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form by the easy availability of appropriate land. Therefore the panel hesitates to look beyond its initial potential mitigation of 240 Mt CO2/yr. In addition, a number of years would be required to build reforestation to its full mitigation potential.

Stimulation of ocean biomass with iron may be feasible and would be a relatively low-cost option. Its application appears to be limited at most to the mitigation of about 7 Gt CO2 equivalent per year (about 1.5 times U.S. annual CO2 emissions). The biological, ecological, and ocean chemical and physical dynamics of this possibility are not well understood and should be investigated further, both theoretically and experimentally. There continue to be questions as to whether iron is the limiting nutrient. Furthermore, the circulation dynamics of the antarctic ocean might severely limit the effect. If feasible, the mitigation potential of the possibility—storage of CO2 in a standing crop and as dissolved CO2 with slow sequestering of carbon to the ocean bottom—could probably be established over several years. If applications of iron were stopped, the standing crop would be expected to die within days or weeks, thus ending the mitigation effect.

Cloud stimulation by provision of cloud condensation nuclei appears to be a feasible and low-cost option capable of being used to mitigate any quantity of CO2 equivalent per year. Details of the cloud physics, verification of the amount of CCN to be added for a particular degree of mitigation, and the possible acid rain or other effects of adding CCN over the oceans need to be investigated before such system is put to use. Once a decision has been made, the system could be mobilized and begin to operate in a year or so, and mitigation effects would be immediate. If the system were stopped, the mitigation effect would presumably cease very rapidly, within days or weeks, as extra CCN were removed by rain and drizzle.

Several schemes depend on the effect of additional dust (or possibly soot) in the stratosphere or very low stratosphere screening out sunlight. Such dust might be delivered to the stratosphere by various means, including being fired with large rifles or rockets or being lifted by hydrogen or hot-air balloons. These possibilities appear feasible, economical, and capable of mitigating the effect of as much CO2 equivalent per year as we care to pay for. (Lifting dust, or soot, to the tropopause or the low stratosphere with aircraft may be limited, at low cost, to the mitigation of 8 to 80 Gt CO2 equivalent per year.) Such systems could probably be put into full effect within a year or two of a decision to do so, and mitigation effects would begin immediately. Because dust falls out naturally, if the delivery of dust were stopped, mitigation effects would cease within about 6 months for dust (or soot) delivered to the tropopause and within a couple of years for dust delivered to the midstratosphere.

Such dust would have a visible effect, particularly on sunsets and sunrises, and would heat the stratosphere at the altitude of the dust. The