6.3.6. Uncertainties
Assignment of formal uncertaintyor the likely (2/3 probability) interval about
the best valueto radiative forcing caused by aircraft perturbations is difficult.
For wellmixed gases (e.g., CO2, CH4)
or for welldefined tropospheric perturbations (subsonic O3),
there is small uncertainty in calculated RF. In these cases, the overall uncertainty
interval lies with calculating the perturbation itself: 25% for CO2,
a factor of 2 for O3, and a factor of 3 for CH4.
For perturbations to stratospheric ozone and water, there is much greater uncertainty
in calculating RF, especially because in these cases radiative forcing at the
tropopause can be substantially different after stratospheric temperatures adjust.
In addition, HSCTinduced ozone and water vapor perturbationswith large variations
in the lower stratospherepresent a much more difficult calculation of RF where
the placement of the modeled tropopause can lead to additional uncertainty.
As an example of the uncertainty in calculating RF values from an adopted ozone
perturbation, the NASA1992 tropospheric ozone perturbation was calculated by
several groups, as shown in Table 64. The instantaneous
RF at the tropopause is consistent across the models, and the stratospheric
adjustment (calculated by two groups) is consistently 0.001 to 0.002 W m2
less. For the HSCT(500) water vapor perturbation, the two groups have significantly
greater disagreement, and the correction following stratospheric adjustment
is a large fraction of tropopause instantaneous RF. This water vapor perturbation
is the result of averaging six model results, and an additional RF is calculated
using the water vapor perturbation calculated with a 3D model that lies at
the lower end of this ensemble (Grossman*; see Table
64). This table highlights the robustness of calculated RF for tropospheric
perturbations and the much greater uncertainty in deriving climate forcing for
stratospheric changes.
Uncertainty ranges about the best values are given in Table
61 for the NASA1992* and FESGa (tech 1) 2050 subsonic scenarios and for
some components of supersonic scenarios HSCT(500) and HSCT(1000). These intervals
are intended to represent the same probability range (67% likelihood), but there
is no uniform statistical model (e.g., Gaussian) for all of them, nor are the
individual RF contributions fully independent; hence, these ranges cannot be
combined into a confidence interval on total RF.
Table 64: Results of RF (W m2)
for the 1992 aviationinduced ozone perturbation and for the water vapor
from the HSCT(500) fleet as calculated by several models (see Section
6.3.1).

NASA1992 Tropospheric Ozone Perturbation 
Type of RF Calculation 
Forster & Haywood

Ponater & Sausen

Grossman

Rind

Wang

Top of atmosphere, instantaneous

+0.014

+0.021

+0.013

+0.011

+0.010

Tropopause, instantaneous

+0.020

+0.026

+0.022


+0.020

Tropopause, after stratospheric adjustment

+0.019

+0.024




HSCT(500) Stratospheric Water Vapor Perturbation 
Type of RF Calculation 
Forster & Haywood

Ponater & Sausen

Grossman

Grossman*


Top of atmosphere, instantaneous

+0.001

+0.001

0.002

0.001


Tropopause, instantaneous

+0.096

+0.049

+0.074

+0.048


Tropopause, after stratospheric adjustment

+0.068

+0.034




* RF calculation for a lower
accumulation rate of H2O (Danilin et al., 1998; Hannegan
et al., 1998).

