that could be sold to those needing to offset emissions in the international cap-and-trade system that seems likely to be negotiated. Results to date are not nearly definitive, and the scientific community has raised a number of concerns about the effectiveness and safety of iron fertilization (e.g., see http://www.scor-int.org/SCOR-GESAMP.pdf). Questions meriting investigation include: whether adding micronutrients in one location to utilize apparently unused macronutrients is just displacing growth of marine life that would have occurred further along the water’s path; whether the carbon that sinks is really kept away from the atmosphere for centuries or is only dissolved a bit deeper in the ocean and so would be released back to the atmosphere within a few decades; and how all of the accounting is being done, because drawing CO2 into the ocean in one location will just lead, by adjustment of the chemical equilibrium between the atmosphere and upper ocean, to out-gassing elsewhere of at least a fraction of the CO2 that is being taken up? In addition, the long-term problems with this approach are that: (a) the higher the ocean carbon content, the more acidic the ocean waters are, thereby adversely affecting the ability of many marine organisms to form carbonate shells; and (b) the warmer the ocean gets, the less the ocean overturns, and so the less nutrient-rich waters are brought to the surface, diminishing the level of natural ocean biological activity and the pumping of carbon to the deep ocean.

There have also been proposals to basically farm the ocean for biomass. Such proposals are at the interface of mitigation and geoengineering in that they are, basically, biologically harvesting the solar energy that falls on the ocean surface. The American Methanol Institute a number of years ago proposed growing algae out in the equatorial Pacific, and augmenting the available ocean nutrients by using a fraction of the resulting biofuel (in their case, methanol) to bring up deeper, nutrient-rich waters. The rest of the resulting methanol was to be used to displace use of fossil fuels, so this ocean-based approach was intended mainly as a way to mitigate fossil fuel emissions. Although not now so focused on sequestration, the present range of activities in this area is quite broad (e.g., see http://www.algaebiofuelsummit.com/), and might well be combined with efforts to enhance fish production, so helping to increase global food stocks.

While most attention has gone to removal of atmospheric CO2, removing other greenhouse gases, especially those that have a high global warming potential (GWP) as compared to CO2, also merits consideration as a geoengineering approach. One proposal, which arose in thinking about the challenge of meeting the goal of the Montreal Protocol to limit depletion of stratospheric ozone, was to use lasers to dissociate, and thus destroy, chlorofluorocarbons before they can contribute to ozone depletion. Such an approach may merit further consideration as a means of limiting the long-acting warming influence of many of the long-lived halocarbons and perfluorocarbons. To make up for their low concentration, the notion was to bounce a laser signal many times between mirrors on nearby mountaintops such that the pathlength was extended and an energetic photon was more likely to actually strike a halocarbon molecule. Finding a way to destroy methane (CH4) in this manner might be particularly useful, given that its 20-year GWP is ~3 times its 100-year value (Forster and Ramaswamy, 2007), meaning that reducing the concentration of methane to well below its present concentration, whether by mitigation or geoengineering, would have an early and significant limiting effect on the warming influence of greenhouse gases.

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