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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 uptake of anthropogenic carbon dioxide (CO2) by the ocean leads to a process known as ocean acidification (OA), which lowers the aragonite saturation state (ΩAr) and pH, and this is poorly documented in coastal environments including fjords due to lack of appropriate observations. Here we use weekly underway data from the Voluntary Observing Ships (VOS) program covering the period 2005-2009 combined with data from research cruises to estimate ΩAr and pH values in several adjacent western Norwegian fjords, and to evaluate how seawater CO2 chemistry drives their variations in response to physical and biological factors. The OA parameters in the surface waters of the fjords are subject to strong seasonal and spatially coherent variations. These changes are governed by the seasonal changes in temperature, salinity, formation and decay of organic matter, and vertical mixing with deeper, carbon-rich coastal water. Annual mean pH and ΩAr values were 8.13 and 2.21, respectively. The former varies from minimum values (≈ 8.05) in late December -- early January to maximum values of around 8.2 during early spring (March-April) as a consequence of the phytoplankton spring bloom, which reduces dissolved inorganic carbon (DIC). In the following months, pH decreases in response to warming. This thermodynamic decrease in pH is reinforced by the deepening of the mixed layer, which enables carbon-rich coastal water to reach the surface, and this trend continues until the low winter values of pH are reached again. ΩAr, on the other hand, reaches its seasonal maximum (> 2.5) in mid- to late summer (July-September), when the spring bloom is over and pH is decreasing. The lowest ΩAr values (≈ 1.3-1.6) occur during winter (January-March), when both pH and sea surface temperature (SST) are low and DIC is its highest. Consequently, seasonal ΩAr variations align with those of SST and salinity normalized DIC (nDIC). We demonstrate that underway measurements of fugacity of CO2 in seawater (fCO2) and SST from VOS lines combined with high frequency observations of the complete carbonate system at strategically placed fixed stations provide an approach to interpolate OA parameters over large areas in the fjords of western Norway. [ABSTRACT FROM AUTHOR]

• A novel stoichiometric approach called the Cseep method was developed and used to. • Predict natural DIC variations around the Goldeneye site in the north-western North Sea. • Establish a process-based baseline DIC concentration (Cb) with minimal variability. • Determine CO2 seepage detection threshold (DT) to reliably differentiate subsea released−CO 2 signal from natural variability. • Quantify DIC concentration of subsea released−CO 2 dissolved in the sampled seawater. Carbon Capture and Storage (CCS) is a potential significant mitigation strategy to combat climate change and ocean acidification. The technology is well understood but its current implementation must be scaled up nearly by a hundredfold to become an effective tool that helps meet mitigation targets. Regulations require monitoring and verification at storage sites, and reliable monitoring strategies for detection and quantification of seepage of the stored carbon need to be developed. The C seep method was developed for reliable determination of CO 2 seepage signal in seawater by estimating and filtering out natural variations in dissolved inorganic carbon (C). In this work, we analysed data from the first-ever subsea CO 2 release experiment performed in the north-western North Sea by the EU STEMM−CCS project. We successfully demonstrated the ability of the C seep method to (i) predict natural C variations around the Goldeneye site over seasonal to interannual time scales; (ii) establish a process-based baseline C concentration with minimal variability; (iii) determine CO 2 seepage detection threshold (DT) to reliably differentiate released−CO 2 signal from natural variability and quantify released−CO 2 dissolved in the sampled seawater. DT values were around 20 % of the natural C variations indicating high sensitivity of the method. Moreover, with the availability of DT value, the identification of released−CO 2 required no pre-knowledge of seepage occurrence, but we used additional available information to assess the confidence of the results. Overall, the C seep method features high sensitivity, automation suitability, and represents a powerful future monitoring tool both for large and confined marine areas. [ABSTRACT FROM AUTHOR]