Greenhouse Effect of Chlorofluorocarbons and Other Trace Gases

We compare the radiative (greenhouse) forcing of the climate system due to changes of atmospheric chlorofluorocarbons and other trace gases. We find that CFCs, defined to include chlorofluorocarbons, chlorocarbons, and fluorocarbons, now provide about one-quarter of current annual increases in anthropogenic greenhouse climate forcing. If the growth rates of CFC production in the early 1970s had continued to the present, current annual growth of climate forcing due to CFCs would exceed that due to CO 2.

parameters are held fixed), computed with a one-dimensional radiative-convective climate model [Lacis et al., 1981]. Spectral intervals for the thermal infrared region are of the width 50 cm 'l. Overlapping absorption by different gases within each spectral interval is approximated as uncorrelated, i.e., the net transmission is based on the sum of all products of individual (k-distribution) transmissions. Absorption by CFCs is assumed to be optically thin, so that CFC absorption can be combined linearly with the k-distribution of the other atmospheric gases and cloud particles.
The full climate response at equilibrium is 2-4 times larger than AT o if global climate sensitivity for doubled CO 2 is in the range 2.5øC to 5øC, as estimated with current global climate models (GCMs). The time required to approach the equilibrium response is estimated to be at least several decades because of the large thermal inertia of the global ocean [Dickinson, 1986;Wigley and Schlesinger, 1985;Hansen et al., 1985]. Uncertainties in the decadal changes of trace gas abundances are typically less than or of the order of 10% for the major species. Additional error in the net CFC climate forcing is due to uncertainties in infrared absorption data. Band strength measurements for the two principal CFC greenhouse gases, CFC-11 and CFC-12, appear to be reproducible within about 10%. But band strengths of other CFCs, most importantly CFC-22 and CFC-113, are more uncertain.
Recent measurements (D. Fisher, private communication, 1989) show band strengths for CFC-22 that are a factor of 2.5 larger than the unpublished values used by Ramanathan et al. [1985] and for CFC-113 35% smaller than the band strengths of Rogers and Stephens [1988]. The uncertainty in the total CFC climate forcing due to inaccuracies of the absorption coefficient data thus is at least of the order of 10%. Because the uncertainties for individual gases are so large, we have included in Table 1 the band strengths which we used for each gas. This allows our results to be scaled as more accurate absorption data become available, because the absorption bands are generally weak and thus in a linear regime. Figure 1 summarizes the results for the radiative forcing of the climate system due to increases of atmospheric CFCs in the 1980s. CFC-12 and CFC-11 account for two-thirds of the CFC climate forcing added in the 1980s. However, CFC-113 and CFC-22, which are growing more rapidly (Table 1), are approaching the magnitude of the CFC-12 and CFC-11 climate forcings. The climate forcings by the other CFCs are individually an order of magnitude smaller (Table 1), but their combined effect is not negligible (Figure 1). The results tabulated in Table 1 (Table 1), it seems likely that CFC-22 will become a substantial greenhouse gas in the future, especially if it becomes a major substitute for CFC-12 and CFC-11.

COMPARISON OF CFC AND OTHER GREENHOUSE FORCINGS
We compare the radiative forcing due to CFCs and other greenhouse gases, for the past few decades and the preceding century. Estimated abundances for the gases are given in  , 1988] and the assumption of simple lifetimes (60 and 120 years for CFC-11 and CFC-12, respectively) with no lag between production and release. The calculated CFC atmospheric amounts are in good agreement with observations over the past several years (perhaps accidentally because failure to include production by nonreporting companies approximately compensates for the time lag between CFC production and release into the atmosphere). Figure 2 compares the decadal increments of climate forcing due to measured greenhouse gas changes since 1850. CO 2 is the dominant greenhouse gas (62%) for the full period 1850-1990, with CH 4 next at 21%, CFCs at 13%, and N20 4%.
The rate of increase of greenhouse forcing has accelerated in recent decades. Indeed, more than 50% of the total added forcing for the period 1850-1990 has been added during the last 30 years. The increase of CFC forcing in the 1980s represents about one-quarter of the total growth in radiative forcing by trace gases (Figure 3). Indeed the CFC contribution to growth of the greenhouse forcing now dearly exceeds that of CH 4 and N20 combined. The CFC component of the greenhouse forcing has continued to increase, despite slowdowns for CFC-11 and CFC-12, because of rapid growth of other CFCs, as shown in Figure 3.
We note that there are at least two other changing greenhouse gases: ozone and stratospheric water vapor.

Water vapor is the dominant greenhouse gas in the Earth's atmosphere. Change of tropospheric water vapor is considered
to be a dimate feedback, rather than a dimate forcing, because the water vapor amount is determined by the climate, especially by the temperature. But stratospheric water vapor may be increasing as a result of the increasing abundance of atmospheric methane [Ehhalt, 1986]. This could cause a significant greenhouse warming; for example, a doubling of stratospheric water vapor from 3 to 6 ppm at altitudes between Because adequate measurements are not available, we do not include either ozone or stratospheric water vapor changes in our results. Based on the discussion above, it seems likely that these two omissions are partially offsetting.

IMPACt OF PRODUC'rION CONSTRAINTS IN 1970S
Prior to 1974 the production of CFC-12 and CFC-11 was increasing by about 8-11% per year (Figure 4a). After public concern was raised that CFCs may destroy stratospheric ozone there was a sharp break in the growth of CFC production, initially because consumers voluntarily turned to alternative products and, shortly thereafter, because legislatures in the United States, Canada and a few European countries passed laws restricting certain CFC uses.
If the growth rates of the early 1970s had continued to the present, the annual increments of greenhouse forcing by CFCs would now exceed that for CO 2 (Figure 4b). Of course, some slowdown in CFC growth probably would have occurred due to economic forces, even without environmental concerns. But it appears that CFC greenhouse forcing would now be much greater than it is, at least comparable in magnitude to that for CO2, if there had been no public concern about possible adverse effects of continued CFC growth.

DISCUSSION
These results illustrate that CFCs are a large fraction, about one quarter, of current additions to greenhouse climate forcing. Thus, if the rate of release of CFCs to the atmosphere can be reduced, there is the potential for a major reduction in the rate of increase of the greenhouse effect. It should be noted that many of the proposed halocarbon substitutes for CFCs contain only fluorine, and, while posing no threat to the ozone layer, they may still contribute to an increased greenhouse effect. Assessment of the greenhouse impact of possible constraints on CFC emissions requires better data for the infrared absorption coefficients of various CFCs. These data are needed especially for compounds, such as CFC-22, which may be substituted for other CFCs.
If the growth of CFC production in the early 1970s had continued to date, the CFCs would now cause a much larger greenhouse effect, greater than that for CO 2. This illustrates that growth trends of greenhouse forcing are not inevitable; in this example decisions of consumers and legislators had a major impact on global climate forcing. This example also shows that, even if emissions are not eliminated, there is eventually a great difference between the greenhouse forcing with continued rapid growth of emissions and the greenhouse forcing with more constant emission rates. Of course, this conclusion applies for CO 2 emissions as well as CFCs. Trace actual climate forcing may be much less than in these "business as usual" scenarios.
Finally, we emphasize that full analysis of greenhouse climate forcing requires better measurements of other atmospheric constituents. The biggest uncertainties appear to arise from the lack of adequate monitoring of ozone in the upper troposphere and lower stratosphere, and of water vapor in the stratosphere.