Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
may not simply disappear (see Section 6.5.1). Moreover, the prospect of large and
canceling RFs, each with large independent uncertainties, greatly increases the probability
of a large residual.
6.6.4. The HSCT Option and Radiative Forcing
The HSCT option of building a fleet beginning in 2015 and capping at 1,000 aircraft in 2040
(scenario Fa1H) has significant impact on total aviation RF, as shown in Figure 6-15c. The
addition of HSCT aircraft (+0.08 W m-2) increases RF for scenario Fa1 by a factor of 1.4,
becoming comparable to RF for the higher demand scenario Fe1 (see also Figure 6-16b).
The HSCT aircraft themselves cause RF of +0.10 W m-2, which is offset only by -0.02 W m-
2 from displaced subsonic aircraft. This large RF from HSCT aircraft is driven mainly by RF
from increased stratospheric H2O. This single component has a large uncertainty (at least a
factor of 3) associated with the calculation of the stratospheric buildup of water and the
value of RF. The differential CO 2 -RF penalty for HSCT aircraft is only 3% of total aviation
RF. In conclusion, the best estimate of RF from HSCTs is about five times as large as RF
from the subsonic fleet they replace; however, with the large confidence intervals on O3 and
H2O, this factor has a very large uncertainty and may range from 0 to 16.
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Figure 6-16: (a) Aviation fossil fuel use relative to the
IS92a fossil fuel use from 1990 to 2050 for the air
traffic scenarios Fc1, Fa1, Fa1H, Fe1, Eab, and Edh;
and (b) from aviation as percentage of total radiative
forcing (without additional cirrus).
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