Current radiative transfer models are able to predict the influence of changes in ozone on ground-level UV irradiances with a high degree of confidence. The ability of these models to treat aerosols is less well developed because of uncertainties in their optical properties. In addition, the geometrical complexity and temporal variability of clouds still pose challenges to accurate radiative transfer modeling. From the standpoint of assessing the effects of future aviation, it appears that changes in atmospheric ozone abundances are the most important driver of changes in UV irradiance. Outcomes predicted for a range of scenarios are summarized below.
A fleet of subsonic aircraft leads to a latitude-dependent reduction in UVery, with the greatest percentage changes in the Northern Hemisphere. In the year 2015, the best estimate of the change in UVery for July associated with a subsonic fleet lies in the range -0.5 to -1.0% poleward of 30�N. When the range of uncertainty is included, the spread in calculated change in irradiance is-1.8 to -0.5% at 45�N. These values are for the year 2015 for a subsonic fleet relative to the year 2015 without this fleet. In the Southern Hemisphere, the corresponding best estimate of change in UVery for January is less than -0.3% from the equator to 30�S and near 0% at 60�S. For 1992, the calculated impacts for aviation are about half those calculated for 2015. In 2050, the calculated impacts of a pure subsonic fleet are about 50% greater than those for 2015.
Scenarios based on fleets containing subsonic and supersonic aircraft imply aviation-related increases in UVery in 2015 and 2050 relative to what would otherwise be expected in those years. The best estimates for both years are similar-very little impact on UVery in the tropics, rising to increases between about 0.2 and 0.8% around 65� in both hemispheres. Although these estimated changes may be considered small, they do have significantly larger uncertainty limits. For the 67% likelihood range, it is estimated that the percentage change in UVery relative to the background atmosphere can be expressed as (-2% + best estimate) to (+3% + best estimate)-a 5% range in the change relative to the background column. As can be seen from the middle panel of Figure 5-9, this range always encompasses the possibility of no change in UVery relative to the corresponding background atmosphere.
The change in aerosol loading expected from increased aircraft operations between the present and 2050 is small relative to both the natural aerosol background and anthropogenic influences other than those related to aviation. The effect of the aircraft-related increase in aerosols is to reduce UV irradiance by less than 0.1%. Calculations indicate that aircraft-related increases in contrails lead to a decrease in UV irradiance of less than 0.5% in regions where 10% of the sky is covered.
The anticipated decline of the chlorine and bromine content of the stratosphere as well as increased emissions of NOx from combustion between 1992 and 2050 are expected to lead to latitude-dependent increases in ozone amounts and consequent decreases in UVery, irrespective of future aviation. For a background atmosphere at 45�N in July, the calculated change in UVery with respect to a 1970 background is +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. Observed changes in total ozone from 1970 to 1992 imply smaller percentage increases in UVery, indicating the degree of uncertainty in the model predictions.
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