Ocean iron fertilization – moving forward in a sea of uncertainty

The consequences of global climate change are profound, and the scientific community has an obligation to assess the ramifications of policy options for reducing greenhouse gas emissions and enhancing CO2 sinks in reservoirs other than the atmosphere (1, 2). Ocean iron fertilization (OIF), one of several ocean methods proposed for mitigating rising atmospheric CO2, 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 programs with small-scale, purposeful releases of iron since 1993 (3, 4). Although these experiments greatly improved our understanding 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 CO2 to the deep sea remains poorly constrained, and we do not understand the intended and unintended biogeochemical and ecological impacts. Environmental perturbations from OIF are non-local and are spread over a large area by ocean circulation, which makes long-term verification and assessment very difficult. Modeling studies have addressed sequestration more directly and have suggested 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 mitigation of global CO2 increases.