The ranges of aircraft perturbations to climate change examined in this report are, of course, small compared to the overall increase in RF expected throughout the next century. Climate modeling of this transient change over the next 50 years cannot readily evaluate aircraft, or any equally small RF, in terms of specific climate changes because of problems in separating "signal" from "noise." Climate parameters that might be attributed to aircraft, such as the increase in global mean surface air temperature, are calculated here using a simple model consistent with previous IS92a relationships.
Figure 6-17: Predicted change in global mean surface
The predicted change in global mean surface temperature is shown in Figure 6-17 for total warming in accord with IS92a (solid) and for the case in which all of aviation's contribution to global warming (scenario Fa1) is cut off (dotted line). Of total global warming of 0.9 K anticipated in 2050, about 0.05 K would be attributable to aviation. The Eab case anticipates a larger aviation component (0.09 K). One caveat is that aircraft may produce a different climate signature, one that is not represented by an increase in global mean temperature.
To evaluate individual energy sectors as part of overall climate forcing, it is necessary to compare their summed radiative forcing from all atmospheric perturbations, not just that from their use of fossil-fuel carbon alone. The radiative forcing index-defined here as the ratio of total radiative forcing to that from CO2 emissions alone-is a measure of the importance of aircraft-induced climate change relative to that from an equivalent sector with the same fossil fuel use but without any effect other than CO2 (see also Section 6.2.3). In 1992, the RFI for aircraft was about 2.7, with an uncertainty of at least ±1.5. The RFI changes to 3.0 by 2015 then drops to 2.6 for the Fa1 scenario (see Table 6-1). This index ranges from 2.2 to 3.4 in the year 2050 for the various E- and F-type scenarios for subsonic aviation and technical options considered here. The RFI increases from 2.6 to 3.4 with the addition of HSCT aircraft (scenario Fa1H), as a result of the effects of stratospheric water vapor. Thus, aircraft-induced climate change with RFI > 1 points to the need for a complete scientific assessment rather than basing the climate impact on the use of fossil fuel alone. 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 much higher for sectors emitting CH4 and N2O without significant fossil fuel use.
The overall positive anthropogenic RF today, leading toward global warming, is caused primarily by an increase in anthropogenic emissions of long-lived greenhouse gases, countered in part by short-lived aerosols. Much of this radiative forcing has built up since the industrial revolution. Emissions from aviation are a relatively new contributor to this RF, although they are potentially a growing sector. In evaluating climate change forced by aviation, as well as that by other industrial sectors, the RFI provides a useful indicator. With an RFI of about 3, aviation's role in climate change involves several important climate perturbations beyond that from its release of fossil carbon alone.
This report presents the first thorough IPCC climate assessment of any industrial or agricultural sector. It includes all known climate forcings, many of which are important and not currently represented by indices such as GWP. Comparison of aviation to other industries must await an equally thorough evaluation of the summed effects of human activities by sector that is not available in IPCC (1996) but is anticipated for IPCC's Third Assessment Report in 2001.
The range of technology options that would attempt to reduce the impact of aviation on climate did not significantly change radiative forcing by the year 2050. A lesson taken from the Second Assessment Report (IPCC, 1996, Figure 6a) is that changes in total radiative forcing, even the wide range of emissions for the IS92a-f scenarios, do not appear much before year 2050. Thus, subsequent climate assessments of 21st century options for civil aviation need to carry out projected changes in greenhouse gases and aerosols, and all chemical and climate feedbacks, to the year 2100.
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