The northern European shelf as an increasing net sink for CO2

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 fCO2in 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 CO2Atlas (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 CO2flux. The surface ocean fCO2trends 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 fCO2increases by 2 μatmyr-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 fCO2in pace with the rise of atmospheric CO2levels. The calculated air-sea CO2fluxes revealed that most regions were net sinks for CO2. Only the southern North Sea and the Baltic Sea emitted CO2to the atmosphere. Especially in the northern regions the sink strength increased during the studied period.