28 Sep 2020

28 Sep 2020

Review status: this preprint is currently under review for the journal BG.

Nordic Seas Acidification

Filippa Fransner1, Friederike Fröb2, Jerry Tjiputra3, Melissa Chierici4, Agneta Fransson5, Emil Jeansson3, Truls Johannessen1, Elizabeth Jones4, Siv K. Lauvset3, Sólveig R. Ólafsdóttir6, Abdirahman Omar3, Ingunn Skjelvan3, and Are Olsen1 Filippa Fransner et al.
  • 1Geophysical Institute, University of Bergen, and Bjerknes Centre for Climate Research, Bergen, Norway
  • 2Max Planck Institute for Meteorology, Hamburg, Germany
  • 3NORCE Norwegian Research Centre, Bjerknes Centre for Climate Research, Bergen, Norway
  • 4Institute of Marine Research, Fram Centre, Tromsø, Norway
  • 5Norwegian Polar Institute, Tromsø, Norway
  • 6Marine and Freshwater Research Institute, Reykjavík, Iceland

Abstract. Being windows to the deep ocean, the Nordic Seas play an important role in transferring anthropogenic carbon, and thus ocean acidification, to the abyss. Due to its location in high latitudes, it is further more sensitive to acidification compared with many other oceanic regions. Here we make a detailed investigation of the acidification of the Nordic Seas, and its drivers, since pre-Industrial to 2100 by using in situ measurements, gridded climatological data, and simulations from one Earth System Model (ESM). In the last 40 years, pH has decreased by 0.11 units in the Nordic Seas surface waters, a change that is twice as large as that between 1850–1980. We find that present trends are larger than expected from the increase in atmospheric CO2 alone, which is related to a faster increase in the seawater pCO2 compared with that of the atmosphere, i.e. a weakening of the pCO2 undersaturation of the Nordic Seas. The pH drop, mainly driven by an uptake of anthropogenic CO2, is significant all over the Nordic Seas, except for in the Barents Sea Opening, where it is counteracted by a significant increase in alkalinity. We also find that the acidification signal penetrates relatively deep, in some regions down to 2000 m. This has resulted in a significant decrease in the aragonite saturation state, which approaches undersaturation at 1000–2000 m in the modern ocean. Future scenarios suggest an additional drop of 0.1–0.4 units, depending on the emission scenario, in surface pH until 2100. In the worst case scenario, RCP8.5, the entire water column will be undersaturated with respect to aragonite by the end of the century, threatening Nordic Seas cold-water corals and their ecosystems. The model simulations suggest that aragonite undersaturation can be avoided at depths where the majority of the cold-water corals live in the RCP2.6 and RCP4.5 scenarios. As these results are based on one model only, we request additional observational and model studies to better quantify the transfer of anthropogenic CO2 to deep waters and its effect on future pH in the Nordic Seas.

Filippa Fransner et al.

Status: final response (author comments only)
Status: final response (author comments only)
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Filippa Fransner et al.

Filippa Fransner et al.


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Short summary
Ocean acidification, a direct consequence of the CO2 release by human activities, is a serious threat to marine ecosystems. In this study we make a detailed investigation of the acidification of the Nordic Seas, from 1850 to 2100, by using a large set of samples taken during research cruises together with numerical model simulations. We estimate the effects of changes in different environmental factors on the rate of acidification, and its potential effects on cold-water corals.