Arctic Ocean acidification over the 21st century co-driven by anthropogenic carbon increases and freshening in the CMIP6 model ensemble

Abstract

Abstract. The uptake of anthropogenic carbon (Cant) by the ocean leads to ocean acidification, causing the reduction of pH and the saturation states of aragonite (Ωarag) and calcite (Ωcalc). The Arctic Ocean is particularly vulnerable to ocean acidification due to its naturally low pH and saturation states and due to ongoing freshening and the concurrent reduction in total alkalinity in this region. Here, we analyse ocean acidification in the Arctic Ocean over the 21st century across 14 Earth system models (ESMs) from the latest Coupled Model Intercomparison Project Phase 6 (CMIP6). Compared to the previous model generation (CMIP5), models generally better simulate maximum sea surface densities in the Arctic Ocean and consequently the transport of Cant into the Arctic Ocean interior, with simulated historical increases in Cant in improved agreement with observational products. Moreover, in CMIP6 the inter-model uncertainty of projected changes over the 21st century in Arctic Ocean Ωarag and Ωcalc averaged over the upper 1000 m is reduced by 44–64 %. The strong reduction in projection uncertainties of Ωarag and Ωcalc can be attributed to compensation between Cant uptake and total alkalinity reduction in the latest models. Specifically, ESMs with a large increase in Arctic Ocean Cant over the 21st century tend to simulate a relatively weak concurrent freshening and alkalinity reduction, while ESMs with a small increase in Cant simulate a relatively strong freshening and concurrent total alkalinity reduction. Although both mechanisms contribute to Arctic Ocean acidification over the 21st century, the increase in Cant remains the dominant driver. Even under the low-emissions Shared Socioeconomic Pathway 1-2.6 (SSP1-2.6), basin-wide averaged Ωarag undersaturation in the upper 1000 m occurs before the end of the century. While under the high-emissions pathway SSP5-8.5, the Arctic Ocean mesopelagic is projected to even become undersaturated with respect to calcite. An emergent constraint identified in CMIP5 which relates present-day maximum sea surface densities in the Arctic Ocean to the projected end-of-century Arctic Ocean Cant inventory is found to generally hold in CMIP6. However, a coincident constraint on Arctic declines in Ωarag and Ωcalc is not apparent in the new generation of models. This is due to both the reduction in Ωarag and Ωcalc projection uncertainty and the weaker direct relationship between projected changes in Arctic Ocean Cant and changes in Ωarag and Ωcalc.

Nutrient and Silicon Isotope Dynamics in the Laptev Sea and Implications for Nutrient Availability in the Transpolar Drift

“…These waters are marked by weaker or no nutrient loss (Δ[ nutrient ] ≈ 0%, Figure 6), suggesting conservative behavior and hence a dominant role of the contributing water masses and their mixing ( L for DSi and AAW for DIN and DIP) in generating these high concentrations. Nutrient supply through erosion of coastal soils (Terhaar et al., 2021 and references therein) does not seem to have a major effect on the nutrient distributions as this would be likely reflected in markedly higher Δ[ nutrient ], given that our analysis does not account for such additional inputs. The dominance of the Lena River in supplying most of the DSi is further supported by low δ 30 Si DSi values near the Lena River Delta (in 2013 reaching +0.91‰ at station 19, Figure 2) approaching those of the summer Lena River end‐member (δ 30 Si DSi = +0.86‰, Sun et al., 2018).…”

“…On a pan‐Arctic scale, replenishment of surface water macronutrient levels ( here : dissolved inorganic nitrogen, DIN; dissolved inorganic phosphorus, DIP; and silicic acid, DSi) from the deep pool via vertical mixing is the main process governing primary productivity (Hill et al., 2013; Randelhoff et al., 2020). In the extensive Arctic shelf areas, terrigenous nutrient inputs from land and nutrient cycling exert additional strong controls on nutrient bioavailability, supporting as much as one third of the current net primary productivity in the Arctic Ocean (Terhaar et al., 2021). In some of these marginal regions, despite the decrease in sea ice cover and opposite to the general trend, primary productivity has decreased in the recent past (Ardyna & Arrigo, 2020; Arrigo & van Dijken, 2015; Demidov et al., 2020; Lewis et al., 2020).…”

“…Pioneering investigations identified the Lena River as the main source of riverine inorganic nutrients to the Laptev Sea and found strong contrasts in nutrient concentrations between surface and bottom waters near the Lena Delta in summer (Cauwet & Sidorov, 1996; Codispoti & Richards, 1968; Létolle et al., 1993). More recently, erosion of coastal soils was suggested to be another strong source of terrestrial nutrients to the water column (Fritz et al., 2017; Holmes et al., 2012; Sanders et al., 2022; Terhaar et al., 2019, 2021). The widely observed depletion of nutrients in the freshwater enriched surface layer has been attributed to water mass mixing and biological utilization by primary producers (Codispoti & Richards, 1968; Gordeev et al., 1996; Kattner et al., 1999; Létolle et al., 1993).…”

“…. In the extensive Arctic shelf areas, terrigenous nutrient inputs from land and nutrient cycling exert additional strong controls on nutrient bioavailability, supporting as much as one third of the current net primary productivity in the Arctic Ocean (Terhaar et al, 2021). In some of these marginal regions, despite the decrease in sea ice cover and opposite to the general trend, primary productivity has decreased in the recent past (Ardyna & Arrigo, 2020;Arrigo & van Dijken, 2015;Demidov et al, 2020;Lewis et al, 2020).…”

Acidification of the Nordic Seas

“…the Arctic Ocean in several CMIP6 models (Terhaar et al, 2021). However, as discussed earlier, the net effect of these freshwater fluxes on pH are minor, as the dilution of A T and C T is similar, but have opposite effects on pH (compare Fig.…”