The ocean contains far more carbon than the atmosphere or land ecosystems. Storage of carbon in the ocean occurs by several mechanisms whose rates can be altered by human activities. In the ocean, CO2 dissolves directly in sea water; CO2 is sequestered when marine plants photosynthesize, and organic carbon ultimately sinks to great depths; and CO2 is also sequestered by conversion to CaCO3 by plankton, invertebrates, and fish (Wilson et al., 2009), CaCO3 that either forms sediments or sinks to deep water after the organism dies. None of these forms of storage is permanent, but sequestration rates can be modified greatly by a variety of factors (e.g., water temperature, pH, and the abundance of fish and plankton), ultimately affecting how much CO2 remains locked away or returns to the sea.
Because the oceans provide such an enormous reservoir for carbon storage, it may be possible to manipulate (i.e., geoengineer—see Chapter 15) ocean ecosystems to cause a transfer of CO2 from the atmosphere to the oceans. Several different approaches have been proposed to achieve this end, most of them involving the introduction of some kind of fertilizer to the upper ocean. The basic hypothesis is that fertilization may stimulate the incorporation of dissolved CO2 into organic matter through phytoplank-ton blooms, which could then sink to the deeper ocean. Some of the carbon that sinks out of the upper ocean should be replaced by CO2 from the atmosphere, thus reducing atmospheric CO2 concentrations.
Most of the attention given to the ocean fertilization hypothesis has focused on iron (Martin and Fitzwater, 1988; see also Limiting the Magnitude of Future Climate Change [NRC, 2010c]). In some parts of the ocean, especially the Southern Ocean and parts of the equatorial Pacific Ocean, marine biological productivity is limited by the availability of iron. The ratios of carbon to iron in marine phytoplankton typically exceed 10,000 to 1, so there is the potential that small amounts of iron could lead to substantial carbon uptake in the form of phytoplankton blooms. While there is still considerable uncertainty, the prevailing view is that this approach could store some carbon, but maximum achievable sustainable rates might be only a small fraction of the total carbon emitted due to fossil fuel emissions (Buesseler et al., 2008). There have been various proposals to fertilize the ocean with other nutrients, such as phosphate or nitrogen, or to fertilize the oceans by bringing up nutrients from the deep ocean, but these approaches have received even less study and attention on either their potential efficacy in reducing atmospheric CO2 or their broader environmental impacts.
In general, significant uncertainties remain about the effectiveness of ocean fertilization at removing CO2 from the atmosphere, as well as the length of time this CO2
would stay isolated from the atmosphere. Furthermore, there is considerable uncertainty about the impact of these manipulations on marine ecosystems and the services they provide to society, particularly since CO2 causes ocean acidification, which is expected to harm marine ecosystems. Much effort has been focused on trying to protect marine ecosystems by keeping CO2 out of the ocean, whereas ocean fertilization proposals seek to do the opposite. Because large parts of the oceans are a global commons, regulation of such activities represents a significant issue that has yet to be addressed. Furthermore, verification of amounts of carbon stored by ocean fertilization activities would be challenging, at best.
In summary, it is feasible that human manipulation of marine ecosystems could store at least some extra CO2 in the oceans. While maximum storage rates are projected to be at most a few percent of total human-generated GHG emissions, significant questions remain regarding exactly how much carbon could be stored, and for how long, using these approaches. Furthermore, considerations such as ocean acidification and the difficulty of predicting responses of marine ecosystems make it doubtful whether such manipulations could contribute to overall environmental risk reduction.
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