It seems that a simple assumption about where to measure CO2 in the ocean surface has drastic implications. via The Hockey Schtick
New paper finds global carbon cycle datasets may be biased
A paper published today in Global Biogeochemical Cycles finds prior calculations of the global carbon cycle may be erroneous because such calculations are based upon partial pressures of CO2 from several meters below the ocean surface instead of CO2 levels at the ocean surface [“the boundary layer”] where CO2 is actually exchanged between the atmosphere and ocean.
The authors find a “strong” CO2 variability between the global datasets measured from several meters below the surface in comparison to the ocean surface that cannot be explained by Henry’s Law alone, and are primarily due to variations in biological activity between these layers. The paper finds higher levels of CO2 in the boundary layer than in the 5 meter deep global datasets, which would suggest that either the oceans are less of a sink for CO2 or a larger source of CO2 to the atmosphere than previously assumed.
The authors recommend, “Observations of pCO2 just beneath the air-sea boundary layer should be further investigated in order to estimate possible biases in calculating global air-sea CO2 fluxes.”
Maria Ll. Calleja et al
The gradient in the partial pressure of carbon dioxide (pCO2) across the air-sea boundary layer is the main driving force for the air-sea CO2 flux. Global data-bases for surface seawater pCO2 are actually based on pCO2 measurements from several meters below the sea surface, assuming a homogeneous distribution between the diffusive boundary layer and the upper top meters of the ocean.
Compiling vertical profiles of pCO2, Temperature and dissolved oxygen in the upper 5-8 meters of the ocean from different biogeographical areas, we detected a mean difference between the boundary layer and 5 [meters below the surface] pCO2 of 13 ± 1 µatm. Temperature gradients accounted for only 11 % of this pCO2 gradient in the top meters of the ocean, thus, pointing to a heterogeneous biological activity underneath the air-sea boundary layer as the main factor controlling the top meters pCO2 variability.
Observations of pCO2 just beneath the air-sea boundary layer should be further investigated in order to estimate possible biases in calculating global air-sea CO2 fluxes.