IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Working Group I: The Physical Science Basis

2.10.2 Direct Global Warming Potentials

All GWPs depend on the AGWP for CO2 (the denominator in the definition of the GWP). The AGWP of CO2 again depends on the radiative efficiency for a small perturbation of CO2 from the current level of about 380 ppm. The radiative efficiency per kilogram of CO2 has been calculated using the same expression as for the CO2 RF in Section 2.3.1, with an updated background CO2 mixing ratio of 378 ppm. For a small perturbation from 378 ppm, the RF is 0.01413 W m–2 ppm–1 (8.7% lower than the TAR value). The CO2 response function (see Table 2.14) is based on an updated version of the Bern carbon cycle model (Bern2.5CC; Joos et al. 2001), using a background CO2 concentration of 378 ppm. The increased background concentration of CO2 means that the airborne fraction of emitted CO2 (Section 7.3) is enhanced, contributing to an increase in the AGWP for CO2. The AGWP values for CO2 for 20, 100, and 500 year time horizons are 2.47 × 10–14, 8.69 × 10–14, and 28.6 × 10–14 W m–2 yr (kg CO2)–1, respectively. The uncertainty in the AGWP for CO2 is estimated to be ±15%, with equal contributions from the CO2 response function and the RF calculation.

Updated radiative efficiencies for well-mixed greenhouse gases are given in Table 2.14. Since the TAR, radiative efficiencies have been reviewed by Montzka et al. (2003) and Velders et al. (2005). Gohar et al. (2004) and Forster et al. (2005) investigated HFC compounds, with up to 40% differences from earlier published results. Based on a variety of radiative transfer codes, they found that uncertainties could be reduced to around 12% with well-constrained experiments. The HFCs studied were HFC-23, HFC-32, HFC-134a and HFC-227ea. Hurley et al. (2005) studied the infrared spectrum and RF of perfluoromethane (CΩF4) and derived a 30% higher GWP value than given in the TAR. The RF calculations for the GWPs for CH4, N2O and halogen-containing well-mixed greenhouse gases employ the simplified formulas given in Ramaswamy et al. (2001; see Table 6.2 of the TAR). Table 2.14 gives GWP values for time horizons of 20, 100 and 500 years. The species in Table 2.14 are those for which either significant concentrations or large trends in concentrations have been observed or a clear potential for future emissions has been identified. The uncertainties of these direct GWPs are taken to be ±35% for the 5 to 95% (90%) confidence range.

Table 2.14. Lifetimes, radiative efficiencies and direct (except for CH4) GWPs relative to CO2. For ozone-depleting substances and their replacements, data are taken from IPCC/TEAP (2005) unless otherwise indicated.

Errata

  Global Warming Potential for Given Time Horizon  
Industrial Designation or Common Name (years)  Chemical Formula   Lifetime (years)  RadiativeEfficiency (W m–2 ppb–1)  SAR (100-yr)  20-yr  100-yr  500-yr 
Carbon dioxide  CO2  See belowa  b1.4x10–5  1  1  1  1 
Methanec  CH4  12c  3.7x10–4  21  72  25  7.6 
Nitrous oxide  N2O  114  3.03x10–3  310  289  298  153 
Substances controlled by the Montreal Protocol  
CFC-11  CCl3F  45  0.25  3,800  6,730  4,750  1,620 
CFC-12  CCl2F2  100  0.32  8,100  11,000  10,900  5,200 
CFC-13  CClF3  640  0.25    10,800  14,400  16,400 
CFC-113  CCl2FCClF2  85  0.3  4,800  6,540  6,130  2,700 
CFC-114  CClF2CClF2  300  0.31    8,040  10,000  8,730 
CFC-115  CClF2CF3  1,700  0.18    5,310  7,370  9,990 
Halon-1301  CBrF3  65  0.32  5,400  8,480  7,140  2,760 
Halon-1211  CBrClF2  16  0.3    4,750  1,890  575 
Halon-2402  CBrF2CBrF2  20  0.33    3,680  1,640  503 
Carbon tetrachloride  CCl4  26  0.13  1,400  2,700  1,400  435 
Methyl bromide  CH3Br  0.7  0.01    17  5  1 
Methyl chloroform  CH3CCl3  5  0.06    506  146  45 
HCFC-22  CHClF2  12  0.2  1,500  5,160  1,810  549 
HCFC-123  CHCl2CF3  1.3  0.14  90  273  77  24 
HCFC-124  CHClFCF3  5.8  0.22  470  2,070  609  185 
HCFC-141b  CH3CCl2F  9.3  0.14    2,250  725  220 
HCFC-142b  CH3CClF2  17.9  0.2  1,800  5,490  2,310  705 
HCFC-225ca  CHCl2CF2CF3  1.9  0.2    429  122  37 
HCFC-225cb  CHClFCF2CClF2  5.8  0.32    2,030  595  181 
Hydrofluorocarbons 
HFC-23  CHF3  270  0.19  11,700  12,000  14,800  12,200 
HFC-32  CH2F2  4.9  0.11  650  2,330  675  205 
HFC-125  CHF2CF3  29  0.23  2,800  6,350  3,500  1,100 
HFC-134a  CH2FCF3  14  0.16  1,300  3,830  1,430  435 
HFC-143a  CH3CF3  52  0.13  3,800  5,890  4,470  1,590 
HFC-152a  CH3CHF2  1.4  0.09  140  437  124  38 
HFC-227ea  CF3CHFCF3  34.2  0.26  2,900  5,310  3,220  1,040 
HFC-236fa  CF3CH2CF3  240  0.28  6,300  8,100  9,810  7,660 
HFC-245fa  CHF2CH2CF3  7.6  0.28    3,380  1030  314 
HFC-365mfc  CH3CF2CH2CF3  8.6  0.21    2,520  794  241 
HFC-43-10mee  CF3CHFCHFCF2CF3  15.9  0.4  1,300  4,140  1,640  500 
Perfluorinated compounds  
Sulphur hexafluoride  SF6  3,200  0.52  23,900  16,300  22,800  32,600 
Nitrogen trifluoride  NF3  740  0.21    12,300  17,200  20,700 
PFC-14  CF4  50,000  0.10  6,500  5,210  7,390  11,200 
PFC-116  C2F6  10,000  0.26  9,200  8,630  12,200  18,200 

