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

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

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.

hese are options that would involve large-scale engineering of our environment in order to combat or counteract the effects of changes in atmospheric chemistry. Most of these options have to do with the possibility of compensating for a rise in global temperature, caused by an increase in greenhouse gases, by reflecting or scattering back a fraction of the incoming sunlight. Other geoengineering possibilities include reforesting the United States to increase the storage of carbon in vegetation, stimulating an increase in oceanic biomass as a means of increasing the storage and natural sequestering of carbon in the ocean, decreasing CO2 by direct absorption, and decreasing atmospheric halocarbons by direct destruction. It is important to recognize that we are at present involved in a large project of inadvertent "geoengineering" by altering atmospheric chemistry, and it does not seem inappropriate to inquire if there are countermeasures that might be implemented to address the adverse impacts.

Control of population growth has the potential to make a major contribution to raising living standards and to easing environmental problems like greenhouse warming. The United States should resume full participation in international programs to slow population growth and should contribute its share to their financial and other support. (p. 65)

Population size and economic activity both affect greenhouse gas emissions. Even with rapid technological advances, slowing global population growth is a necessary component of a long-term effort to control worldwide emissions of greenhouse gases. Reducing population growth alone, however, may not reduce emissions of greenhouse gases because it may also stimulate growth in per capita income. If the nature of economic activity (especially energy use) changes, some growth will be possible with far less greenhouse gas emissions.

Encouraging voluntary population control programs is of considerable benefit for slowing future emissions of greenhouse gases. In addition, countries vulnerable to the possible impacts of climate change would be better able to adapt to those changes if their populations were smaller and they had higher per capita income.