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into the atmosphere, each greenhouse gas is removed from the atmosphere (e.g., by transport to the oceans or by chemical reaction), but no account is taken of the extent to which one greenhouse gas (including ozone and water vapor in the stratosphere) may be affected by or introduced as a by-product of a chemical reaction that depletes another. In particular, in accord with the uncertainties attending the fate of CO2 emissions (Emanuel et al., 1989), the procedure is approximately consistent with the observations of the past century; i.e., approximately 60 percent of the CO2 emissions introduced into the atmosphere are removed promptly, and the remaining 40 percent contribute to long-term (i.e., several century) enhancement of the CO2 concentration. The current concentrations, current emission rates, and lifetimes of the most important of the greenhouse gases that were considered are given in Table 17.1, and projected concentrations are shown in Figure 17.1. The radiative forcing associated with each of these gases is depicted as a function of its concentration level in Figure 17.2.

As indicated in Chapter 18, the Effects Panel agrees that it is plausible to expect that the increase in the equilibrium global mean temperature of our climatic system that might be implied by an equivalent CO2 doubling would

TABLE 17.1 1990 Atmospheric Concentrations, Emissions, and Lifetimes of Key Greenhouse Gases