Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Table 4-6:Relative sensitivity (%) of global ozone perturbations
from aircraft emissions.
Sensitivity
Case
IMAGES/
BISA
ECHAm 3 /
CHEM
Tm 3 /
KNMI UiO
Lightning -16
Surface
emissions IS92a
NASA-ANCAT -20
NMHC chemistry -35 -10
Exclusion of N2O5 removal on aerosol
-10 0
Interannual variability ±6.3
Scenario G - F +44
The reduction in CH4 lifetime would lead to a nearly uniform CH4 reduction globally because of the relatively long residence time computed for CH4 . This result would
be in contrast to O3 , for which changes would occur on large regional scales. Finally, it should be noted that for computational reasons, the experiments in this
assessment were performed using fixed CH4 concentrations at the Earth's surface (see Section 4.2.2.1), and CH4 concentrations at the surface were not allowed to
adapt to higher OH abundances (positive feedback). Hence, even larger CH4 decreases are to be expected if CH4 ground flux boundary conditions are used. However,
such calculations are much more computationally intensive. IPCC (1995) and Fuglestvedt et al. (1996) showed that the feedback factor is uncertain and modeldependent
but is estimated to be in the range 1.2-1.5. Adopting a factor of 1.4 increases the percentage changes, in the CH4 lifetimes shown in Table 4-5, to -2.2 to -
4.1% in 2015 and -3.2 to -6.0% in 2050. Changes in CH 4 lifetime of this order will lead to global average radiative forcing (see Chapter 6) similar to global average
radiative forcing perturbations from aircraft-induced O 3 changes, but with an opposite sign (CH 4 will be reduced).
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