6.1.4. Aviation's Contribution to the CO2 Budget

Carbon dioxide released from fossil fuel combustion rapidly equilibrates among atmosphere, surface ocean, and parts of the biosphere, leaving behind excess atmospheric CO2 that decays slowly over the following century (see carbon cycle discussion in IPCC, 1996). Thus, for CO2 radiative forcing, it makes no difference whether the fossil fuel is burned in aircraft or other transportation/energy sectors, and the relative role of aircraft can be found by comparing the history of fuel burned by aviation with that of total anthropogenic carbon emissions.

Comparing projected IS92a carbon emissions from fossil fuels in Figure 6-6, CO2 emissions from aircraft in 1990 account for about 2.4% of the total; they are projected to grow to about 3% (Fa1) or more than 7% (Eab) of all fossil fuel carbon emissions by 2050. Sustained growth in air traffic demand (5%/yr compounded) envisaged in Edh would lead to an aviation fraction of more than 10% by 2050. By comparison, the entire transportation sector is currently about 25% of the total (see discussion in Chapter 8). Clearly, different economic projections, as well as uncertainties in predicting demand for air travel and aviation's ability to meet that demand, can alter this aviation fraction by more than a factor of 2. Technology option 2 (low-NOx engines, Fa2) increases this fuel fraction slightly to 3.2%, and the HSCT option increases this fraction from 3.1% (Fa1) to 3.6% (scenario Fa1H, assuming no change in air traffic demand) by the year 2050.

The cumulative history of CO2 emissions allows us to calculate the excess atmospheric CO2 concentration attributable to aircraft, as shown in Figure 6-8. These calculations are contributed by Jain, Wigley, and Schumann using carbon cycle models consistent with IPCC (1996) and the IS92 scenarios therein. Aviation is estimated to be responsible for about 1 ppmv of the 80 ppmv rise in CO2 from 1860 to 1990. Uncertainty in the prediction of atmospheric CO2 is estimated to be ±25%. Resulting CO2 radiative forcing is only one part of aviation's climate impact. Other changes in greenhouse gases, radiatively important aerosols, and clouds-as noted in Figure 6-1 and broken out in Table 6-1-must be included. The remaining sections of this chapter examine aviation's total role in climate change over the next 50 years for the example scenarios given in Table 6-3.

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