The northern European shelf as an increasing net sink for CO2

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 fCO(2) 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 mu atm in the North Sea, 0 +/- 16 mu atm along the Norwegian Coast, 0 +/- 19 mu atm in the Barents Sea and 2 +/- 42 mu atm in the Baltic Sea. We used these maps to investigate trends in fCO(2), pH and air-sea CO2 flux. The surface ocean fCO(2) 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 fCO(2) increases by 2 mu atm 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 fCO(2) 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.