Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
By the year 2050, the number of aircraft flying in the upper troposphere is expected to have
increased significantly (see Chapter 9). In scenario Fa1, global annual aviation fuel
consumption in 2050 will have increased by a factor of 3.2 compared with 1992, with a
larger increase (factor of 4.3) above 500 hPa. Scenarios Fc1, Fe1, and Eah (see Chapter 9)
assume increases by factors of 1.8, 5, and 14, respectively, in total fuel consumption
compared to 1992. The frequency of contrail formation is expected to increase with traffic
because large regions of the atmosphere are humid and cold enough to allow persistent
contrails to form and because such regions are not at present fully covered with optically
thick cirrus or contrail clouds (see Sections 3.4.1 and 3.4.3). The number of aircraft may
grow slightly less rapidly than fuel consumption when smaller aircraft are replaced by larger
ones. This factor is important because the amount of persistent contrail cover may depend
mainly on the number of aircraft triggering contrails and less on fuel consumption.
As aircraft engines become more fuel efficient, contrails will form more frequently at lower
flight levels because exhaust plumes of more efficient engines are cooler for the same water
content (see Section 3.2.4.1). The overall efficiency h (Cumpsty, 1997) with which engines
convert fuel combustion heat into propulsion of cruising subsonic aircraft was close to 0.22
in the 1950s, near 0.37 for modern engines in the early 1990s, and may reach 0.5 for new
engines to be built by 2010 (see Figure 3-22). An increase of h from 0.3 to 0.5 in a standard
atmosphere increases the threshold formation temperature of contrails by about 2.8 K
(equivalent to 700-m lower altitude) (Schumann, 1996a).
The change in persistent contrail coverage because of changed traffic has been determined
using thermodynamic analysis of meteorological data from 1983 to 1992 and fuel
consumption data (Sausen et al., 1998) (see Section 3.4). This method has been used to
estimate future changes in contrail cover resulting from changes in air traffic, assuming a
fixed climate, fuel consumption scenarios, and expected engine performance specifications
(Gierens et al., 1998). The computed contrail cover (Figure 3-23) for the 2050 Fa1 fuel
scenario using present analysis data and h of 0.5 shows a global and annual mean contrail
cover of 0.47%, with values of 0.26% and 0.75% for scenarios Fc1 and Fe1. Values may be
as high as 1 to 2% for scenario Eah, which does not specify the spatial distribution of future
Figure 3-25: Annually and zonally averaged perturbation of sulfate
aerosol surface area density (in µm2 cm-3) caused by an HSCT fleet
of 500 aircraft flying at Mach 2.4 according to AER 2-D model
(Weisenstein et al., 1997). A sulfur emission index of 0.2 g kg-1 and a
10% conversion to sulfate particles with 10-nm radius in the plume
are assumed in these calculations.
traffic and in which contrail cover may become limited by the amount of cloud-free ice-supersaturated air masses. In comparison, values are 0.087% for the 1992 DLR
fuel inventory with h of 0.3 and 0.38% for the 2050 scenario with h of 0.3. Hence, contrail cover is expected to increase by a factor of about 5 over present cover for a
3.2-fold increase in annual aviation fuel consumption from 1992 to 2050, even under constant climate conditions. Increased efficiency of propulsion by future engines
causes about 20% of the computed increase in contrail cover. In the year 2050, the maximum contrail coverage is expected to occur over Europe (4.6%, 4 times more
than 1992), the United States of America (3.7%, 2.6 times more), and southeast Asia (1.2%, 10 times more). Contrail-induced increases in cirrus cloud cover may
depend also on wind shear, vertical motions, and existing cirrus cover, which this thermodynamic analysis does not take into account. In addition, changes in climate
conditions may influence future contrail formation conditions.
Radiative forcing from contrails was calculated for 2050 using the Fa1 fuel scenario and the same method as described in Section 3.6.3 (Minnis et al., 1999). For the
contrail cover shown in Figure 3-23, values of SW, LW, and net forcing were found to be about 6 times larger than in 1992 (see Table 3-8). The increase in radiative
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