The SAR referred to Pinnock et al. (1995), who obtained a higher radiative forcing for CFC-11 than used in previous IPCC reports, but refrained from changing the recommended value pending further investigations. Since then several papers have investigated CFC-11, confirming the higher forcing value (Christidis et al., 1997; Hansen et al., 1997a; Myhre and Stordal, 1997; Good et al., 1998; Myhre et al., 1998b; Jain et al., 2000) with a range from 0.24 to 0.29 Wm^-2 ppbv^-1. As mentioned above, Christidis et al. (1997) found a large discrepancy in the absorption data for CFC-11 in the literature. Other causes for the difference in the radiative forcing are different treatments of the decrease in mixing ratio in the stratosphere and the fact that some estimates are performed with a single global mean column atmospheric profile. Taking these effects into account, a radiative efficiency due to CFC-11 of 0.25 Wm^-2 ppbv^-1 is used, the same value as in WMO (1999). For the present concentration of CFC-11, this yields a forcing of 0.07 Wm^-2 since pre-industrial times. In previous IPCC reports, radiative forcing due to CFCs and their replacements have been given relative to CFC-11. CFC-11 is now revised and this introduces a complicating factor since the radiative forcing for the CFCs and CFC replacements are given as absolute values in some studies, but relative to CFC-11 in others. WMO (1999) updated several of the halocarbons giving radiative forcing in absolute values (in Wm^-2 ppbv^-1).

CFC-12 is investigated in Hansen et al. (1997a), Myhre et al. (1998b), Minschwaner et al. (1998), Good et al. (1998) and Jain et al. (2000). The difference in the results is up to 20% which is due to differing impact of clouds, absorption cross-section data, and the vertical profile of decay of the mixing ratio in the stratosphere. The radiative forcing due to CFC-12 of 0.32 Wm^-2 ppbv^-1 used in WMO (1999) is retained, which is slightly higher than the SAR value. The present radiative forcing due to CFC-12 is therefore 0.17 Wm^-2, which is the third highest forcing among the well-mixed greenhouse gases.

Radiative forcing values for well-mixed greenhouse gases with non-negligible contributions at present are included in Table 6.1. Several recent studies have investigated various CFC replacements (Imasu et al., 1995; Gierczak et al., 1996; Barry et al., 1997; Christidis et al., 1997; Grossman et al., 1997; Papasavva et al., 1997; Good et al., 1998; Heathfield et al., 1998b; Highwood and Shine, 2000; Ko et al., 1999; Myhre et al., 1999; Jain et al., 2000; Li et al., 2000; Naik et al., 2000; Shira et al., 2001). For some CFC replacements not included in Table 6.1, the radiative forcings are shown in Tables 6.7 and 6.8 (Section 6.12).

The values of CFC-115 and CCl₄ have been substantially revised since the IPCC (1994) report, with a lower and higher radiative forcing estimate, respectively. Highwood and Shine (2000) calculated a radiative forcing due to chloroform (CHCl₃) which is much stronger than the SAR value. They suggest that this is due to the neglect of bands outside 800 to 1,200 cm^-1 in previous studies of chloroform. Highwood and Shine (2000) found a radiative forcing due to HFC-23 which is substantially lower than the value given in the SAR.

Table 6.1: Pre-industrial (1750) and present (1998) abundances of well-mixed greenhouse gases and the radiative forcing due to the change in abundance. Volume mixing ratios for CO2 are in ppm, for CH4 and N2O in ppb, and for the rest in ppt.
Gas	Abundance (Year 1750)	Abundance (Year 1998)	Radiative forcing (Wm-2)
Gases relevant to radiative forcing only
CO2	278	365	1.46
CH4	700	1745	0.48
N2O	270	314	0.15
CF4	40	80	0.003
C2F6	0	3	0.001
SF6	0	4.2	0.002
HFC-23	0	14	0.002
HFC-134a	0	7.5	0.001
HFC-152a	0	0.5	0.000
Gases relevant to radiative forcing and ozone depletion
CFC-11	0	268	0.07
CFC-12	0	533	0.17
CFC-13	0	4	0.001
CFC-113	0	84	0.03
CFC-114	0	15	0.005
CFC-115	0	7	0.001
CCl4	0	102	0.01
CH3CCl3	0	69	0.004
HCFC-22	0	132	0.03
HCFC-141b	0	10	0.001
HCFC-142b	0	11	0.002
Halon-1211	0	3.8	0.001
Halon-1301	0	2.5	0.001

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