Climate Change 2001:
Working Group I: The Scientific Basis
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6.12.2 Direct GWPs

The CO2 response function used in this report is the same as that in WMO (l999) and the SAR and is based on the "Bern" carbon cycle model (see Siegenthaler and Joos, 1992; Joos et al., l996) run for a constant future mixing ratio of CO2 over a period of 500 years. The Bern carbon cycle model was compared to others in IPCC (l994), where it was shown that different models gave a range of as much as 20% in the CO2 response, with the greatest differences occurring over time-scales greater than 20 years.

The radiative efficiency per kilogram of CO2 has been updated compared to previous IPCC assessments (IPCC, 1994; SAR). Here we employ the approach discussed in WMO (l999) using the simplified formula presented in Table 6.2. We assume a background CO2 mixing ratio of 364 ppmv, close to the present day value (WMO (1995) used 354 ppmv). For this assumption, this expression agrees well with the adjusted total-sky radiative forcing calculations of Myhre and Stordal (1997); see also Myhre et al. (1998b). The revised forcing is about 12% lower than that in the SAR. For a small perturbation in CO2 from 364 ppmv, the radiative efficiency is 0.01548 Wm-2/ppmv. This value is used in the GWP calculations presented here. We emphasise that it applies only to GWP calculations and cannot be used to obtain the total radiative forcing for this key gas since pre-industrial times, due to time-dependent changes in mixing ratio as noted above. Because of this change in the CO2 forcing per mass, the CO2 AGWPs are 0.207, 0.696, and 2.241 Wm-2/yr /ppmv for 20, 100, and 500 year time horizons, respectively. These are smaller than the values used in the SAR by 13%. AGWPs for any gas can be obtained from the GWP values given in the tables presented here by multiplying by these numbers.

The decreases in the CO2 AGWPs will lead to proportionately larger GWPs for other gases compared to previous IPCC assessments, in the absence of other changes. In Tables 6.7 and 6.8, GWPs (on a mass basis) for 93 gases are tabulated for time horizons of 20, 100, and 500 years. The list includes CH4, N2O, CFCs, HCFCs, HFCs, hydrochlorocarbons, bromocarbons, iodocarbons, fully fluorinated species, fluoroalcohols, and fluoroethers. The radiative efficiencies per kilogram were derived from the values given per ppbv in Section 6.3. Several of these have been updated since the SAR, most notably that of CFC-11. Since the radiative forcings of the halocarbon replacement gases are scaled relative to CFC-11 in GWP calculations, the GWPs of those gases are also affected by this change. As discussed further in Section 6.3, the change in radiative forcing for CFC-11 reflects new studies (Pinnock et al., 1995; Christidis et al., l997; Hansen et al., l997a; Myhre and Stordal, 1997; Good et al., 1998) suggesting that the radiative forcing of this gas is about 0.25 Wm-2 ppbv-1, an increase of about 14% compared to the value adopted in earlier IPCC reports, which was based on the study of Hansen et al. (l988).

Table 6.7: Direct Global Warming Potentials (mass basis) relative to carbon dioxide (for gases for which the lifetimes have been adequately characterised).
Gas
Radiative efficiency (Wm-2 ppb-1) (from (b) unless indicated)
Lifetime (years) (from Chapter 4 unless indicated)
Global Warming Potential
Time horizon
   
   
   
