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Methane is a long-lived, well-mixed greenhouse gas. It has an atmospheric lifetime of about 9 years. The tropospheric chemical models used to evaluate the impact of the subsonic fleet found unanimously that CH4 lifetime was reduced by aircraft emissions (see Chapter 4). This instantaneous change (-1.3% in 1992, -2.6% in 2015, and -3.9% in 2050 for scenario FSEGa (tech1)) needs to be increased further by a factor of 1.4 to include the feedback of CH4 concentrations on lifetime (Prather, 1994; IPCC, 1996). It is then applied to the IS92a CH4 abundance to calculate the reduction in CH4 concentration that can be attributed to aircraft. The CH4 perturbation is assumed to be instantaneous; in reality, however, it takes a couple of decades to appear and will lag the O3 perturbation. For the purposes of interpolating between RF points in Table 6-1, we assume that the CH4 perturbation, like the O3 perturbation, is proportional to NOx emissions.
Water vapor is a potent greenhouse gas that is highly variable in the troposphere, with a short average residence time controlled by the hydrological cycle. In the stratosphere, the slow turnover of air and extreme dryness make precipitation and clouds a rare phenomena, leading to smoothly varying concentrations ranging from 3 to 6 ppmv as CH4 is oxidized to H2O (Dessler et al., 1994; Harries et al., 1996). The contribution of aircraft to atmospheric H2O is directly from the H in the fuel (assumed to be 14% by mass). Most of the subsonic fleet's fuel is burned in the troposphere, where this additional source of water is swamped by the hydrological cycle. A smaller fraction is released in the stratosphere, where longer residence times may lead to greater accumulation. However, because flight routes are close to the tropopause and reach at most into the lowermost stratosphere, this effluent is rapidly returned to the troposphere with little expected accumulation (Holton et al., 1995; see also Section 3.3.4).
Although the uncertainty of predicting the current subsonic RF for water vapor is large-a factor of 3-the absolute number in 1992 is estimated to be sufficiently small, +0.0015 W m-2, making this factor a minor uncertainty in subsonic climate forcing. It is assumed that this value scales linearly with fuel use (see Table 6-1). This value is consistent with earlier studies: Schumann (1994) and Fortuin et al. (1995) estimated that present air traffic enhances background H2O by less than 1.5% for regions most frequently used by aircraft; likewise, Ponater et al. (1996) and Rind et al. (1996) used GCM studies to conclude that the direct radiative effect on the climate of water vapor emissions from 1992 air traffic is negligibly small.
The projected HSCT fleet, however, would cruise at 20-km altitude and build up much greater H2O enhancements in the stratosphere. The stratospheric models described in Chapter 4 predicted excess stratospheric water vapor from an HSCT fleet of 500 aircraft (designated HSCT(500) in Table 6-1). This perturbation is difficult to calculate, and the likely (2/3 probability) range includes a factor of 2 higher and lower. Furthermore, RF modeling of this stratospheric H2O perturbation adds further uncertainty, as indicated in Table 6-1. All results suggest that this effect is the dominant HSCT climate impact, with RF equal to +0.05 (0.017 to 0.15) W m-2 for 500 aircraft, increasing to +0.10 (0.03 to 0.30) W m-2 for a mature fleet of 1,000 aircraft (HSCT(1000)). Although it takes several years to accumulate this excess stratospheric water vapor, it is assumed that this RF is instantaneously proportional to the HSCT fleet size.
In a GCM study, Rind and Lonergan (1995) looked for climate change caused by H2O accumulation from a fleet of 500 HSCTs. They found no statistically significant change in surface temperature. Their result is consistent with this assessment and with water vapor as the dominant HSCT climate impact because the magnitude of this radiative forcing from the fleet, +0.05 W m-2, would induce a mean global warming that would be difficult to detect above natural climate variability.
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