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 NO_x and non-methane hydrocarbons

The short lifetimes and complex non-linear chemistries of NO_x 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 NO_x 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 NO_x 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 NO_x emitted from aircraft. These studies suggest GWPs of the order of 450 for aircraft NO_x 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 NO_x from aircraft are characterised by far greater GWPs than those of surface sources, due mainly to the longer lifetime of the emitted NO_x 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

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