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Further thought is likely to lead to simpler, more automatic, and cheaper means for the distribution of iron fertilizer.

Some Problems

Peng and Broecker (1991) argue that on dynamic considerations the scheme of fertilization by iron is unlikely to succeed as a continuing full-scale pump of CO2 from the atmosphere into the ocean. They argue that vertical transport in the antarctic is sufficiently sluggish that the flow of CO2 into the ocean from the atmosphere would rapidly saturate the surface waters with CO2, and, since the circulation would not carry the saturated water away fast enough, the transfer of CO2 from the atmosphere into the ocean would stop. This presumably would leave the storage of CO2 (as carbon) represented by the standing crop of algae in place as long as the fertilization were to continue. They estimate that 100 years of fertilization would result in a lowering of the atmospheric CO2 content by 30 ± 15 ppm. It is possible that small-scale experiments might be devised to test this by fertilizing a limited area for a period of time and studying the results. Recently reported observations by De Baar et al. (1990) suggest that iron may not be the sole limiting factor in antarctic phytoplankton growth.

In addition to using microalgae to assimilate CO2, use of macroalgae (seaweed) has also been proposed. Advantages of macroalgae include a faster rate of sedimentation, as well as their value as a biomass fuel and a source of chemicals and feeds. Their use, however, would require an engineered system of production to achieve large areas of cultivation.

There are numerous questions to be answered pertaining to the use of both microalgae and macroalgae for CO2 assimilation before better estimates of costs of carbon removal can be made for either system. Key aspects associated with cost projections include productivity rates in open oceans, nutrient recycle, micronutrient (especially iron) limitations, and the detailed design of a system for the controlled delivery of millions of tons of iron over large areas of open oceans.

As with other mitigation options, important environmental questions raised by the use of algae to assimilate CO2 include those concerning food chain effects, the introduction and proliferation of nonindigenous species (especially for macroalgae), anaerobic decomposition of algae to CH4, and the possible formation of large amounts of haloforms and dimethyl sulfide. As noted in the discussion of cloud stimulation below, the natural organisms whose growth would be stimulated by the addition of iron might be expected to produce dimethyl sulfide, which would form cloud condensation nuclei. At times and places of little cloudiness, the area fertilized might be expected to become more cloudy, which, might in turn have a further cooling effect. There is also concern about the possibility of creating an anoxic layer in the shallow subsurface.