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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

   

1990 Emissions

 

Species

1990 Atmospheric Concentration

Natural

Anthropogenica

Assumed Lifetime (years)

CO2

354 ppmv

6 Pg C/yr

b

CH4

1.72 ppmv

200 Tg/yr

340 Tg/yr

10

N2O

310 ppbv

9.3 Tg N/yr

4 Tg N/yr

150

CCl4

146 pptv

119 Gg/yr

50

CH3CCl3

158 pptv

738 Gg/yr

7

CFC-11

280 pptv

361 Gg/yr

60

CFC-12

484 pptv

428 Gg/yr

130

CFC-113

60 pptv

202 Gg/yr

90

CFC-114

15 pptv

15.7 Gg/yr

200

CFC-115

5 pptv

6.9 Gg/yr

400

HCFC-22

122 pptv

179 Gg/yr

15

Halon-1301

2 pptv

7 Gg/yr

110

aOnly anthropogenic emissions are assumed to increase or decrease because of future policy and technological developments.

bThere is no simple method for calculating CO2 lifetime.

SOURCE: Courtesy of Michael C. MacCracken.



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