Climate Change 2001:
Working Group I: The Scientific Basis
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6.12.3 Indirect GWPs

We next consider discuss indirect effects in more detail, and present GWPs for other gases, including estimates of their impacts. While direct GWPs are usually believed to be known reasonably accurately (±35%), indirect GWPs can be highly uncertain. A number of different processes contribute to indirect effects for various molecules; many of these are also discussed in Section 6.6.

6.12.3.1 Methane

Four types of indirect effects due to the presence of atmospheric CH4 have been identified (see Chapter 4 and Section 6.6). The largest effect is potentially the production of O3 (25% of the direct effect, or 19% of the total, as in the SAR). This effect is difficult to quantify, however, because the magnitude of O3 production is highly dependent on the abundance and distribution of NOx (IPCC, 1994; SAR). Other indirect effects include the production of stratospheric water vapour (assumed here to represent 5% of the direct effect, or 4% of the total, as in the SAR), the production of CO2 (from certain CH4 sources), and the temporal changes in the CH4 adjustment time resulting from its coupling with OH (Lelieveld and Crutzen, 1992; Brühl, 1993; Prather, 1994, 1996; SAR; Fuglestvedt et al., 1996). Here we adopt the values for each of these terms as given in the SAR, with a correction for the updated CO2 AGWPs and adopting the perturbation lifetime given in Chapter 4. It should be noted that the climate forcing caused by CO2 produced from the oxidation of CH4 is not included in these GWP estimates. As discussed in the SAR, it is often the case that this CO2 is included in national carbon production inventories. Therefore, depending on how the inventories are combined, including the CO2 production from CH4 could result in double counting this CO2.

6.12.3.2 Carbon monoxide

CO has a small direct GWP but leads to indirect radiative effects that are similar to those of CH4. As in the case of CH4, the production of CO2 from oxidised CO can lead to double counting of this CO2 and is therefore not considered here. The emission of CO perturbs OH, which in turn can then lead to an increase in the CH4 lifetime (Fuglestvedt et al., l996; Prather, 1996; Daniel and Solomon, 1998). This term involves the same processes whereby CH4 itself influences its own lifetime and hence GWP values (Prather, l996) and is subject to similar uncertainty. This term can be evaluated with reasonable accuracy using a box model, as shown by Prather (l996) and Daniel and Solomon (1998). Emissions of CO can also lead to the production of O3 (see Chapter 4), with the magnitude of O3 formation dependent on the amount of NOx present. As with CH4, this effect is quite difficult to quantify due to the highly variable and uncertain NOx distribution (e.g., Emmons et al., 1997). Because of the difficulty in accurately calculating the amount of O3 produced by CO emissions, an accurate estimate of the entire indirect forcing of CO requires a three-dimensional chemical model. Table 6.9 presents estimates of the CO GWP due to O3 production and to feedbacks on the CH4 cycle from two recent multi-dimensional model studies (in which a "slab" emission of CO was imposed), along with the box-model estimate for the latter term alone from Daniel and Solomon (l998), which is based on the analytical formalism developed by Prather (l996). Table 6.9 shows that the 100-year GWP for CO is likely to be 1.0 to 3.0, while that for shorter time horizons is estimated at 2.8 to 10. These estimates are subject to large uncertainties, as discussed further in Chapter 4.

Table 6.8: Direct Global Warming Potentials (mass basis) relative to carbon dioxide (for gases for whose lifetime has been determined only via indirect means, rather than laboratory measurements, or for whom there is uncertainty over the loss processes). Radiative efficiency is defined with respect to all sky.
Gas
Radiative efficiency (Wm-2 ppb-1) (from (b) unless indicated)
Estimated lifetime (years)
Global Warming Potential
Time horizon
20 years
100 years
500 years
NF3  
0.13
740 (a)
7700
10800
13100
SF5CF3  
0.57 (d)
>1000 *
>12200
>17500
>22500
c-C3F6  
0.42
>1000 *
>11800
>16800
>21600
             
HFE-227ea CF3CHFOCF3
0.40
11 (c)
4200
1500
460
HFE-236ea2 CF3CHFOCHF2
0.44
5.8 (c)
3100
960
300
HFE-236fa CF3CH2OCF3
0.34
3.7 (c)
1600
470
150
HFE-245fa1 CHF2CH2OCF3
0.30
2.2 (c)
940
280
86
HFE-263fb2 CF3CH2OCH3
0.20
0.1 (c)
37
11
3
             
HFE-329mcc2 CF3CF2OCF2CHF2
0.49
6.8 (c)
2800
890
280
HFE-338mcf2 CF3CF2OCH2CF3
0.43
4.3 (c)
1800
540
170
HFE-347mcf2 CF3CF2OCH2CHF2
0.41
2.8 (c)
1200
360
110
HFE-356mec3 CF3CHFCF2OCH3
0.30
0.94 (c)
330
98
30
HFE-356pcc3 CHF2CF2CF2OCH3
0.33
0.93 (c)
360
110
33
HFE-356pcf2 CHF2CF2OCH2CHF2
0.37
2.0 (c)
860
260
80
HFE-365mcf3 CF3CF2CH2OCH3
0.27
0.11 (c)
38
11
4
             
(CF3)2CHOCHF2  
0.41
3.1 (c)
1200
370
110
(CF3)2CHOCH3  
0.30
0.25 (c)
88
26
8
             
-(CF2)4CH(OH)-  
0.30
0.85 (c)
240
70
22
(a) Molina et al. (1995).
(b) WMO (1999).
(c) Imasu et al. (1995).
(d) Sturges et al. (2000).
* Estimated lower limit based upon perfluorinated structure.
Table 6.9: Estimated indirect Global Warming Potentials for CO for time horizons of 20, 100, and 500 years.
 
Indirect Global Warming Potentials
Time horizon
20 years
100 years
500 years
Daniel and Solomon (1998): box model considering CH4 feedbacks only
2.8
1.0
0.3
Fuglestvedt et al. (1996): two-dimensional model including CH4 feedbacks and tropospheric O3 production by CO itself
10
3.0
1.0
Johnson and Derwent (1996): two-dimensional model including CH4 feedbacks and tropospheric O3 production by CO itself
2.1


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