Climate Change 2001:
Working Group I: The Scientific Basis
Other reports in this collection

6.12.3.3 Halocarbons

In addition to their direct radiative forcing, chlorinated and brominated halocarbons can lead to a significant indirect forcing through their destruction of stratospheric O3 (Section 6.4). By destroying stratospheric O3, itself a greenhouse gas, halocarbons induce a negative indirect forcing that counteracts some or perhaps all (in certain cases) of their direct forcing. Furthermore, decreases in stratospheric O3 act to increase the ultraviolet field of the troposphere and hence can increase OH and deplete those gases destroyed by reaction with the OH radical (particularly CH4); this provides an additional negative forcing. Quantifying the magnitude of the negative indirect forcing is quite difficult for several reasons. As discussed in Section 6.4, the negative forcing arising from the O3 destruction is highly dependent on the altitude profile of the O3 loss. The additional radiative effect due to enhanced tropospheric OH is similarly difficult to quantify (see e.g., WMO, l999). While recognising these uncertainties, estimates have been made of the net radiative forcing due to particular halocarbons, which can then be used to determine net GWPs (including both direct and indirect effects). This was done by Daniel et al. (1995), where it was shown that if the enhanced tropospheric OH effect were ignored, and the negative forcing due to O3 loss during the 1980s was -0.08 Wm-2, the net GWPs for the bromocarbons were significantly negative, illustrating the impact of the negative forcing arising from the bromocarbon-induced ozone depletion. While the effect on the chlorocarbon GWPs was less pronounced, it was significant as well. Table 6.10 updates the results from Daniel et al.'s "constant-alpha" case A as in WMO (l999), where the effectiveness of bromine for O3 loss relative to chlorine (called alpha) has been increased from 40 to 60. The updated radiative efficiency of CO2 has also been included. An uncertainty in the 1980 to 1990 O3 radiative forcing of -0.03 to -0.15 Wm-2 has been adopted based upon Section 6.4, and these correspond (respectively) to the maximum and minimum GWP estimates given in Table 6.10.

6.12.3.4 NOx and non-methane hydrocarbons

The short lifetimes and complex non-linear chemistries of NOx and NMHC make calculation of their indirect GWPs a challenging task subject to very large uncertainties (see Chapter 4). However, IPCC (l999) has probed in detail the issue of the relative differences in the impacts of NOx upon O3 depending on where it is emitted (in particular, surface emissions versus those from aircraft). Higher altitude emissions have greater impacts both because of longer NOx residence times and more efficient tropospheric O3 production, as well as enhanced radiative forcing sensitivity (see Section 6.5). Two recent two-dimensional model studies (Fuglestvedt et al., l996; Johnson and Derwent, l996) have presented estimates of the GWPs for NOx emitted from aircraft. These studies suggest GWPs of the order of 450 for aircraft NOx emissions considering a 100-year time horizon, while those for surface emissions are likely to be much smaller, of the order of 5. While such numerical values are subject to very large quantitative uncertainties, they illustrate that the emissions of NOx from aircraft are characterised by far greater GWPs than those of surface sources, due mainly to the longer lifetime of the emitted NOx at higher altitudes.

Table 6.10: Net Global Warming Potentials (mass basis) of selected halocarbons (updated from Daniel et al., 1995; based upon updated strato-spheric O3 forcing estimates, lifetimes, and radiative data from this report).
Species
Time horizon = 2010 (20 years)
Time horizon = 2090 (100 years)
 
 
Direct
Min
Max
Direct
Min
Max
CFC-11
6300
100
5000
4600
-600
3600
CFC-12
10200
7100
9600
10600
7300
9900
CFC-113
6100
2400
5300
6000
2200
5200
HCFC-22
4800
4100
4700
1700
1400
1700
HCFC-123
390
100
330
120
20
100
HCFC-124
2000
1600
1900
620
480
590
HCFC-141b
2100
180
1700
700
-5
570
HCFC-142b
5200
4400
5100
2400
1900
2300
CHCl3
450
-1800
10
140
-560
0
CCl4
2700
-4700
1300
1800
-3900
660
CH3Br
16
-8900

-1700

5
-2600
-500
Halon-1211
3600
-58000
-8600
1300
-24000
-3600
Halon-1301
7900
-79000
-9100
6900
-76000
-9300


Other reports in this collection