Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Aviation and the Global Atmosphere
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6.2.2. Global Warming Potential
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Global warming potential (GWP; see Shine et al., 1990, for a formal definition) is an index that attempts to integrate the overall climate impacts of a specific action (e.
g., emissions of CH4, NOx or aerosols). It relates the impact of emissions of a gas to that of emission of an equivalent mass of CO 2 . The duration of the perturbation is
included by integrating radiative forcing over a time horizon (e.g., standard horizons for IPCC have been 20, 100, and 500 years). The time horizon thus includes the
cumulative climate change and the decay of the perturbation.
GWP has provided a convenient measure for policymakers to compare the relative climate impacts of two different emissions. However, the basic definition of GWP
has flaws that make its use questionable, in particular, for aircraft emissions. For example, impacts such as contrails may not be directly related to emissions of a
particular greenhouse gas. Also, indirect RF from O3 produced by NO x emissions is not linearly proportional to the amount of NOx emitted but depends also on
location and season. Essentially, the buildup and radiative impact of short-lived gases and aerosols will depend on the location and even the timing of their emissions.
Furthermore, the GWP does not account for an evolving atmosphere wherein the RF from a 1-ppm increase in CO 2 is larger today than in 2050 and the efficiency of
NOx at producing tropospheric O3 depends on concurrent pollution of the troposphere.
In summary, GWPs were meant to compare emissions of long-lived, well-mixed gases such as CO 2 , CH4, N2O, and hydrofluorocarbons (HFC) for the current
atmosphere; they are not adequate to describe the climate impacts of aviation.
Nevertheless, some researchers have calculated a GWP, or modified version, for aircraft NOx emissions via induced ozone perturbation (e.g., Michaelis, 1993;
Fuglestvedt et al., 1996; Johnson and Derwent, 1996; Wuebbles, 1996). The results vary widely as a result of model differences, varying scenarios for NOx emission,
and the ambiguous GWP definition for short-lived gases. There is a basic impossibility of defining a GWP for "aircraft NOx" because emissions during takeoff and
landing would have one GWP; those at cruise, another; those in polar winter, another; and those in the upper tropical troposphere, yet another. Different chemical
regimes will produce different amounts of ozone for the same injection of NOx, and the radiative forcing of that ozone perturbation will vary by location (Fuglesvedt et
al., 1999). In view of all these problems, we will not attempt to derive GWP indices for aircraft emissions in this study. The history of radiative forcing, calculated for the
changing atmosphere, is a far better index of anthropogenic climate change from different gases and aerosols than is GWP.
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