Factors limiting the reduction of atmospheric CO2 by iron fertilization

A limit on the reduction in atmospheric CO2 partial pressure (pCO2) in the next century resulting from purposeful Fe fertilization of the Antarctic Ocean is estimated with an advection-diffusion model calibrated with transient tracer distributions. To evaluate the possible increase in atmospheric CO2 with and without fertilization, the authors adopt a business-as-usual scenario of anthropogenic CO2 emission. Such increase is computed from the atmospheric pCO2 in the ocean-atmosphere total C system as it responds to this emission scenario. Assuming completely successful Fe fertilization, the authors calculate an 8% atmospheric CO2 reduction for a case with a 3 cm2 s-1 vertical diffusivity and 17.4 Sv upwelling flux, as derived from distribution of bomb-14c in the ocean. Hence, if atmospheric pCO2 reaches 800 μatm in the next century, the maximum possible reduction is ∼64 μatm. Doubling of upwelling flux to 34.8 Sv results in a reduction of 96 μatm. If they assume the surface area of the Antarctic Ocean is 16% of the total ocean area instead of 10% as used in the standard case, the reduction is ∼71 μatm. As they hold the surface water PO4 content at a near-zero value, it makes no difference at what depth the organic material is oxidized. Changes in the gas exchange rate over the Antarctic Ocean also do not have a significant effect on the magnitude of atmospheric CO2 drawdown. Doubling the gas exchange rate in the Antarctic region after fertilization results in a reduction of 68 μatm. Doubling of vertical diffusivity to 6 cm2 s-1 in Antarctic deep water yields a reduction of 75 μatm. The key parameters are the rate of upwelling in the Antarctic and the fate of this upwelled water