Factors limiting the reduction of atmospheric CO₂ by iron fertilization

Abstract

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, we 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, we 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 we 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. These results are independent of the respiration function adopted. As we hold the surface water PO4 content at a near-zero value, it makes no difference at what depth the organic material is oxidized (or whether it falls to the bottom and accumulates). 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. The length of the productive season influences the extent of pCO2 reduction. For 8 months of productive fertilization our model yields a reduction of 60 µatm, for 4 months a reduction of 50 µatm, and for 2 months a reduction of 40 µatm. The maximum O2 consumption in our standard case is estimated to be 133 µmol kg‒1 at a depth of 600 m. However, O2 consumption depends on the reoxidation function in the subsurface water. If the organic flux reoxidizes completely in the upper 2,000 m, the maximum consumption of O2 at 500 m could reach 500 µmol kg‒1. Hence, depending on the reoxidation function, an anoxic Antarctic thermocline could result from Fe fertilization. Both calculations regarding the seasonality of production and those regarding oxygen reduction are highly sensitive to parameters over which we have little control. They are included only to emphasize their potential importance.