|
6.3.3 Halocarbons
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
|
|
|
