Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
forcing from 1992 to 2050 is larger than the increase in contrail cover (factor of 5) during the same period because additional contrails in the subtropics and over Asia
over relatively warm and cloud-free surfaces are more effective in increasing radiative forcing. The global distribution of radiative forcing calculated with this procedure
is shown in Figure 3-24 for an assumed optical depth of 0.3. Radiative forcing grows more strongly globally than in regions of present peak traffic. Global mean forcing
is 0.1 W m-2 in this computation, with maximum values of 3.0 and 1.4 W m-2 (3.3 and 2.4 times more than 1992) over northeast France and the eastern United States
of America, respectively.
For an optical depth of 0.3, the best-estimate value of the global radiative forcing in 2050 (scenario Fa1) is 0.10 W m -2 . The uncertainty range is a little larger than in
1992, and estimated to amount to a factor of 4. Hence, the likely range of forcing extends from 0.03 to 0.4 W m -2 (see Table 3-9). The forcing for other scenarios has
not been computed in detail, but rough estimates scale with the fuel consumption. The climatic consequences of this forcing are discussed in Chapter 6.
An estimate of the range of aviation-induced cirrus cloudiness in 2050, as required for this assessment, is not available in the scientific literature. For 1992, a range for
the best estimate of the additional aviation-induced cirrus clouds was derived from decadal trends in high fuel-use regions (0-0.2% global cover; Section 3.5.1.5). For
the 2050 time period, a different approach is required. Observed contrail frequencies and trends in cirrus occurrence have been found to correlate with aviation fuel
consumption (see Figures 3-14 and 3-18). Therefore, the aviation-induced cirrus cloudiness between 1992 and 2050 is projected to grow in proportion to the total
aviation fuel consumption in the upper troposphere. This fuel consumption grows by a factor of 4 between 1992 and 2050 in scenario Fa1. Hence, the best-estimate of
additional global cirrus cover in 2050 would range from 0 to 0.8%. For the same radiative sensitivity as in 1992, the associated radiative forcing could be between 0
and 0.16 W m -2 or up to 1.6 times the value given for line-shaped contrail cirrus in 2050 (see Table 3-9). The forcing could be outside this range if future aviation
causes strong changes in the optical properties of the cirrus clouds. Saturation effects (Sausen et al., 1999) will likely limit any increase in cirrus cover in heavy air
traffic regions. Because of these uncertainties the status of understanding of radiative forcing from additional aviation-induced cirrus clouds in 2050 is very poor. The
assessment of the other indirect effects (Section 3.6.5) is beyond the scope of present understanding.
Modern subsonic aircraft cruise most efficiently at flight altitudes of 9 to 13 km. Trends in aircraft cruise altitudes are discussed in Chapter 7. If mean flight levels of
global air traffic were to increase, the frequency of persistent contrails in the troposphere at mid-latitudes would be reduced and the frequency in the upper troposphere
in the tropics would be increased. In addition, the formation of polar stratospheric clouds in the lower polar stratosphere (Peter et al., 1991) may be enhanced as a
result of increased emissions in the stratosphere. At mid-latitudes, a 1-km flight-level increase causes a moderate reduction of contrail cover because of increased
flights in the dry stratosphere (e.g., 12% less contrail cover over the North Atlantic compared with the nominal-altitude cover). Despite these changes, the global
change in contrail cover from an altitude increase is small because of compensating changes in the tropics. The stronger increase of contrail cover in the tropics may
cause a stronger positive radiative forcing because of the warmer surface in the tropics compared with mid-latitudes (see Table 3-7). A reduction in flight levels
generally has the opposite effect (more contrails at high latitudes and fewer contrails in the tropics). Results for Europe and parts of the United States of America are
different in that computed contrail coverage decreases slightly for both an increase and a decrease in mean flight levels because air traffic currently occurs in the cold
and humid upper troposphere in those regions, and a shift toward the drier stratosphere or the warmer mid-troposphere reduces contrail coverage (Sausen et al.,
1998). A change in mean altitude of contrails may change their radiative impact even for constant areal coverage. A contrail at higher altitude in the troposphere will
likely contain less ice mass and produce less radiative forcing, therefore, despite lower ambient temperatures (see Table 3-7).
Trends in soot emissions would be important if soot influences ice particle formation (see Section 3.4) or the chemistry of ozone (see Chapter 2). A soot mass emission
index of 0.5 to 1 g kg-1 (and larger) is not uncommon for older aircraft engines. The soot mass emission index of the present aircraft fleet is estimated as 0.04 g kg-1 (see Chapter 7). The soot emission index decreased with new engine technology until about 1980 but has showed no significant trend thereafter (Döpelheuer, 1997).
The mass of soot emitted may decrease despite increases in fuel consumption. No data exist on trends for the number and surface area of soot aerosol emissions.
Atmospheric models and fuel consumption scenarios suggest that aircraft emissions contribute little to the tropospheric mass of sulfate and soot aerosol in today's
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