2.6.2 Radiative Forcing Estimates for Persistent Line-Shaped Contrails
Aircraft produce persistent contrails in the upper troposphere in ice-supersaturated air masses (IPCC, 1999). Contrails are thin cirrus clouds, which reflect solar radiation and trap outgoing longwave radiation. The latter effect is expected to dominate for thin cirrus (Hartmann et al., 1992; Meerkötter et al., 1999), thereby resulting in a net positive RF value for contrails. Persistent contrail cover has been calculated globally from meteorological data (e.g., Sausen et al., 1998) or by using a modified cirrus cloud parametrization in a GCM (Ponater et al., 2002). Contrail cover calculations are uncertain because the extent of supersaturated regions in the atmosphere is poorly known. The associated contrail RF follows from determining an optical depth for the computed contrail cover. The global RF values for contrail and induced cloudiness are assumed to vary linearly with distances flown by the global fleet if flight ambient conditions remain unchanged. The current best estimate for the RF of persistent linear contrails for aircraft operations in 2000 is +0.010 W m–2 (Table 2.9; Sausen et al., 2005). The value is based on independent estimates derived from Myhre and Stordal (2001b) and Marquart et al. (2003) that were updated for increased aircraft traffic in Sausen et al. (2005) to give RF estimates of +0.015 W m–2 and +0.006 W m–2, respectively. The uncertainty range is conservatively estimated to be a factor of three. The +0.010 W m–2 value is also considered to be the best estimate for 2005 because of the slow overall growth in aviation fuel use in the 2000 to 2005 period. The decrease in the best estimate from the TAR by a factor of two results from reassessments of persistent contrail cover and lower optical depth estimates (Marquart and Mayer, 2002; Meyer et al., 2002; Ponater et al., 2002; Marquart et al., 2003). The new estimates include diurnal changes in the solar RF, which decreases the net RF for a given contrail cover by about 20% (Myhre and Stordal, 2001b). The level of scientific understanding of contrail RF is considered low, since important uncertainties remain in the determination of global values (Section 2.9, Table 2.11). For example, unexplained regional differences are found in contrail optical depths between Europe and the USA that have not been fully accounted for in model calculations (Meyer et al., 2002; Ponater et al., 2002; Palikonda et al., 2005).
Table 2.9. Radiative forcing terms for contrail and cirrus effects caused by global subsonic aircraft operations.
| ||Radiative forcing (W m–2)a |
| ||1992 IPCCb ||2000 IPCCc ||2000d |
|CO2d ||0.018 ||0.025 |
| Persistent linear contrails ||0.020 ||0.034 |
(0.006 to 0.015)
|Aviation-induced cloudiness without persistent contrails ||0 to 0.040 ||n.a. |
|Aviation-induced cloudiness with persistent contrails || || |
(0.010 to 0.080)