Nordic Seas Acidification

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.