|
6.3.2. Radiative Forcing for CO2
Carbon dioxide has a long atmospheric residence time (on the order of many
decades); hence, aircraft CO2 becomes well mixed within
the atmosphere and can be treated together with other anthropogenic CO2
emissions in conventional global warming simulations (e.g., Washington and Meehl,
1989; Cubasch et al., 1992; Murphy and Mitchell, 1995). The aircraft influence
depends on the temporal evolution of the amount of the CO2
increase that can be attributed to aircraft emissions, which is directly proportional
to the amount of fuel burned. See Section 6.1.2 and Table
6-2 for the calculation of CO2 increases attributed
to aviation.
Over the period 1990 to 2050, under IS92a we expect an increase in atmospheric
CO2 of about 155 ppmv from burning of fossil fuels, cement
production, and other anthropogenic activities that release biospheric carbon.
By 2050, F-type aviation scenarios produce a 5-7 ppmv increase, and the high-growth
Edh scenario leads to a 13 ppmv increase. Thus, aviation in these scenarios
would be responsible for 3-8% of the total anthropogenic increase in CO2
from 1990 to 2050.
The RF for aviation CO2 in 1992 is estimated to be +0.018
W m-2, with a likely range of ±30% that includes uncertainties
in the carbon cycle and in radiative calculations (see WMO, 1999). Uncertainties
and confidence intervals discussed here do not include possible errors in predicting
future scenarios. By 2050, the different aviation scenarios have a range of
+0.06 to +0.16 W m-2. The technology option 2 scenario
(Fa2) leads to a 0.1 ppmv increase in CO2 by 2050, with only a small increase
in CO2-RF.
The HSCT option, F1aH, has 18% greater fuel use but only 8% greater CO2
concentrations by 2050, with a corresponding increase in CO2-RF
from +0.074 to +0.080 W m-2. Because the HSCT fleet has
just reached maturity in 2040, the extra fuel consumption of the HSCT aircraft
is barely felt in terms of the accumulation of CO2. Similarly,
the CO2 impact of new subsonic technologies that are
introduced linearly between 2015 and 2050 is not fully effected by 2050. A fuller
evaluation would have to extend the assessment beyond 2050, when the cumulative
effects of mature fleets would be felt (e.g., Sausen and Schumann, 1999).
|