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.