THE BOTTLENECK OF BIOLOGICAL CARBON PUMP FEEDBACKS ON ATMOSPHERIC PCO2

The biological carbon pump (BCP), i.e. the production of organic matter in the surface ocean, its export to and degradation in the ocean interior, contributes significantly to the surface-to-deep marine inorganic carbon gradient and hence provides an important control on atmospheric CO2 and climate on long time scales. The strength of the biological pump (i.e. the amount of respired carbon stored in the ocean interior) may change if either the export of organic matter, the flux attenuation in the ocean interior or the residence time of respired carbon in the ocean interior changes. For the transient period of human-induced climate change (e.g. 1850 to 2100), current models suggest that the strength of the biological pump actually increases despite a decrease in export production, as is evident from a decrease in ocean oxygen and an increase in apparent oxygen utilization. This transient change in biological pump strength, however, does not translate 1:1 to a respective carbon exchange with the atmosphere. Using an Earth System model of intermediate complexity we show that the erosion of the surface ocean CO2-buffer capacity, a direct effect of the uptake of anthropogenic CO2 by the ocean, is a gate-keeper of the CO2 flux attributable to changes of the biological pump. For different CO2-emission scenarios, the effective atmosphere-ocean carbon flux attributable to BCP-change is 2-5 times smaller compared to what might be expected from the change in BCP strength. We propose that the buffer capacity erosion affecting all atmosphere-ocean carbon fluxes provides a general bottleneck of marine ecological feedbacks on atmospheric pCO2 under transient climate conditions.