
Figure 2a: Estimates of the globally and annually
averaged radiative forcing (Wm^{2})
(see Footnote 4) from subsonic aircraft emissions in 1992 (2a) and in 2050
for scenario Fa1 (2b). The scale in Figure 2b is greater than the scale
in 2a by about a factor of 4. The bars indicate the best estimate of forcing
while the line associated with each bar is a twothirds uncertainty range
developed using the best knowledge and tools available at the present time.
(The twothirds uncertainty range means that there is a 67% probability
that the true value falls within this range.) The available information
on cirrus clouds is insufficient to determine either a best estimate or
an uncertainty range; the dashed line indicates a range of possible best
estimates. The estimate for total forcing does not include the effect of
changes in cirrus cloudiness. The uncertainty estimate for the total radiative
forcing (without additional cirrus) is calculated as the square root of
the sums of the squares of the upper and lower ranges for the individual
components. The evaluations below the graph ("good," "fair," "poor," "very
poor") are a relative appraisal associated with each component and indicates
the level of scientific understanding. It is based on the amount of evidence
available to support the best estimate and its uncertainty, the degree of
consensus in the scientific literature, and the scope of the analysis. This
evaluation is separate from the evaluation of uncertainty range represented
by the lines associated with each bar. This method of presentation is different
and more meaningful than the confidence level presented in similar graphs
from Climate Change 1995: The Science of Climate Change.
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