Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
aircraft RF components from the NASA-1992* scenario in Figure 6-14b (note the change of
scale; see also Table 6-1). Aircraft RF is qualitatively different from overall anthropogenic
RF: The CH4 RF is negative, the O3-RF is greater than the co 2 -RF, and the aerosol-cloud
effects are positive (because of persistent contrails) rather than negative. Within the error
bars, the effects of co 2 , O3, and contrails are comparable; that of methane is also
comparable, but negative. A caveat here is that we are unable to derive a best estimate for
the indirect effects from aircraft on "natural" clouds; these effects might be negligible or may
be comparable to that of contrails. Thus, for 1992 our best estimate is that aviation causes
RF (without additional cirrus) of approximately +0.05 W m-2 and is responsible for about
3.5% of the total anthropogenic RF of about +1.4 W m-2 for greenhouse gases plus
aerosols (+2.7 W m-2 for greenhouse gases alone).
Bar charts for the 2050 atmosphere are presented in Figure 6-15a. The same qualitative
differences apply. However, in 2050 the contribution by contrails is almost twice as large as
the contribution from aircraft co 2 or O3. Thus, in 2050 for the Fa1 scenario, our best
estimate is that aviation RF (without additional cirrus) grows to about +0.19 W m-2, which is
5% of the total anthropogenic radiative forcing anticipated for IS92a of +3.8 W m-2 for
greenhouse gases plus aerosols (+5.8 W m-2 for greenhouse gases alone). This fraction is
more than doubled for E-type scenarios (Table 6-1). There is considerable scientific
uncertainty in these estimates-a factor of 2 or more at the 67% confidence level-that is
separate from the potential error in projecting future scenarios for aviation and IS92a.
Individual RF components in 2050 attributable to the impact of adding a supersonic fleet are
shown in Figure 6-15c (see also Table 6-1). The white bars denote the direct RF of HSCTs;
the black bars denote the RF from displaced subsonic air traffic. As already discussed, RF
from stratospheric H 2 O contributes the largest component, comparable in magnitude to the
big three RFs CO 2 , O3, contrails) from the subsonic fleet (Figure 6-15b). RFs for the
combined fleet (Fa1H) are listed separately in Tables 6-1 and 6-2.
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below the graph are relative appraisals of the level of scientific
understanding associated with each component.
Figure 6-14b: Bar charts of radiative forcing from aviation effects in
1992. Note scale change from (a) to (b). In (b), best estimate (bars)
and high-low 67% probability intervals (whiskers) are given. No best
estimate is shown for the cirrus clouds; rather, the dashed line
indicates a range of possible estimates. The evaluations below the
graph are relative appraisals of the level of scientific understanding
associated with each component.