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Ocean iron fertilization (OIF) is being considered as a strategy for mitigating rising atmospheric CO2 concentrations. One model for implementation is the sale of carbon offsets. Model- ing studies predict that OIF has the potential to produce a material difference in the rise of atmos- pheric CO2 over the next several decades, but this could only be attained by alteration of the ecosys- tems and biogeochemical cycles of much of the world's oceans. The efficacy of OIF on this scale has not been proven. However, the consequences of successful implementation must be considered now, for 2 important reasons: (1) to determine if the environmental effects would be predictable and veri- fiable, and if so, acceptable; and (2) to establish whether the basis for valuing carbon offsets — an accurate audit of net reductions in cumulative greenhouse gas potential over 100 yr — can be met. Potential side-effects of widespread OIF that must be considered include a reduced supply of macronutrients to surface waters downstream of fertilized regions, increased emissions of the potent greenhouse gases nitrous oxide and methane, and changes in the extent or frequency of coastal hypoxia. Given the uncertainties inherent in ocean models, predictions of environmental effects must be backed up by measurements. Thus, to go forward with confidence that the effects of rising CO2 could indeed be mitigated through OIF over the next century, and to establish the foundations for auditing carbon offsets, it must be explicitly demonstrated that methods exist to predict and detect downstream effects of OIF against the background of both climate variability and global warming. We propose that until the side-effects of widespread OIF can be shown to be verifiable — and there is good reason to believe that they cannot — OIF should not be considered a viable technology for climate mitigation.

We present a simple method for describing the influence of variable attenuation of spec- tral irradiance, Kd(λ), on water-column photosynthesis and its inhibition by ultraviolet radiation (UV). The approach is based on weighted water transparency, a calculation introduced by Vincent et al. 1998 (Ann Glaciol 27:691-696). Key results of a depth- and spectrally-resolved model of photosyn- thesis can be reproduced for a broad range of water types by simple parameterizations using a refer- ence solar irradiance spectrum at the surface and water transparency (1/Kd(λ)) weighted spectrally for biological effectiveness. Transparency that has been weighted spectrally by the normalized prod- uct of irradiance and photosynthetic absorption (PUR-weighted transparency, T W) describes spec- tral effects on photosynthesis in the water column. An empirical parameterization of transparency weighted by the product of surface irradiance and the biological weighting function for inhibition of photosynthesis (T W PIR), along with T W PUR, describes the inhibition of water-column photosynthesis rela- tive to the uninhibited rate. Our approach is directly compared with an analysis that used weighted transparency as an indicator of the potential for inhibition of photosynthesis by UV as influenced by variations in chromophoric dissolved organic matter associated with climate change over the past 6000 yr (Pienitz & Vincent 2000, Nature 404:484-487). Results demonstrate how weighted trans- parency, used as an indicator of potential inhibition, can be transformed into a predictor of biological effects.