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Due to low calcium carbonate saturation states, and winter mixing that brings anthropogenic carbon to the deep ocean, the Nordic Seas and their cold-water corals are vulnerable to ocean acidification. Here, we present a detailed investigation of the changes in pH and aragonite saturation in the Nordic Seas from preindustrial times to 2100, by using in situ observations, gridded climatological data, and projections for three different future scenarios with the Norwegian Earth System Model (NorESM1-ME). During the period of regular ocean biogeochemistry observations from 1981–2019, the pH decreased with rates of 2–3 × 10 -3 yr -1 in the upper 200 m of the Nordic Seas. In some regions, the pH decrease can be detected down to 2000 m depth. This resulted in a decrease in the aragonite saturation state, which is now close to undersaturation in the depth layer of 1000–2000 m. The model simulations suggest that the pH of the Nordic Seas will decrease at an overall faster rate than the global ocean from the preindustrial era to 2100, bringing the Nordic Seas' pH closer to the global average. In the esmRCP8.5 scenario, the whole water column is projected to be undersaturated with respect to aragonite at the end of the 21st century, thereby endangering all cold-water corals of the Nordic Seas. In the esmRCP4.5 scenario, the deepest cold-water coral reefs are projected to be exposed to undersaturation. Exposure of all cold-water corals to corrosive waters can only be avoided with marginal under the esmRCP2.6 scenario. Over all timescales, the main driver of the pH drop is the increase in dissolved inorganic carbon (CT) caused by the raising anthropogenic CO 2 , followed by the temperature increase. Thermodynamic salinity effects are of secondary importance. We find substantial changes in total alkalinity (AT) and CT as a result of the salinification, or decreased freshwater content, of the Atlantic water during all time periods, and as a result of an increased freshwater export in polar waters in past and future scenarios. However, the net impact of this decrease (increase) in freshwater content on pH is negligible, as the effects of a concentration (dilution) of CT and AT are canceling. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The Ocean Weather Station M (OWSM) is situated at a fixed position in the Norwegian Sea, one of the major basins of the Nordic Seas, which represents an important area for uptake of atmospheric CO 2 as well as deep water formation. At OWSM, the inorganic carbon cycle has been regularly monitored since 2001, and significant interannual changes of the carbonate system have been determined. Data collected at this site since the 1990s have been included, and over the 28 last years the surface fugacity of CO 2 (f CO 2) has increased by 2.92 ± 0.37 μatm yr−1, while surface pH and aragonite saturation (Ω Ar) have decreased by −0.0033 ± 0.0005 yr−1 and −0.018 ± 0.003 yr−1, respectively. This corresponds to a surface pH change of −0.092 over 28 years, which is comparable to the global mean pH decrease of −0.1 since the onset of the industrial revolution. Our estimates suggest that 80% of the surface pH trend at OWSM is driven by uptake of CO 2 from the atmosphere. In the deepest layer, Ω Ar has decreased significantly (−0.006 ± 0.001 yr−1) over the last 28 years, now occasionally reaching undersaturated values (Ω Ar < 1). As a rough estimate, the saturation horizon has shoaled by 7 m yr−1 between 1994 and 2021. The increase in surface f CO 2 is confirmed by semi-continuous measurements of CO 2 from the site (2.69 ± 0.14 μatm yr−1), and thus, the area has become less of a net sink for atmospheric CO 2 , taking into consideration an atmospheric CO 2 increase at OWSM of 2.27 ± 0.08 μatm yr−1. • pH of the Norwegian Sea surface has decreased significantly over the last two decades. • ocean acidification is affecting the full water column (0–2100 m). • the acidification is primarily due to uptake of CO 2 from the atmosphere. • the area is a decreasing sink for atmospheric CO 2. • the saturation horizon, which is currently around 2000 m depth, is shoaling. [ABSTRACT FROM AUTHOR]