Abstract
Abstract. 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 CO2, 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.
Highlights
Since 1850, human activities have released 650 ± 65 Gt of carbon to the atmosphere, of which about 25 % have been taken up by the oceans (Friedlingstein et al, 2020), where it has been added to the CT pool
Because the relation between CT and AT is a proxy for the buffer capacity, we decided to look at their combined effect on pH, meaning that both changes in CT and AT are included in the calculations
Thereafter, we describe regional changes from the preindustrial era to the present day (Sect. 5.3), present-day changes (Sect. 5.4), and changes from the present day to the future (Sect. 5.5) and assess its impacts on cold-water corals (Sect. 5.6)
Summary
Since 1850, human activities have released 650 ± 65 Gt of carbon to the atmosphere, of which about 25 % have been taken up by the oceans (Friedlingstein et al, 2020), where it has been added to the CT pool. The increasing CT has resulted in a surface seawater pH decline of approximately 0.1 in the global ocean from the preindustrial era to the present day, which corresponds to an approximately 30 % increase in hydrogen ion (H+) concentration (e.g., Doney et al, 2009; Gattuso and Hansson, 2011; Jiang et al, 2019). Depending on the CO2 concentration pathway, future projections suggest further reductions in the surface ocean pH of 0.1–0.3 from the 1990s until the end of the 21st century (Bopp et al, 2013). While global average acidification rates for surface waters, both from preindustrial times to the present day and as projected for the future, are investigated in several studies (e.g., Caldeira and Wickett, 2003; Raven et al, 2005; Kwiatkowski et al, 2020), less is known about acidification rates on regional scales, especially below the surface
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