|
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
|
|
| 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
|
—
|
|
|
