Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
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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