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an increase in greenhouse gas concentrations equivalent to a doubling of the preindustrial level of atmospheric CO2 would produce global average temperature increases between 1.9° and 5.2°C (3.4° and 9.4°F). The larger of these temperature increases would mean a climate warmer than any in human history. The consequences of this amount of warming are unknown and could include extremely unpleasant surprises.

During the last 100 years the average global temperature has increased between 0.3° and 0.6°C (0.5° and 1.1°F). This temperature rise could be attributable to greenhouse warming or to natural climate variability; with today's limited understanding of the underlying phenomena, neither can be ruled out.

Increases in atmospheric greenhouse gas concentrations probably will be followed by increases in average atmospheric temperature. We cannot predict how rapidly these changes will occur, how intense they will be for any given atmospheric concentration, or, in particular, what regional changes in temperature, precipitation, wind speed, and frost occurrence can be expected. So far, no large or rapid increases in the global average temperature have occurred, and there is no evidence yet of imminent rapid change. But if the higher GCM projections prove to be accurate, substantial responses would be needed, and the stresses on this planet and its inhabitants would be serious.

It is against this backdrop that prudent, necessarily international, plans should be made and actions undertaken. These plans and actions should start with responses justified by the current credibility of the threat. They also should include preparatory measures that can set the stage for more far-reaching responses if the evidence of need becomes persuasive. It is in this setting that the Synthesis Panel performed its analyses and developed recommendations for action by the United States.

The principal findings and conclusions of the panel are summarized in Chapter 8, and its recommendations are in Chapter 9. Appendix A, "Questions and Answers About Greenhouse Warming," discusses relevant issues in a format the panel believes may be especially useful to the reader.

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