Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
July of 2015 at 45°N is -0.9%. Accordingly, the 67% likelihood range for this particular change is -1 .8
to -0.5%; because it is a calculation for 2050, our confidence in this result is "poor" as prescribed by
Chapter 4. Similarly, in the middle panel of Figure 5-9, the impact of the hybrid fleet at the equator in
July of 2050 is shown as -0.2%. This impact is based on a calculation of ozone changes by the AER
2-D model. From the discussion above, we determined the 67% likelihood range for this impact to be -
2.2 to +2.8%, and our estimate of the confidence is "good." At 65°N in July of 2050 (middle panel of
Figure 5-9), the calculated impact of the hybrid fleet on UVery is +0.5%, which gives -1 .5 to +3.5% as
the 67% likelihood range with an estimated confidence of "fair."
As discussed in Section 5.4.1.1, the calculations shown in Figures 5-6 to 5-9 include percent changes
in UVery for the background atmosphere using 1970 as the reference year. At 45°N in July, these
changes are +8, +3, and -3% for 1992, 2015, and 2050, respectively. At 45°S in January, the
calculated changes are +9, +4.5, and 0% for 1992, 2015, and 2050, respectively. For comparison, the
computed change due to observed ozone depletion at 35-50°N in July is about 4% over the period
1970-1992 (WMO, 1999). The corresponding change for 35-50°S in January is 8%.
5.4.2.4. Contribution of Persistent Contrails, Cirrus Clouds, and Aerosols to the
Impact of Aviation on UV
The geographic and temporal variability associated with clouds and aerosols complicates attempts to
make general statements concerning their effects on UV irradiance. Nonetheless, this section
considers some highly simplified scenarios to place bounds on the influence of altered contrails,
cirrus, and aerosol amounts. There are three issues to address: The effect of regional, persistent
contrails; the effect of observed trends in cirrus that may result from a variety of processes, including
aviation; and the effect of aviation-produced aerosols.
To estimate the effects of persistent contrails on ground-level UV, calculations assume a 5% area
coverage, appropriate to the local maximum over the eastern United States of America (see Section
3.4.3), and a contrail optical thickness of 0.3. For latitude 45°N summer, local noon, this scenario
leads to a reduction in UVery of 0.2% relative to clear skies. If persistent contrail coverage were to
increase to 10%, the corresponding reduction in UVery would be 0.4%.
The observed trends in cirrus presented in Chapter 3 include the effects of aviation, as well as any
other influences that may be operative. The estimated response of UVery to trends in the cirrus
background is based on the following assumptions: The cirrus have a scattering optical thickness of
0.3, and initially 23% of the land area is covered by cirrus at an altitude of 11 km. Starting from this
condition, an increase of 3.5% per decade in the land area covered by cirrus, appropriate to the
United States of America in spring (Figure 3-19), leads to a decline in UVery of approximately 0.1%
per decade for local noon at latitude 45°N. If changes in persistent contrails and cirrus, especially on
regional scales, differ substantially from the estimates adopted above, the resulting changes in UVery
would have to be modified accordingly. The numbers given here point to the magnitude of reasonable
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Figure 5-8: Calculated ozone and UVery at 45°N in July
referred to the calculated background values for 1970.