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Posted: Wed Jul 11, 2007 12:29 am Post Subject: Policy Implications of Greenhouse Warming: Mitigation, Adapt
Policy Implications of Greenhouse Warming: Mitigation, Adaptation, and the Science Base (1992)
National Academy of Sciences, National Academy of Engineering, Institute of Medicine
Page 459 of the National Academy of Sciences 1992 paper discussing geoengineering to mitigate the rapid rise of greenhouse gases in the atmosphere...
Quote:
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 CO 2 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 CO 2 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 CO 2 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 heating would have an effect on the chemistry of the stratospheric ozone layer, and this possibility must be considered before major use of such a mitigation system. The amount of dust to be added is within the range of that added from time to time by volcanic eruption, so the effects on climate would not be expected to go beyond those experienced naturally. However, either the natural or the artificial effects on the chemistry might be very serious under conditions of increased CFC chlorine in the stratosphere, and the result of having these effects continuously must be considered, so the option might not be usable. Better specification of dust characteristics and size for best effect and better data on the fallout rate of dust from various altitudes as well as on chlorine chemistry are needed. It will be important to observe the effects on stratospheric chemistry of any volcanic eruptions that occur, with special attention to separating the effects of dust, aerosol, and hydrochloric acid.
Of these systems to alter the planetary albedo, the increase of low-level marine clouds by increasing CCN and the delivery of dust to the stratosphere by using large rifles seem the most promising. The rifle system appears to be inexpensive, to be relatively easily managed, and to require few launch sites. However, the possible effect of the additional stratospheric dust on ozone chemistry may be a serious problem, and the noise of the rifles would have to be managed. Balloons also appear to be a good possibility, but the return of the balloons to ground level would require management.
Sunlight screening systems would not have to be put into practice until shortly before they were needed for mitigation, although research to understand their effects, as well as design and engineering work, should be done now so that it will be known whether these technologies are available if wanted.
Perhaps one of the surprises of this analysis is the relatively low costs at which some of the geoengineering options might be implemented. If, however, further analyses support the preliminary conclusions, it will bear further inquiry to decide if they can produce the targeted responses without unacceptable additional effects. The level at which we are currently able to evaluate the cost-effectiveness of engineering the global mean radiation balance leaves great uncertainty in both technical feasibility and environmental consequences. This analysis does suggest that further inquiry is appropriate.
