Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Aviation and the Global Atmosphere
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6.6.2. Uncertainties and Confidence Intervals
Other reports in this collection
Throughout this report, we focus on "best estimates" for each component of atmospheric perturbations caused by aircraft and then of subsequent climate forcing or
ultraviolet change. We also try to understand the confidence that we have in these estimates using uncertainty ranges deduced in Chapters 2, 3, and 4 and those from
the modeling and combining of RF in this chapter.
Uncertainties in estimating aviation's RF values are addressed with a confidence interval (indicated by error bars or whiskers about each best value) and a description
("good," "fair," "poor," "very poor") of the level of scientific understanding of the physical processes, models, and data on which the calculation is based. The
confidence intervals shown in Figures 6-14b and 6-15b define a likelihood range such that the probability that the true value falls within the interval is 2/3. The interval
and the quality-of-the-science descriptions are, to a large extent, independent measures covering different aspects of uncertainty.
The likelihood range is defined consistently within this report as the 2/3 or 67% probability range. These probability ranges are meant to be symmetric about the best
value; hence, the best value is not always the mean of the upper and lower values. In this case, the probability that the value is less than the lower value is 16%, and
the probability that it is less than the upper value is 84%. The range between the low and high values is equivalent to the "1-sigma" range of a normal (i.e., Gaussian)
probability distribution. Unfortunately, derivation of these confidence intervals lies with the expert judgment of the scientists contributing to each chapter and may
include a combination of objective statistical models and subjective expertise. Thus, the 67% confidence intervals do not imply a specific statistical model and, for
example, cannot be used to infer the probability of extreme events beyond the stated confidence interval.
The confidence interval in RF stated here combines uncertainty in calculating atmospheric perturbation to greenhouse gases and aerosols with that of calculating
radiative forcing. It includes, but is not based solely on, the range of best values from different research groups. For example, the interval for the HSCT(1000) impact
on O3 was derived from high- and low-end calculations using different combinations of atmospheric models and chemical assumptions. The range in RF from these
stratospheric O3 perturbations was expanded further in this chapter to account for the difficulty in calculating RF for stratospheric perturbations. The tropospheric O3
perturbation from the subsonic fleet (scenario Fa1) was presented with the 67% confidence interval as a factor of 2 higher and lower than the best value. In this case,
the RF calculation did not significantly add to the uncertainty because tropospheric perturbations can be more accurately calculated. The confidence interval for
aviation-induced CH4 changes is believed to be about 1.5 times larger (log-scale) than that for tropospheric O3, but potential errors in both are highly correlated. The
confidence interval for contrails is taken directly from Chapter 3; the RF from additional cirrus clouds is highly uncertain and no probability range is given.
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