20 years
100 years
500 years
Carbon dioxide CO2 See Section 6.12.2 See Section 6.12.2
1
1
1
Methane CH4 3.7x10-4 12.0*
62
23
7
Nitrous oxide N2O 3.1x10-3 114*
275
296
156
Chlorofluorocarbons
CFC-11 CCl3F 0.25 45
6300
4600
1600
CFC-12 CCl2F2 0.32 100
10200
10600
5200
CFC-13 CClF3 0.25 640 (c)
10000
14000
16300
CFC-113 CCl2FCClF2 0.30 85
6100
6000
2700
CFC-114 CClF2CClF2 0.31 300
7500
9800
8700
CFC-115 CF3CClF2 0.18 1700
4900
7200
9900
Hydrochlorofluorocarbons
HCFC-21 CHCl2F 0.17 2.0 (d)
700
210
65
HCFC-22 CHClF2 0.20§ 11.9
4800
1700
540
HCFC-123 CF3CHCl2 0.20 1.4 (a)
390
120
36
HCFC-124 CF3CHClF 0.22 6.1 (a)
2000
620
190
HCFC-141b CH3CCl2F 0.14 9.3
2100
700
220
HCFC-142b CH3CClF2 0.20 19
5200
2400
740
HCFC-225ca CF3CF2CHCl2 0.27 2.1 (a)
590
180
55
HCFC-225cb CClF2CF2CHClF 0.32 6.2 (a)
2000
620
190
Hydrofluorocarbons
HFC-23 CHF3 0.16§ 260
9400
12000
10000
HFC-32 CH2F2 0.09§ 5.0
1800
550
170
HFC-41 CH3F 0.02 2.6
330
97
30
HFC-125 CHF2CF3 0.23§ 29
5900
3400
1100
HFC-134 CHF2CHF2 0.18 9.6
3200
1100
330
HFC-134a CH2FCF3 0.15§ 13.8
3300
1300
400
HFC-143 CHF2CH2F 0.13 3.4
1100
330
100
HFC-143a CF3CH3 0.13§ 52
5500
4300
1600
HFC-152 CH2FCH2F 0.09 0.5
140
43
13
HFC-152a CH3CHF2 0.09§ 1.4
410
120
37
HFC-161 CH3CH2F 0.03 0.3
40
12
4
HFC-227ea CF3CHFCF3 0.30 33.0
5600
3500
1100
HFC-236cb CH2FCF2CF3 0.23 13.2
3300
1300
390
HFC-236ea CHF2CHFCF3 0.30 10.0
3600
1200
390
HFC-236fa CF3CH2CF3 0.28 220
7500
9400
7100
HFC-245ca CH2FCF2CHF2 0.23 5.9
2100
640
200
HFC-245fa CHF2CH2CF3 0.28& 7.2
3000
950
300
HFC-365mfc CF3CH2CF2CH3 0.21 (k) 9.9
2600
890
280
HFC-43-10mee CF3CHFCHFCF2CF3 0.40 15
3700
1500
470
Chlorocarbons
CH3CCl3   0.06 4.8
450
140
42
CCl4   0.13†† 35
2700
1800
580
CHCl3   0.11§ 0.51 (a)
100
30
9
CH3Cl   0.01 1.3 (b)
55
16
5
CH2Cl2   0.03 0.46 (a)
35
10
3
Bromocarbons
CH3Br   0.01 0.7 (b)
16
5
1
CH2Br2   0.01 0.41 (i)
5
1
<<1
CHBrF2   0.14 7.0 (i)
1500
470
150
Halon-1211 CBrClF2 0.30 11
3600
1300
390
Halon-1301 CBrF3 0.32 65
7900
6900
2700
Iodocarbons
CF3I   0.23 0.005 (a)
1
1
<<1
Fully fluorinated species
SF6   0.52 3200
15100
22200
32400
CF4   0.08 50000
3900
5700
8900
C2F6   0.26§ 10000
8000
11900
18000
C3F8   0.26 2600
5900
8600
12400
C4F10   0.33 2600
5900
8600
12400
c-C4F8   0.32§ 3200
6800
10000
14500
C5F12   0.41 4100
6000
8900
13200
C6F14   0.49 3200
6100
9000
13200
Ethers and Halogenated Ethers
CH3OCH3   0.02 0.015 (e)
1
1
<<1
(CF3)2CFOCH3   0.31 3.4 (l)
1100
330
100
(CF3)CH2OH   0.18 0.5 (m)
190
57
18
CF3CF2CH2OH   0.24 0.4 (m)
140
40
13
(CF3)2CHOH   0.28 1.8 (m)
640
190
59
HFE-125 CF3OCHF2 0.44 150
12900
14900
9200
HFE-134 CHF2OCHF2 0.45 26.2
10500
6100
2000
HFE-143a CH3OCF3 0.27 4.4
2500
750
230
HCFE-235da2 CF3CHClOCHF2 0.38 2.6 (i)
1100
340
110
HFE-245cb2 CF3CF2OCH3 0.32 4.3 (l)
1900
580
180
HFE-245fa2 CF3CH2OCHF2 0.31 4.4 (i)
1900
570
180
HFE-254cb2 CHF2CF2OCH3 0.28 0.22 (h)
99
30
9
HFE-347mcc3 CF3CF2CF2OCH3 0.34 4.5 (l)
1600
480
150
HFE-356pcf3 CHF2CF2CH2OCHF2 0.39 3.2 (n)
1500
430
130
HFE-374pc2 CHF2CF2OCH2CH3 0.25 5.0 (n)
1800
540
170
HFE-7100 C4F9OCH3 0.31 5.0 (f)
1300
390
120
HFE-7200 C4F9OC2H5 0.30 0.77 (g)
190
55
17
H-Galden 1040x CHF2OCF2OC2F4OCHF2 1.37(j) 6.3
5900
1800
560
HG-10 CHF2CHF2OCF2OCHF2 0.66 12.1
7500
2700
850
HG-01 CHFOCFCFCHFOCFCFOCHF2 0.87 6.2
4700
1500
450

