Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Persistent contrails in the stratosphere are not likely because of the low ambient relative humidity. Radiative forcing from contrails from the proposed HSCT fleet may
therefore be neglected in future scenarios of radiative forcing (except for the 11% reduction of subsonic RF from air traffic displaced by HSCT aircraft).
6.4.5. Uncertainties
It is difficult to constrain the direct and indirect climate effects of aerosols from aircraft. Measurements at altitude are not adequate to define the aircraft contribution
today, so models representing emissions and subsequent chemical and physical transformations have adopted differing parameterizations to estimate atmospheric
concentrations. Furthermore, different aerosol size distributions, the hygroscopic nature of some of the aerosol constituents, and mixing of different species of aerosol
all lead to uncertainties in the radiative properties of aerosols. These uncertainties in the burden and radiative properties of aerosols emitted by aircraft lead to
associated uncertainties in radiative forcing that are much larger than for a well-mixed, directly emitted greenhouse gas such as carbon dioxide.
RF from aircraft emissions of greenhouse gases and aerosols can be constrained when the amount of radiatively important species is directly limited by emissions (e.
g., CO 2 , H2O, sulfate, BC). However, it is more difficult to place upper limits on RF when the climate impact occurs indirectly, as in aircraft NOx production of O3 or-
specifically here-in perturbations to naturally occurring clouds by aircraft aerosols.
The indirect effect of aerosols from aircraft emissions on naturally occurring clouds cannot be quantified at present owing to the complexity of several processes such
as ice-cloud nucleation and the dependence of albedo and emissivity on the size of the ice crystals. With our present knowledge, this uncertainty must be considered
significant. Radiative forcing from contrails is estimated to grow disproportionately with fuel use, and extrapolation to large values in 2050 is an important uncertainty in
the future radiative impact of aviation. Further uncertainty arises from the fact that all estimates of contrail RF were made for the present climate, hence do not account
for future climate change (see discussion in Section 3.7.1).
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