Greenhouse gases in the atmosphere have an important influence on the climate of our planet. Simply stated, greenhouse gases impede the outward flow of infrared radiation more effectively than they impede incoming solar radiation. Because of this asymmetry, the earth, its atmosphere, and its oceans are warmer than they would be in the absence of such gases.
The major greenhouse gases are water vapor, carbon dioxide (CO2), methane (CH4), chlorofluorocarbons (CFCs) and hydrogenated chlorofluorocarbons (HCFCs), ozone (O3), and nitrous oxide (N2O). Without the naturally occurring greenhouse gases (principally water vapor and CO2), the earth's average temperature would be about 33°C (59°F) colder than it is, and the planet would be much less suitable for human habitation.
Human activity has contributed to increased atmospheric concentrations of CO2, and CH4, and CFCs. The increased atmospheric concentrations of greenhouse gases may increase average global temperatures. The possible warming due to increased concentrations of these gases is called ''greenhouse warming." The atmospheric concentration of CO2 in 1990 was 353 parts per million by volume (ppmv), about 25 percent greater than it was before the Industrial Revolution (about 280 ± 10 ppmv prior to 1750). Atmospheric CO2 is increasing at about 0.5 percent per year. The concentration of CH4 was 1.72 ppmv in 1990, or slightly more than twice that before 1750. It is rising at a rate of 0.9 percent per year. CFCs do not occur naturally and were not found in the atmosphere until production began a few decades ago. Continued increases in atmospheric concentrations of greenhouse gases would affect the earth's radiative balance and might cause a significant amount of additional greenhouse warming.
General circulation models (GCMs) are the principal tools used to project climatic changes. At their present level of development, GCMs project that
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.