11 JANUARY 2008 VOL 319 SCIENCE www.sciencemag.org 162 POLICY FORUM T he consequences of global climate change are profound, and the scien-tific community has an obligation to assess the ramifications of policy options for reducing greenhouse gas emissions and enhancing CO 2 sinks in reservoirs other than the atmosphere ( 1 , 2 ). Ocean iron fertilization (OIF), one of sev-eral ocean methods proposed for mitigating rising atmospheric CO 2 , involves stimulating net phytoplankton growth by releasing iron to certain parts of the surface ocean. The international oceanographic community has studied OIF, including 12 major field pro-grams with small-scale, purposeful releases of iron since 1993 ( 3 , 4 ). Although these experiments greatly improved our under-standing of the role of iron in regulating ocean ecosystems and carbon dynamics, they were not designed to characterize OIF as a carbon mitigation strategy. The efficacy by which OIF sequesters atmospheric CO 2 to the deep sea remains poorly constrained, and we do not understand the intended and un-intended biogeochemical and ecological impacts. Environmental perturbations from OIF are nonlocal and are spread over a large area by ocean circulation, which makes long-term verification and assessment very diffi-cult. Modeling studies have addressed sequestration more directly and have sug-gested that OIF in areas of persistent high nutrients (so-called high-nutrient, low-chlorophyll areas) would be unlikely to sequester more than several hundred million tons of carbon per year. Thus, OIF could make only a partial contribution to mitiga-tion of global CO 2 increases. Despite these uncertainties in the science,
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