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Factors limiting the reduction of atmospheric CO 2 by iron fertilization
Author(s) -
Peng TsungHung,
Broecker Wallace S.
Publication year - 1991
Publication title -
limnology and oceanography
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.7
H-Index - 197
eISSN - 1939-5590
pISSN - 0024-3590
DOI - 10.4319/lo.1991.36.8.1919
Subject(s) - upwelling , advection , atmosphere (unit) , iron fertilization , flux (metallurgy) , environmental science , atmospheric pressure , atmospheric sciences , drawdown (hydrology) , climatology , chemistry , oceanography , meteorology , geology , phytoplankton , physics , thermodynamics , aquifer , geotechnical engineering , organic chemistry , nutrient , groundwater
A limit on the reduction in atmospheric CO 2 partial pressure ( p CO 2 ) 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 2 with and without fertilization, we adopt a “business‐as‐usual” scenario of anthropogenic CO 2 emission. Such increase is computed from the atmospheric p CO 2 in the ocean‐atmosphere total C system as it responds to this emission scenario. Assuming completely successful Fe fertilization, we calculate an 8% atmospheric CO 2 reduction for a case with a 3 cm 2 s ‒1 vertical diffusivity and 17.4 Sv upwelling flux, as derived from distribution of bomb‐ 14 C in the ocean. Hence, if atmospheric p CO 2 reaches 800 µ atm in the next century, the maximum possible reduction is ∼64 µ atm. Doubling of upwelling flux to 34.8 Sv results in a reduction of 96 µ atm. 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 µ atm. These results are independent of the respiration function adopted. As we hold the surface water PO 4 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 2 drawdown. Doubling the gas exchange rate in the Antarctic region after fertilization results in a reduction of 68 µ atm. Doubling of vertical diffusivity to 6 cm 2 s ‒1 in Antarctic deep water yields a reduction of 75 µ atm. 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 p CO 2 reduction. For 8 months of productive fertilization our model yields a reduction of 60 µ atm, for 4 months a reduction of 50 µ atm, and for 2 months a reduction of 40 µ atm. The maximum O 2 consumption in our standard case is estimated to be 133 µ mol kg ‒1 at a depth of 600 m. However, O 2 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 O 2 at 500 m could reach 500 µ mol 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.