Aviation and the Global Atmosphere

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6.1.2. Aircraft-Induced Climate Change

Figure 6-3: CO2 concentration over the past 1000
years from ice core records (D47, D57, Siple, and
South Pole) and (since 1958) from Mauna Loa,
Hawaii, measurement site.

Aircraft emissions are expected to modify the Earth's radiative budget and climate as a result of several processes (see also Figure 6-1): emission of radiatively active substances (e.g., CO2 or H2O); emission of chemical species that produce or destroy radiatively active substances (such as NOx, which modifies O3 concentration, or SO2, which oxidizes to sulfate aerosols); and emission of substances (e.g., H2O, soot) that trigger the generation of additional clouds (e.g., contrails).

The task of detecting climate change is already difficult; the task of detecting the aircraft contribution to the overall change is more difficult because aircraft forcing is a small fraction of anthropogenic forcing as a whole. However, aircraft perturb the atmosphere in a specific way because their emissions occur in the free troposphere and lower stratosphere, and they trigger contrails, so the aircraft contribution to overall climate change may have a particular signature. At a minimum, the aircraft-induced climate change pattern would have to be significantly different from the overall climate change pattern in order to be detected.

The climatic impact of aircraft emissions is considered on the background of other anthropogenic perturbations of climate. The present assessment is based on the IS92 scenarios used to represent alternative futures in the Second Assessment Report (IPCC, 1996); it does not incorporate recent observed trends in methane (Dlugokencky et al., 1998), nor any implications from the Kyoto Protocol. The scenarios for aircraft flight patterns, fuel burn, and emissions are described in Chapter 9. Atmospheric perturbations are taken from detailed atmospheric models (Chapters 2, 3, and 4), except for CO2 accumulation (which is discussed in this chapter).

The largest, single, known radiative forcing change over the past century is from the increase in CO2, driven primarily by the burning of fossil fuel. Figure 6-3 shows the change in CO2 over the past 1,000 years; the CO2 concentration increased from about 280 ppmv in 1850 to about 360 ppmv in 1990. Figure 6-4 shows six IPCC projections for anthropogenic emissions of CO2 (labeled IS92a-f) and predicted atmospheric CO2 concentrations. We take the central case, IS92a, as the future scenario with which to compare aircraft effects. The central aircraft scenario (Fa1) assessed here matches the economic assumptions of IS92a. Radiative forcing for IS92a is shown in Figure 6-5a, including total radiative forcing and individual contributions.

Figure 6-4: a) Total anthropogenic CO2 emissions (Gt C yr-1) under the IS92 emission scenarios.

Figure 6-4: b) the resulting atmospheric CO2 concentrations (ppmv) calculated using the "Bern" carbon cycle model (IPCC, 1996, Figure 5).

Figure 6-5: a) Radiative forcing components resulting from the IS92a emission scenario for 1990 to 2100. The "Total Non-CO2 Trace Gases" curve includes the radiative forcing from CH4 (including CH4-related increases in stratospheric water vapor), N2O, tropospheric O3, and the halocarbons (including the negative forcing effect of stratospheric O3 depletion). Halocarbon emissions have been modified to take account of the Montreal Protocol and its Adjustments and Amendments. The three aerosol components are direct sulfate, indirect sulfate, and direct biomass burning.


Figure 6-5: b) Non-CO2 trace gas radiative forcing components. "Cl/Br direct" is the direct radiative forcing resulting from Cl- and Br-containing halocarbons; emissions are assumed to be controlled under the Montreal Protocol and its Adjustments and Amendments. The indirect forcing from these compounds (through stratospheric O3 depletion) is shown separately (Strat. O3). All other emissions follow the IS92a scenario. The tropospheric O3 forcing (Trop. O3) takes account of concentration changes resulting only from the indirect effect from CH4 (IPCC, 1996, Figure 6b-c).

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