Biological Sequestration in the Oceans

In equation (3), this option promotes CO2 uptake in the ocean component of FSq. Ocean Fertilization

The efficiency and duration of carbon storage by ocean fertilization remain poorly defined and strongly depend on the oceanic region and fertilizer (iron, nitrogen, phosphorus) used (Bakker, Chapter 26, this volume). The maximum potential of iron fertilization has been estimated as 1 PgC yr-1 by continuous fertilization of all oceanic waters south of 30°S (Valparaiso, Cape Town, Perth) for 100 years (Sarmiento and Orr 1991). This model study, however, probably strongly overestimates the potential for carbon storage by its assumption of complete nutrient depletion (Bakker, Chapter 26). Furthermore, roughly half of the stored carbon would be rapidly released to the atmosphere upon termination of the fertilization. Verification of actual C sequestration remains an issue.

Enhanced algal growth upon ocean fertilization of the surface oceans will decrease oxygen levels and may create anoxic (near-zero oxygen) conditions at intermediate depths (Fuhrman and Capone 1991; Sarmiento and Orr 1991), which will promote production of N2O and methane. The release of these potent greenhouse gases to the atmosphere will partly offset or even outweigh the reduction in radiative forcing by atmospheric CO2 mitigation, especially for the equatorial Pacific Ocean (Jin and Gruber 2002; Jin et al. 2002; Bakker, Chapter 26). Increases in biological production may also result in the release of dimethyl sulphide (DMS) (Turner et al. 1996) and halocar-bons (Chuck 2002) to the atmosphere, which will create powerful feedbacks on atmospheric chemistry and global climate.

Large-scale iron fertilization will profoundly change oceanic ecosystems, ocean bio-geochemistry, and the composition of oceanic sediments (Watson et al. 2000a; Duck-low et al. 2003; Bakker, Chapter 26). A shift toward larger algal species was observed in iron fertilization experiments (Coale et al. 1996; Boyd et al. 2000). A comparison to mariculture and coastal seas suggests that harmful algal blooms may occur in intensively managed systems (Bakker, Chapter 26). Fertilization might affect fish stocks, and thence fisheries and other economic sectors.

The public is very concerned about large-scale fertilization manipulations of the surface ocean for carbon storage. International law with respect to fertilization is ambiguous.

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