Decadal trends in Ocean Acidification from the Ocean Weather Station M in the Norwegian Sea

The Ocean Weather Station M (OWSM) is situated at a fixed position in the Norwegian Sea, one of the major basins of the Nordic Seas, which represents an important area for uptake of atmospheric CO₂ as well as deep water formation. At OWSM, the inorganic carbon cycle has been regularly monitored since 2001, and significant interannual changes of the carbonate system have been determined. Data collected at this site since the 1990s have been included, and over the 28 last years the surface fugacity of CO₂ (fCO₂) has increased by 2.92 ± 0.37 μatm yr⁻¹, while surface pH and aragonite saturation (Ω_Ar) have decreased by −0.0033 ± 0.0005 yr⁻¹ and −0.018 ± 0.003 yr⁻¹, respectively. This corresponds to a surface pH change of −0.092 over 28 years, which is comparable to the global mean pH decrease of −0.1 since the onset of the industrial revolution. Our estimates suggest that 80% of the surface pH trend at OWSM is driven by uptake of CO₂ from the atmosphere. In the deepest layer, Ω_Ar has decreased significantly (−0.006 ± 0.001 yr⁻¹) over the last 28 years, now occasionally reaching undersaturated values (Ω_Ar < 1). As a rough estimate, the saturation horizon has shoaled by 7 m yr⁻¹ between 1994 and 2021. The increase in surface fCO₂ is confirmed by semi-continuous measurements of CO₂ from the site (2.69 ± 0.14 μatm yr⁻¹), and thus, the area has become less of a net sink for atmospheric CO₂, taking into consideration an atmospheric CO₂ increase at OWSM of 2.27 ± 0.08 μatm yr⁻¹.