Table 2.14 (continued)

        Global Warming Potential for Given Time Horizon  
Industrial Designation or Common Name (years)   Chemical Formula   Lifetime (years)  RadiativeEfficiency (W m–2 ppb–1)  SAR‡ (100-yr)  20-yr  100-yr  500-yr 
Perfluorinated compounds (continued)  
PFC-218  2,600  0.26  7,000  6,310  8,830  12,500 
PFC-318  3,200  0.32  8,700  7,310  10,300  14,700 
PFC-3-1-10  2,600  0.33  7,000  6,330  8,860  12,500 
PFC-4-1-12  4,100  0.41    6,510  9,160  13,300 
PFC-5-1-14  3,200  0.49  7,400  6,600  9,300  13,300 
PFC-9-1-18  >1,000d  0.56    >5,500  >7,500  >9,500 
trifluoromethyl sulphur pentafluoride 800  0.57    13,200  17,700  21,200 
Fluorinated ethers  
HFE-125  136  0.44    13,800  14,900  8,490 
HFE-134  26  0.45    12,200  6,320  1,960 
HFE-143a  4.3  0.27    2,630  756  230 
HCFE-235da2  2.6  0.38    1,230  350  106 
HFE-245cb2  5.1  0.32    2,440  708  215 
HFE-245fa2  4.9  0.31    2,280  659  200 
HFE-254cb2  2.6  0.28    1,260  359  109 
HFE-347mcc3  5.2  0.34    1,980  575  175 
HFE-347pcf2  7.1  0.25    1,900  580  175 
HFE-356pcc3  0.33  0.93    386  110  33 
HFE-449sl (HFE-7100)  3.8  0.31    1,040  297  90 
HFE-569sf2 (HFE-7200)  0.77  0.3    207  59  18 
HFE-43-10pccc124 (H-Galden 1040x)  6.3  1.37    6,320  1,870  569 
HFE-236ca12 (HG-10)  12.1  0.66    8,000  2,800  860 
HFE-338pcc13 (HG-01)  6.2  0.87    5,100  1,500  460 
Perfluoropolyethers  
PFPMIE  800  0.65    7,620  10,300  12,400 
Hydrocarbons and other compounds – Direct Effects  
Dimethylether  0.015  0.02    1  1  <<1 
Methylene chloride  0.38  0.03    31  8.7  2.7 
Methyl chloride  1.0  0.01    45  13  4 

Notes:

a The CO2 response function used in this report is based on the revised version of the Bern Carbon cycle model used in Chapter 10 of this report (Bern2.5CC; Joos et al. 2001) using a background CO2 concentration value of 378 ppm. The decay of a pulse of CO2 with time t is given by

Where a0 = 0.217, a1 = 0.259, a2 = 0.338, a3 = 0.186, τ1 = 172.9 years, τ2 = 18.51 years, and τ3 = 1.186 years.

b The radiative efficiency of CO2 is calculated using the IPCC (1990) simplified expression as revised in the TAR, with an updated background concentration value of 378 ppm and a perturbation of +1 ppm (see Section 2.10.2).

c The perturbation lifetime for methane is 12 years as in the TAR (see also Section 7.4). The GWP for methane includes indirect effects from enhancements of ozone and stratospheric water vapour (see Section 2.10.3.1).

d Shine et al. (2005c), updated by the revised AGWP for CO2. The assumed lifetime of 1,000 years is a lower limit.

e Hurley et al. (2005)

f Robson et al. (2006)

g Young et al. (2006)