Factors limiting the reduction of atmospheric CO2 by iron fertilization

A limit on the reduction in atmospheric CO, partial pressure (PC03 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 CO, with and without fertilization, we adopt a “business-as-usual” scenario of anthropogenic CO, emission. Such increase is computed from the atmospheric pC0, in the oceanatmosphere 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-I vertical diffusivity and 17.4 Sv upwelling flux, as derived from distribution of bomb-14C in the ocean. Hence, if atmosphericpC0, reaches 800 patm in the next century, the maximum possible reduction is - 64 patm. Doubling of upwelling flux to 34.8 Sv results in a reduction of 96 palm. 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 - 7 1 patm. These results are independent of the respiration function adopted. As we hold the surface water PO, 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 CO, drawdown. Doubling the gas exchange rate in the Antarctic region after fertilization results in a reduction of 68 patm. Doubling of vertical diffusivity to 6 cm2 s-l in Antarctic deep water yields a reduction of 75 patm. 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 ofpC0, reduction. For 8 months of productive fertilization our model yields a reduction of 60 patm, for 4 months a reduction of 50 patm, and for 2 months a reduction of 40 patm. The maximum 0, consumption in our standard case is estimated to be 133 pmol kg -I at a depth of 600 m. However, 0, 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 0, at 500 m could reach 500 pmol kg-‘. Hence, depending on the reoxidation function, an anoxic Antarctic thermocline could result from