Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 6 8 2009 10.5194/bg-6-1603-2009 http://www.biogeosciences.net/6/1603/2009/ http://www.biogeosciences.net/6/1603/2009/bg-6-1603-2009.html http://www.biogeosciences.net/6/1603/2009/bg-6-1603-2009.pdf 1603 1613 2009-08-11 Impact of atmospheric and terrestrial CO₂ feedbacks on fertilization-induced marine carbon uptake A. Oschlies aoschlies@ifm-geomar.de IFM-GEOMAR, Leibniz-Institut für Meereswissenschaften, Kiel, Düsternbrooker Weg 20, 24105 Kiel, Germany 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 <i>p</i>CO₂, 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 <i>p</i>CO₂ 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. 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