It is well established that atmospheric carbon dioxide (CO₂) levels are increasing due to human activities, and it is expected that this will result in a substantial global warming. Several ideas for removing excess CO₂ from the atmosphere have been proposed, such as sequestering the CO₂ in the deep ocean. One technique for accomplishing this could be through the fertilization of phytoplankton (algae) blooms with soluble iron, since iron is the limiting factor for phytoplankton growth in certain oceanic regions. The additional phytoplankton growth would draw CO₂ out of the atmosphere and convert it into carbohydrates. Most of this carbon is converted back to CO₂ when the phytoplankton are then consumed by zooplankton and other grazers, but a fraction of the organic material is deposited as sediment on the ocean floor. This idea has led to patent applications and business ventures with the goal of making use of oceanic iron fertilization to gain carbon credits on the world carbon trading market.

There are problems, however, with this mitigation proposal. Besides the sheer logistical issues of supplying the oceans with sufficient iron to result in a significant removal of CO₂, there may be several undesired side effects. The effects on marine ecology were pointed out last year in the literature (Chisholm, S. W., et al., Science, 12 Oct. 2001, p. 309-310). These concerns have now been expanded by Max Planck Institute for Chemistry researcher Mark Lawrence to include a number of previously undiscussed effects on atmospheric chemistry and the climate.

Phytoplankton are responsible for the production of several gases which are important in the atmosphere, including dimethyl sulfide, which leads to the formation of cloud condensation nuclei, and carbonyl sulfide and volatile organohalogens, which are believed to contribute to stratospheric ozone depletion; all of these gases have been investigated extensively by Max Planck Institute for Chemistry researchers in the Atmospheric Chemistry and Biogeochemistry departments over the past three decades. Accelerated production of these gases resulting from iron fertilization would lead to changes in the atmospheric composition and climate, which could offset the beneficial effects of CO₂ removal. The additional photosynthetic activity would also be expected to lead to a significant warming of the ocean's surface waters, which may have important consequences for oceanic circulation and the climate, especially in regions such as the Southern Oceans, where much of the iron fertilization effort would be focused. It is argued that the known potential for significant side effects is sufficient that iron fertilization should not be made eligible for carbon trading credits. Research on the mechanisms controlling phytoplankton productivity and its link to the climate and atmospheric composition should continue, but this should be driven by basic science, rather than market interests.

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