Aviation and the Global Atmosphere

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6.2.2. Global Warming Potential

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 CO2. 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 NOx 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 CO2 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 CO2, 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.

6.2.3. Alternative Indexing of Aviation's Climate Impact-RF Index

A new alternative index to measure the role of aviation in climate change is introduced here: the radiative forcing index (RFI), which is defined as the ratio of total radiative forcing to that from CO2 emissions alone. Total radiative forcing induced by aircraft is the sum of all forcings, including direct emissions (e.g., CO2, soot) and indirect atmospheric responses (e.g., CH4, O3, sulfate, contrails). RFI is a measure of the importance of aircraft-induced climate change other than that from the release of fossil carbon alone. RFI ranges between 2.2 and 3.4 for the various E- and F-type scenarios for subsonic aviation and technical options considered here (see Section 6.6). Thus, aircraft-induced climate change with RFI > 1 highlights the need for a thorough climate assessment of this sector as performed here. For comparison, in the IS92a scenario the RFI for all human activities is about 1; for greenhouse gases alone, it is about 1.5, and it is even higher for sectors that emit CH4 and N2O without significant fossil fuel use.

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