Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
aircraft operations (see Section 3.4.3). The mean values of the computed contrail coverage in the two regions are 0.04 and 1.4%, respectively (Figure 3-16). Satellite
data and surface observations show larger increases in cirrus cloudiness where contrails are expected to occur most frequently than in all other areas (Table 3-5).
Satellite data show that, over land, the increase in high cloudiness in contrail regions was almost four times that in other areas. Over ocean, the difference, though less
significant statistically, amounts to a differential increase of 1.6% cover per decade in contrail areas with respect to non-contrail areas. Cirrus trends over contrail
regions as derived from surface observations are 0.8 to 2.3% per decade greater than those in the remainder of the globe for the two periods from 1971-92. The single
1971-92 surface data set shows a significant increase (1.6%) in cirrus over land contrail regions compared to remaining land areas. Over oceans, the relative
difference is negative but insignificant because of the small number of samples and large variance. Although the differences in these trends are significant at
confidence levels of 95% only for the land and global ISCCP data and the 1971-81 land surface data, they show consistent tendencies.
3.5.1.3. HIRS Observations
High-Resolution Infrared Radiation Sounder (HIRS) data from NOAA satellites have been analyzed for the period June 1989 to February 1997 to determine total and
high cloud cover (Wylie and Menzel, 1999). During this period, high clouds observed by the NOAA-10 and -12 satellites increased by 4-5% over land and ocean in the
Northern Hemisphere (from 23 to 65°N) but only by about 2% in the tropics. High clouds increased by about 3% over southern mid-latitude oceans. The trend values
inferred from the NOAA-11 and -14 satellites are different and somewhat more uncertain because of orbit drift. Over oceans, they also indicate a larger high cloud
increase in northern mid-latitudes (3.3%) than over the tropics (-0.4%). Further analysis of HIRS data is required to determine the extent of any contrail impact.
3.5.1.4. SAGE Observations
Data from the Stratospheric Aerosol and Gas Experiment (SAGE) II satellite instrument indicate that the frequency of subvisible cirrus clouds near 45°N is twice that at
45°S (Wang et al., 1996). Aviation, as well as hemispheric differences in atmospheric conditions and background aerosol (Chiou et al., 1997; Rosen et al., 1997), may
contribute to such differences (Sausen et al., 1998).
3.5.1.5. Upper Bound for Aviation-Induced Changes in Cirrus Clouds
The line-shaped contrail cover and global extrapolation described in Section 3.4.3 (see Figure 3-16; Sausen et al., 1998) provide only a lower bound for aviationinduced
changes in cirrus cloud cover because they are based on satellite observations that identify only contrails and additional cirrus clouds that are line-shaped.
Although estimates of an upper bound of aviation-induced cirrus cover have not yet been established, evidence for a correlation of long-term increases in cirrus
cloudiness with air traffic has been published (Liou et al., 1990; Frankel et al., 1997; Boucher, 1998, 1999). Here, observations of cloudiness changes described above
are used to provide a preliminary estimate of this upper bound.
Differences in trends derived from observations indicate a stronger mean increase of cirrus amounts in regions with large computed contrail cover than in regions with
low computed contrail cover, at least over land (see Table 3-5). The trend difference values vary and are of different statistical significances. The values are considered
more meaningful over land because there is less air traffic over the oceans. Of regions with large computed contrail cover, only 14% occurs over oceans, and most of
these regions occur near the coasts. In addition, less correlation is expected between computed and observable contrail cover over oceans because actual flight tracks
often deviate significantly from idealized great-circle routes. Over land, surface-based observations for 1971-81 suggest a differential increase in cirrus cover of 2.3%
per decade. The 1982-91 trend difference is smaller but still positive, and the combined 22-year trend difference is 1.6% per decade. The 7 years of ISCCP satellite
data suggest even larger trend differences, both globally and over land. If a trend difference of 1.6% per decade is adopted as most representative of available data,
and if that trend is assumed to have persisted for the 3 decades since the beginning of the jet aircraft era (the end of the 1960s), then the current increase in cirrus
coverage from aircraft is 4.8% in areas with contrail cover greater than 0.5%. This value is about three times the currently computed linear-contrail cover of 1.4% in
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