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1.  Does it appear feasible that engineered systems could actually mitigate the effects of greenhouse gases?

2.  Does it appear that the proposed systems might be carried out by feasible technical means at reasonable costs?

3.  Do the proposed systems have effects, besides the sought-after effects, that might be adverse, and can these be accepted or dealt with?

An exhaustive literature search and analysis has not been completed, but it has been possible to find useful material in the literature and to make first-order estimates that suggest positive answers to these first two questions. This being the case, it seems appropriate to continue consideration of the range of geoengineering possibilities and to pursue answers to question 3 above. In virtually all cases, there are significant missing pieces of scientific understanding.

Carrying the examination further would first require more detailed understanding, theoretical modeling, and simulation analyses of the physics, chemistry, and biology in the light of what is known about the geophysical, geochemical, climate, and ecological systems. If these further analyses suggest that the answers to the questions continue to be positive, experiments could then be carried out. These would not be full-scale climate mitigation experiments, but rather experiments intended to answer questions that might still remain after theoretical analysis, e.g., questions concerning optical effects and properties of various kinds of dust or aerosols, lifetimes and cloud stimulation properties of tropospheric sulfate aerosol, and so on. There is also a need for more detailed design, development, and cost analysis of the proposed deployment systems, perhaps including experimentation with specific hardware for deployment. Such work would give much more information with which to decide whether such systems could be deployed at a reasonable cost, and whether they would be likely to work as suggested by the preliminary evaluations included below.

If the theoretical analyses, experiments, and development work show that these mitigation ideas continue to have promise, the possibility of actual deployment would raise additional issues. The global climate and geophysical, geochemical, and biological systems under examination are all highly nonlinear systems involving the interaction of many complicated feedback systems. Such systems are likely to exhibit various forms of instability, including dynamic chaos, as well as various unintended side effects. These possibilities must be seriously considered before deployment of any mitigation system, and the risks involved weighed against alternatives to the proposed system.

Would attempts to mitigate greenhouse warming using one of these geoengineering systems result in putting a global system into some unintended and undesired state? Effects that have been suggested as possible



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