Macovei, V.A., Petersen, W., Brix, H., & Voynova, Y.G. (2021): Reduced ocean carbon sink in the south and central North Sea (2014–2018) revealed from FerryBox observations. Geophysical Research Letters, 48, e2021GL092645, doi:10.1029/2021GL092645
Surface seawater carbon dioxide partial pressure (pCO2) in the south-central North Sea was measured between 2014 and 2018 using FerryBox-integrated membrane sensors on ships-of-opportunity. Average annual pCO2 variability was biologically controlled, with thermal effects modulating its amplitude. Deseasonalized winter trends of seawater pCO2 were positive (4.4 ± 2.0–8.4 ± 2.9 µatm yr−1), biogeochemically driven, stronger than the atmospheric pCO2 trend, and more pronounced than previous analyses. The trends calculated including all deseasonalized monthly averages were even higher (9.7 ± 2.8–12.2 ± 1.4 µatm yr−1). During our investigation, the southern study area became a stronger source and the northern part became a weaker sink for atmospheric carbon. Overall, average sea-air CO2 flux in our study area, from the Skagerrak to the Southern Bight (53°N), changed from −0.75 ± 0.61 mmol m−2 day−1 in 2014 to +0.20 ± 0.96 mmol m−2 day−1 in 2018.
Plain Language Summary:
Oceans and seas take up one fourth of the carbon dioxide (CO2) produced by industrial and agricultural activities, and thus lower the amount left in the atmosphere that can contribute to climate change. This uptake of man-made CO2 can only be understood by taking the natural carbon cycle into account, which is subject to distinct variability. The ability of seawater to remove carbon from the atmosphere can be limited if too much CO2 is absorbed by the oceans and coastal seas. In this study, we show that between 2014 and 2018, the levels of carbon dioxide have increased faster in the North Sea surface waters than in the atmosphere. As a consequence, regions where atmospheric CO2 is taken up (carbon sinks) have become less effective and regions where CO2 is normally released back into the atmosphere (carbon sources) have become more effective. These findings help us understand how carbon cycling in the North Sea works in the context of still increasing atmospheric CO2 concentrations. It also highlights the importance of sustained observations in dynamic coastal environments such as the North Sea.