold-water coral distribution.
Figure 2: Ocean acidification at the Central Irminger Station.
Figure 3: Circulation of the excess of [CO32−] over aragonite saturation and of [H+] in the subpolar North Atlantic.

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Acknowledgements

The OVIDE research project was co-funded by the Institut Français de Recherche pour l’Exploitation de la Mer (IFREMER) and CNRS/Institut National des Sciences de l’Univers (INSU)/Les Enveloppes Fluides et l’Environnement (LEFE). H.M. was supported by CNRS. This is a contribution to the AtlantOS project funded by the European Union’s Horizon 2020 research and innovation programme under grant agreement 633211. This study is also a contribution to the BOCATS project (CTM2013-41048-P) supported by the Spanish Ministry of Economy and Competitiveness and co-funded by the Fondo Europeo de Desarrollo Regional 2014–2020 (FEDER). We thank the captain of the research vessel Sarmiento Gamboa—A. Campos—and her crew for help that made possible the success of the BOCATS cruise. We thank D. Barton for revision of the manuscript.

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F.F.P., H.M. and P.L. designed the program and executed the fieldwork. F.F.P, M.F., M.I.G-I., A.V., M.P., F.A-P., E.F.G. and X.A.P. contributed to the chemical determination of nutrients and carbon system. H.M., P.L. and P.Z. contributed to the measurement of currents and the inverse-model results. All authors contributed to discussion and writing.

Corresponding author

Correspondence to Fiz F. Perez.

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Reviewer Information Nature thanks J. Dunne, M. Roberts and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Figure 1 Anthropogenic CO2 along the 2016 Ovide section.

Vertical distribution of anthropogenic CO2 (colour bar and white isolines; in μmol kg−1) along the Ovide section of the North Atlantic (Fig. 1b) in 2016. The black line marks the isopycnal that delimits the upper and lower limbs of the AMOC. Dots represent samples. Dark shaded area represents the sea bed.

Extended Data Figure 2 Projections of atmospheric CO2 and temperature changes as modelled by the five shared socioeconomic pathways (SSPs)24.

At present (bottom left of each graph), Earth has experienced a 1 °C warming with respect to preindustrial temperatures, and has an atmospheric CO2 concentration ([CO2atm]) of 400 p.p.m. (ref. 21); this represents an atmospheric CO2 excess (xc[CO2atm]) of 120 p.p.m. with respect to preindustrial times (xc[CO2atm] = [CO2atm] − 280 p.p.m., where 280 p.p.m. is the preindustrial [CO2atm]). If atmospheric Cant were to double, so too would xc[CO2atm] (to 480–520 p.p.m.; dotted red line in main graph); this is within the projection range for a warming of 2 °C. According to SSP5 (pink shaded area), this temperature could be reached in 2040–2052 (see insets). SSP5 is the only one of these five scenarios that results in a radiative forcing as high as that projected by the previous IPCC business-as-usual scenario (RCP8.5)54.

Extended Data Figure 3 Evolution of ocean acidification from preindustrial to the doubling of atmospheric Cant scenario.

ac, Along the Ovide section (Fig. 1b) are shown H+ concentrations (colour bar and white isolines; in nmol kg1) for preindustrial times (colour bar and white isolines) (a), the present day (b; 2016), and in a scenario involving doubling of atmospheric Cant (c). The black line shows the isopycnal delimiting the upper and lower limbs of the AMOC. Dots represent samples. Dark shaded area denotes the sea bed.

Extended Data Figure 4 Progression of anthropogenic CO2 in surface waters versus excess of atmospheric CO2 at different temperatures and CO2 disequilibriums.

ad, The concentration of anthropogenic CO2 ([Cant], in μmol kg−1; blue dots), the Cant ratio (navy encircled dots) and the Hant ratio (red dots) are plotted against the excess of atmospheric CO2 that has resulted from anthropogenic activity during 1959–2016 for: a, a 2 °C warming and 5% of CO2 disequilibrium; b, a 2 °C warming and 15% of CO2 disequilibrium; c, a 20 °C warming and 5% of CO2 disequilibrium; and d, a 20 °C warming and 15% of CO2 disequilibrium. The Cant ratio (or Hant ratio) is the ratio between the [Cant] (or [H+]ant) at any time and the average [Cant] ([H+]ant) for the period covered by the OVIDE cruises (2002–2016). The values computed for the doubling of atmospheric Cant scenario (that is, for an excess of atmospheric CO2 of 220 ± 20 p.p.m.) are also shown (blue arrowheads). The progression of the Hant ratio fits to a straight line, while the Cant ratio decreases when increasing the atmospheric excess CO2 owing to the buffer capacity of the marine CO2 system (the Revelle factor). An increase of 72 ± 3% in Cant above the average [Cant] for the Ovide period (2002–2016) is computed for the doubling of atmospheric Cant scenario. This is practically independent of the seawater temperature and the degree of CO2 disequilibrium41.

Extended Data Figure 5 Mean velocities orthogonal to Ovide.

Mean velocities (in m s−1) orthogonal to the Ovide section for the period 2002–2016. The Ekman velocities—estimated from wind stress averaged over the months of the cruises and equally distributed over the first 30 m—were added to the geostrophic velocities derived from a box inverse model17,51,52,53. Positive (or negative) velocities indicate northeastward (or southwestward) currents. The horizontal grey lines are the isopycnals marking σ0 = 27.80 kg m−3, σ1 = 32.15 kg m−3, σ2 = 36.94 kg m−3 and σ4 = 45.85 kg m−3. At the bottom are shown distances from Greenland. Grey shaded area denotes the sea bed. The acronyms indicate topographic features: the Reykjanes Ridge (RR), the Eriador Seamount (ESM), and the Azores–Biscay Ridge (ABR).

Extended Data Figure 6 Circulation of CO32− ions and anthropogenic CO2 in the subpolar North Atlantic.

Mean transports for 2002–2016 between the Ovide section and the Nordic Sills are shown for: sea water (green values, in Sv), as well as natural (preindustrial; light coloured arrows) and observed (dark coloured arrows) [CO32−] (black values) and Cant (white values) (both in kmol s−1). Uncertainties are the errors of the mean transports across the eight occupations of the Ovide section. Transports at the Nordic Sills are derived from refs 18, 19. The grey dotted line represents the limit between the upper and lower limbs of the AMOC. Grey shaded area denotes the sea bed.

Extended Data Table 1 Aragonite saturation at CWC locations
Extended Data Table 2 Transport of sea water, xc[CO32−], [H+], Cant and [CO32−] across the Ovide section
Extended Data Table 3 Transports at the Nordic Sills

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Perez, F., Fontela, M., García-Ibáñez, M. et al. Meridional overturning circulation conveys fast acidification to the deep Atlantic Ocean. Nature 554, 515–518 (2018). https://doi.org/10.1038/nature25493

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