Ocean acidification continues apace, with hardly anyone noticing

From Nature Climate Change:

The uptake of anthropogenic CO₂ by the ocean decreases seawater pH and carbonate mineral aragonite saturation state (Ω_arag), a process known as Ocean Acidification (OA). This can be detrimental to marine organisms and ecosystems^{1, 2}. The Arctic Ocean is particularly sensitive to climate change³ and aragonite is expected to become undersaturated (Ω_arag < 1) there sooner than in other oceans⁴. However, the extent and expansion rate of OA in this region are still unknown. Here we show that, between the 1990s and 2010, low Ω_arag waters have expanded northwards at least 5°, to 85° N, and deepened 100 m, to 250 m depth. Data from trans-western Arctic Ocean cruises show that Ω_arag < 1 water has increased in the upper 250 m from 5% to 31% of the total area north of 70° N. Tracer data and model simulations suggest that increased Pacific Winter Water transport, driven by an anomalous circulation pattern and sea-ice retreat, is primarily responsible for the expansion, although local carbon recycling and anthropogenic CO₂ uptake have also contributed. These results indicate more rapid acidification is occurring in the Arctic Ocean than the Pacific and Atlantic oceans^{5, 6, 7, 8}, with the western Arctic Ocean the first open-ocean region with large-scale expansion of ‘acidified’ water directly observed in the upper water column.