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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. Despite their theoretical potential, there is convincing evidence that the stratospheric particle options contribute to depletion of the ozone layer. 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 through stimulating growth of biological organisms. The panel recommends that research projects be undertaken to improve understanding of both the potential of these options to offset global warming and their possible side effects. Such assessments should involve international cooperation. This is not a recommendation for implementing these options at this time.

Exercising International Leadership

As the largest source of current greenhouse gas emissions, the United States should exercise leadership in addressing responses to greenhouse warming.

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.