* The values for CH4 and N2O are adjustment times including feedbacks of emission on lifetimes (see Chapter 4).
From the formulas given in Table 6.2, with updated constants based on the IPCC (l990) expressions.

Note: For all gases destroyed by reaction with OH, updated lifetimes include scaling to CH3CCl3 lifetimes, as well as an estimate of the stratospheric destruction. See references below for rates along with Chapter 4 and WMO (l999).
(a) Taken from the SAR (b) Taken from WMO (l999) (c) Taken from WMO (1995) (d) DeMore et al. (1997)
(e) Good et al. (1998) (f) Wallington et al. (1997) (g) Christensen et al. (1998) (h) Heathfield et al. (1998a)
(i) Christidis et al. (1997) (j) Gierczak et al. (1996) (k) Barry et al. (1997) (l) Tokuhashi et al. (1999a)
(m) Tokuhashi et al. (1999b) (n) Tokuhashi et al. (2000)    
       
Myhre et al. (l998b) †† Jain et al. (2000) § Highwood and Shine (2000) & Ko et al. (l999).
See Cavalli et al. (l998) and Myhre et al. (l999)

The lifetimes and adjustment times used in Tables 6.7 and 6.8 come from Chapter 4 except where noted. For some gases (including several of the fluoroethers), lifetimes have not been derived from laboratory measurements, but have been estimated by various other means. For this reason, the lifetimes for these gases, and hence the GWPs, are considered to be much less reliable, and so these gases are listed separately in Table 6.8. NF3 is listed in Table 6.8 because, although its photolytic destruction has been characterised, other loss processes may be significant but have not yet been characterised (Molina et al., 1995). Note also that some gases, for example, trifluoromethyl iodide (CF3I) and dimethyl ether (CH3OCH3) have very short lifetimes (less than a few months); GWPs for such very short-lived gases may need to be treated with caution, because the gases are unlikely to be evenly distributed globally, and hence estimates of, for example, their radiative forcing using global mean conditions may be subject to error.

Uncertainties in the lifetimes of CFC-11 and CH3CCl3 are thought to be about 10%, while uncertainties in the lifetimes of gases obtained relative to CFC-11 or CH3CCl3 are somewhat larger (20 to 30%) (SAR; WMO, l999). Uncertainties in the radiative forcing per unit mass of the majority of the gases considered in Table 6.7 are approximately ± 10%. The SAR suggested typical uncertainties of ± 35% (relative to the reference gas) for the GWPs, and we retain this uncertainty estimate for gases listed in Table 6.7. In addition to uncertainties in the CO2 radiative efficiency per kilogram and in the response function, AGWPs of CO2 are affected by assumptions concerning future CO2 abundances as noted above. Furthermore, as the CO2 mixing ratios and climate change, the pulse response function changes as well. In spite of these dependencies on the choice of future emission scenarios, it remains likely that the error introduced by these assumptions is smaller than the uncertainties introduced by our imperfect understanding of the carbon cycle (see Chapter 3). Finally, although any induced error in the CO2 AGWPs will certainly affect the non-CO2 GWPs, it will not affect intercomparisons among non-CO2 GWPs.



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