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Evaluating Geoengineering Options

"Several geoengineering options appear to have considerable potential for offsetting global warming and are much less expensive than other options being considered."

"Because these options have the potential to affect the radiative forcing of the planet, because some of them cause or alter a variety of chemical reactions in the atmosphere, and because the climate system is poorly understood, such options must be considered extremely carefully. These options might be needed if greenhouse warming occurs, climate sensitivity is at the high end of the range considered in this report, and other efforts to restrain greenhouse gas emissions fail."

"The first set of geoengineering options screens incoming solar radiation with dust or soot in orbit about the earth or in the atmosphere. The second set changes cloud abundance by increasing cloud condensation nuclei through carefully controlled emissions of particulate matter."

"The stratospheric particle options should be pursued only under extreme conditions or if additional research and development removes the concern about these problems. The cloud stimulation option should be examined further and could be pursued if concerns about acid rain could be managed through the choice of materials for cloud condensation nuclei or by careful management of the system. The third class increases ocean absorption of CO2 (carbon dioxide) through stimulating growth of biological organisms."

Screening Out Some Sunlight

"Another option for mitigating a global warming would be to try to control the global radiation balance by limiting the amount of incoming radiation from the sun."

"This could be done by increasing the reflectivity of the earth, i.e., the albedo. Proposals for increasing the whiteness of roofs and surface features would have some effect, but only a fraction of incident solar radiation reaches the earth's surface and a purposeful change in albedo would have more impact if done high in the atmosphere. According to Ramanathan (1988), an increase in planetary albedo of just 0.5 percent is sufficient to halve the effect of a CO2 doubling.

Placing a screen in the atmosphere or low earth orbit could take several forms: it could involve changing the quantity or character of cloud cover, it could take the form of a continuous sheet, or it could be divided into many ''mirrors" or a cloud of dust. Preliminary characterizations of some of the possibilities that might be considered are provided below."

"Aircraft Exhaust Penner et al. (1984) suggested that emissions of 1 percent of the fuel mass of the commercial aviation fleet as particulates, between 40,000- and 100,000-foot (12- to 30-km) altitude for a 10-year period, would change the planetary albedo sufficiently to neutralize the effects of an equivalent doubling of CO2."

"They proposed that retuning the engine combustion systems to burn rich during the high-altitude portion of commercial flights could be done with negligible efficiency loss."

"Using Reck's estimates of extinction coefficients for particulates (Reck, 1979a, 1984), they estimated a requirement of about 1.168 × 1010 kg of particulates, compared with the panel's estimate of 1010 kg, based upon Ramaswamy and Kiehl (1985). They then estimated that if 1 percent of the fuel of aircraft flying above 30,000 feet is emitted as soot, over a 10-year period the required mass of particulate material would be emitted.

However, current commercial aircraft fleets seldom operate above 40,000 feet (12 km), and the lifetimes of particles at the operating altitudes will be much shorter than 10 years. An estimate (National Research Council, 1985) for the half-life of smoke is 1.4 × 10-7/s.14 This gives a half-life of 83 days, or a little less than one-quarter of a year. Thus the amount of fuel to be turned into soot continuously for complete mitigation (1012 t C) is closer to 40 percent than to 1 percent. That seems impractical. However, if the amount of mitigation required is equivalent to the 1989 U.S. emissions of greenhouse gases equivalent to CO2 (8 × 109 t CO2), the amount of soot required would be 500 times smaller, and the required soot corresponds to less than 0.1 percent of the fuel burned. If 1 percent of the fuel were used, about 25 × 109 t CO2/yr could be mitigated.

In 1987, 16 percent of the cash operating expenses of airlines were spent on fuel (U.S. Bureau of the Census, 1988). Because the operating revenue in that year was $45,339 million, the approximate cost of the particulate emissions from jet engines for mitigation of the 1989 U.S. CO2 equivalent emissions would be about $7 million, or about $0.001/t CO2/yr plus the capital costs of adjusting the aircraft engines. This provides a cost range of $0.001 to $0.1/t CO2/yr."

"An alternate possibility is simply to lease commercial aircraft to carry dust to their maximum flight altitude, where they would distribute it."

"To make a cost estimate, a simple assumption is made that the same amount of dust assumed above for the stratosphere would work for the tropopause (the boundary between the troposphere and the stratosphere). The results can be scaled for other amounts. The comments made above about the possible effect of dust on stratospheric ozone apply as well to ozone in the low stratosphere, but not in the troposphere. The altitude of the tropopause varies with latitude and season of the year. In 1987, domestic airlines flew 4,339 million ton-miles of freight and express, for a total express and freight operating revenue of $4,904 million.

(U.S. Bureau of the Census, 1988). This gives a cost of slightly more than $1 per ton-mile for freight. If a dust distribution mission requires the equivalent of a 500-mile flight (about 1.5 hours), the delivery cost for dust is $500/t, and ignoring the difference between English and metric tons, a cost of $0.50/kg of dust. If 1010 kg must be delivered each 83 days, (provided dust falls out at the same rate as soot), 5 times more than the 1987 total ton-miles will be required."

"The question of whether dedicated aircraft could fly longer distances at the same effective rate should be investigated."

"However, if the requirement is to mitigate the 1989 U.S. emissions of CO2, 500 times less dust is needed, the cost is about $10 million per year, and implementation would require about 1 percent of the ton-miles flown in 1987. If 10 percent of the ton-miles flown in 1987 were used, the system could mitigate 80 Gt CO2. These costs should probably be increased by the cost of delivered dust (say, $0.50/kg) and of delivery systems in the aircraft, but better-than-average freight rates could probably be arranged. Thus the costs appear to be about $0.0025/t CO2."

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Changing Cloud Abundance

"Independent studies estimated that an approximately 4 percent increase in the coverage of marine stratocumulus clouds would be sufficient to offset CO2 doubling (Reck, 1978; Randall et al., 1984). Albrecht (1989) suggests that the average low-cloud reflectivity could be increased if the abundance of cloud condensation nuclei (CCN) increased due to emissions of SO2."

"It is proposed that CCN emissions should be released over the oceans, that the release should produce an increase in the stratocumulus cloud albedo only, and that the clouds should remain at the same latitudes over the ocean where the surface albedo is relatively constant and small."

"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 compounds 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."

"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."

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