The northern European shelf as an increasing net sink for CO<sub>2</sub>

Abstract

Abstract. We developed a simple method to refine existing open-ocean maps and extend them towards different coastal seas. Using a multi-linear regression we produced monthly maps of surface ocean fCO2 in the northern European coastal seas (the North Sea, the Baltic Sea, the Norwegian Coast and the Barents Sea) covering a time period from 1998 to 2016. A comparison with gridded Surface Ocean CO2 Atlas (SOCAT) v5 data revealed mean biases and standard deviations of 0 ± 26 µatm in the North Sea, 0 ± 16 µatm along the Norwegian Coast, 0 ± 19 µatm in the Barents Sea and 2 ± 42 µatm in the Baltic Sea. We used these maps to investigate trends in fCO2, pH and air–sea CO2 flux. The surface ocean fCO2 trends are smaller than the atmospheric trend in most of the studied regions. The only exception to this is the western part of the North Sea, where sea surface fCO2 increases by 2 µatm yr−1, which is similar to the atmospheric trend. The Baltic Sea does not show a significant trend. Here, the variability was much larger than the expected trends. Consistently, the pH trends were smaller than expected for an increase in fCO2 in pace with the rise of atmospheric CO2 levels. The calculated air–sea CO2 fluxes revealed that most regions were net sinks for CO2. Only the southern North Sea and the Baltic Sea emitted CO2 to the atmosphere. Especially in the northern regions the sink strength increased during the studied period.

Reduced Ocean Carbon Sink in the South and Central North Sea (2014–2018) Revealed From FerryBox Observations

“…While the long‐term observational trends in the North Sea analyzed by Becker et al. (2021) were similar to, or below the currently reported atmospheric trend of 2.3 µatm yr −1 (Friedlingstein et al., 2019), their shortest length trends calculated over the most recent years were higher (2–3.5 µatm yr −1 ), confirming the recent accelerated rise in seawater p CO 2 . Since all our seawater p CO 2 trends are stronger than the atmospheric trend over the 2014–2018 study period, we expect a weakening of the carbon uptake capacity of the North Sea.…”

“…We concluded that our accelerated trends are determined by the fortuitous choice of start and end dates as well as the much higher data resolution, allowing the identification of statistically significant trends in regions where data are otherwise scarce (details in the Text S1). While the long-term observational trends in the North Sea analyzed by Becker et al (2021) were similar to, or below the currently reported atmospheric trend of 2.3 µatm yr −1 (Friedlingstein et al, 2019), their shortest length trends calculated over the most recent years were higher (2-3.5 µatm yr −1 ), confirming the recent accelerated rise in seawater pCO 2 . Since all our seawater pCO 2 trends are stronger than the atmospheric trend over the 2014-2018 study period, we expect a weakening of the carbon uptake capacity of the North Sea.…”

“…The variability of the carbonate chemistry of the North Sea, a large temperate shelf sea, has recently been investigated as part of large‐scale studies of the Northwestern European Shelf (Becker et al., 2021; Hartman et al., 2019; Kitidis et al., 2019). Other studies focused on seawater p CO 2 from ship‐based observations in the North Sea have either been performed over short periods (Omar et al., 2010) or were restricted to the northern North Sea (Omar et al., 2019).…”

A seamless ensemble-based reconstruction of surface ocean <i>p</i>CO<sub>2</sub> and air–sea CO<sub>2</sub> fluxes over the global coastal and open oceans

“…This contrasts with other coastal regions (e.g. southern North Sea and Baltic Sea) where the respiration of terrestrial particulate organic carbon from river run-off contributes to making these areas a strong seasonal source of CO 2 (Borgesa and Gypensb, 2010;Becker et al, 2021). The Subtropical Atlantic (StA) is characterized by weak to moderate mean flux densities per unit area (open, 0.733 ± 0.036 molC m −2 yr −1 ; coastal, 0.457 ± 0.064 molC m −2 yr −1 ).…”

A monthly surface pCO<sub>2</sub> product for the California Current Large Marine Ecosystem

“…Briefly, statistical interpolations (Takahashi et al, 1993;Rödenbeck et al, 2014;Jones et al, 2015;Shutler et al, 2016), multiple linear regressions (Schuster et al, 2013;Iida et al, 2015;Becker et al, 2021), machine-learning-based regression methods (Landschützer et al, 2013;2014;2016;Nakaoka et al, 2013;Zeng et al, 2014;Laruelle et al, 2017;Ritter et al, 2017;Gregor et al, 2017;Chen et al, 2019;Denvil-Sommer et al, 2019), and biogeochemical-model-based approaches (Valsala and Maksyutov, 2010;Majkut et al, 2014;Verdy and Mazloff, 2017) have been common tactics, each one with its own strengths and weaknesses. Recently, ensemble averages of multiple data-or model-based approaches have become a popular option as well (Gregor et al, 2019;Lebehot et al, 2019;Fay et al, 2021).…”