Abstract
Abstract. The sensitivity of oceanic CO2 uptake to alterations in the marine biological carbon pump, such as brought about by natural or purposeful ocean fertilization, has repeatedly been investigated by studies employing numerical biogeochemical ocean models. It is shown here that the results of such ocean-centered studies are very sensitive to the assumption made about the response of the carbon reservoirs on the atmospheric side of the sea surface. Assumptions made include prescribed atmospheric pCO2, an interactive atmospheric CO2 pool exchanging carbon with the ocean but not with the terrestrial biosphere, and an interactive atmosphere that exchanges carbon with both oceanic and terrestrial carbon pools. The impact of these assumptions on simulated annual to millennial oceanic carbon uptake is investigated for a hypothetical increase in the C:N ratio of the biological pump and for an idealized enhancement of phytoplankton growth. Compared to simulations with interactive atmosphere, using prescribed atmospheric pCO2 overestimates the sensitivity of the oceanic CO2 uptake to changes in the biological pump, by about 2%, 25%, 100%, and >500% on annual, decadal, centennial, and millennial timescales, respectively. The smaller efficiency of the oceanic carbon uptake under an interactive atmosphere is due to the back flux of CO2 that occurs when atmospheric CO2 is reduced. Adding an interactive terrestrial carbon pool to the atmosphere-ocean model system has a small effect on annual timescales, but increases the simulated fertilization-induced oceanic carbon uptake by about 4%, 50%, and 100% on decadal, centennial, and millennial timescales, respectively, for pCO2 sensitivities of the terrestrial carbon storage in the middle range of the C4MIP models (Friedlingstein et al., 2006). For such sensitivities, a substantial fraction of oceanic carbon uptake induced by natural or purposeful ocean fertilization originates, on timescales longer than decades, not from the atmosphere but from the terrestrial biosphere.
Highlights
The oceanic control on atmospheric CO2 is the result of a complex interplay of physical, chemical, and biological processes
To explore the sensitivity of the results reported above to background pCO2 typical for historical and expected future CO2 levels, two model runs corresponding to COUPLED and ATCO2const were performed for prescribed increasing C:N ratios and increasing background atmospheric CO2 levels accounting for anthropogenic emissions
This study was motivated by the considerable differences in published model-derived estimates of the impact of enhanced biological production on oceanic carbon uptake
Summary
The oceanic control on atmospheric CO2 is the result of a complex interplay of physical, chemical, and biological processes. Different results have been reported for model studies investigating the impact of atmospheric dust supply and the associated fertilization with the micronutrient iron: Forcing a model with different dust scenarios including switches from interglacial to glacial conditions, Moore et al (2006) reported that, on decadal timescales, 45% to 60% of a dust-related increase in carbon export by the marine biological pump was exporting carbon removed from the atmospheric CO2 pool via air-sea exchange. The simulated ratio of changes in air-sea CO2 flux to changes in biotic carbon export, which has been referred to as atmospheric uptake efficiency (Jin et al, 2008), differs by a factor of about 2 or more among different published model results. It is probably of less relevance to explain differences between the relatively
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