Acidification of the Nordic Seas

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.

Ocean Acidification and Long‐Term Changes in the Carbonate System Properties of the South Atlantic Ocean

“…However, while useful, these projections do not take into account the possible changes that may occur in the biological pump and the ocean circulation, highlighting that a model‐based projection such as the one carried out by Fransner et al. (2022) in the Nordic Seas is necessary for the South Atlantic.…”

“…For the SACW, a total unsaturation with respect to aragonite only was observed for the emissions projected under the SSP5-8.5 in the year 2060, close to the estimates of Fontela et al (2021) for the Argentine Basin. However, while useful, these projections do not take into account the possible changes that may occur in the biological pump and the ocean circulation, highlighting that a model-based projection such as the one carried out by Fransner et al (2022) in the Nordic Seas is necessary for the South Atlantic.…”

Biogeochemical Timescales of Climate Change Onset and Recovery in the North Atlantic Interior Under Rapid Atmospheric CO2 Forcing

“…The anthropogenic DIC invasion predominantly controls the evolution of Ω c , which is reflected by the spatial patterns seen between Figures 2e‐2j‐2o and S13b,c,d. The anthropogenic carbon enters NADW through the Nordic and Labrador Seas before propagating southward through the return flow of overturning circulation (see AMOC stream function Figure ; Fransner et al., 2022; Tjiputra, Assmann, & Heinze, 2010). This is why the domains strongly influenced by NADW have the lowest Trec (Figure 5d) following the end of Ramp‐down, as watermasses now equilibrated once again with PI CO 2 levels start entering the deep ocean.…”

Impact of climate change on Arctic macroalgal communities

“…Ocean acidification is magnified in high latitude regions because cold polar waters favor the dissolution of CO2 and low calcium carbonate saturation states (Fransner et al, 2022). Even if warming limits CO2 absorption, indirect effects of the increase in sea temperature such as ice loss further promote this process by enhancing air-sea gas exchange, including CO2 (Yamamoto-Kawai et al, 2009).…”

“…Also, mixing with river runoff can increase or reduce the buffering capacity of Arctic waters depending on the local geology and releases a large amount of dissolved and particulate carbon from thawing permafrost (Polukhin 2019). With a pH decrease of up to 0.45 units projected over the 21st century (Terhaar et al, 2021), model simulations estimate that the Arctic will undergo the greatest acidification at the global scale (Fransner et al, 2022). At the current emission rate, surface waters of the Arctic Ocean will be locally undersaturated with respect to aragonite within a decade (Yamamoto et al, 2012, Fransner et al, 2022.…”