Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
The computed average change in cirrus occurrence (1987-91 relative to 1982-86) as a function of aviation fuel use at the observation locations is shown in Figure 3-
18. The results indicate a statistically significant (97% confidence level) increase in cirrus occurrence in the North Atlantic flight corridor compared with the rest of the
North Atlantic Ocean (Boucher, 1998, 1999).
From the same source of observations, cirrus cover was analyzed for the periods 1971-81 and 1982-91 and for a combined data set extending from 1971-92 (Minnis et
al., 1998b). Data were averaged for gross air traffic regions (ATRs) and the rest of the globe. ATRs are rectangular areas on the globe that contain most air traffic
routes and constitute 26% of the available regions with data. The ATR trends are larger for cirrus clouds than for total cloudiness and are most significant in the first
period (1971-81), with the largest annual values found in the United States of America (3%), western Asia (1.6%), and the North Pacific (1.7%). No significant ATR
trends were found over Europe. Changes for the rest of the globe were significant only over land but with values less than those over the United States of America and
western Asia. Averaged over all ATRs, the cirrus cover increase between 1971 and 1981 amounts to 1.5% per decade, compared with 0.1% for the rest of the globe.
From 1982-91, cirrus cover increased over all areas except over non-ATR land and North Atlantic regions, where it changed by -0.4 and 0%, respectively.
The combined 1971-92 cirrus cover data set shows somewhat different results from the two separate data sets. The mean trend for land was 0% per decade for ATRs,
compared to -1.1% per decade for other land regions. Over the United States of America, however, the ATR trend is significant at 1.2% per decade. Over oceans, the
mean ATR trend is 1.2% per decade and 0.6% per decade over the rest of the oceans. The Pacific ATR trend is strongest, at 1.5% per decade. Averaging of apparent
trends from the two separate data sets gives results that differ from the mean trends in the combined data set. However, the relative difference in the mean trends
between the ATR regions and the rest of the globe is roughly 1.1% in both cases, indicating that cirrus coverage in areas with significant air traffic is increasing relative
to that over the remainder of the globe.
Using ground-based data sets, Figure 3-19 shows the seasonal trends in cirrus coverage over the United States of America and Europe. The average trend of the two
separate data sets is compared with the seasonal cycle of contrail occurrence frequency over the United States of America and cover over Europe. Contrail occurrence
over the United States of America was derived from 1 year of surface station observations (Minnis et al., 1997); contrail coverage over mid-Europe was computed from
satellite data (Mannstein et al., 1999). Over the United States of America, the seasonal trends in cirrus coverage (statistically significant at least at the 75% confidence
level) are roughly in phase with the seasonal cycle of contrail occurrence (see Figure 3-19a), suggesting that cirrus changes are related to contrail occurrence. In
contrast, the seasonal cycle of cirrus cover trends for Europe does not resemble the seasonal cycle of contrail cover (see Figure 3-19b). The European data, which are
not statistically significant, show that the trends observed for the United States of America are not so obvious for other air traffic regions.
3.5.1.2. ISCCP Observations
Data from the International Satellite Cloud Climatology Project (ISCCP, C2) (Rossow and Schiffer, 1991) between 50°S and 70°N from 1984 to 1990 have also been
inspected for trends in total and high cloud cover over land and ocean regions (Minnis et al., 1998b). The trends are similar to those found in the 1982-91 surface
observations, though details differ. Over all land areas, total cloudiness decreased by 0.5% per decade; high cloud cover grew at a rate of 1.2% per decade. Over the
United States of America, total cloudiness decreased by 2.3% per decade, but high cloud cover increased by 5.5% per decade. The trends over Europe were of the
same sign but half the magnitude. Over Asia, total cloud cover increased by 4.8% per decade, and high cloudiness grew by 2.7% per decade. Thus, the satellite data
suggest a relatively stronger increase in cirrus over the United States of America than surface observations suggest. Over ocean, total cloudiness decreased by 1.2%
per decade, whereas high cloud cover was enhanced by 3.7% per decade overall. Some of the discrepancies between the satellite and surface data may originate
from the different spatial sampling patterns, calibration, and orbit drift issues peculiar to the satellite data (Klein and Hartmann, 1993; Brest et al., 1997). The satellite
covers all regions almost equally, whereas surface observations are from fixed inhabited locations or from well-traveled ship routes.
Further analysis of the ISCCP satellite and the surface data sets was undertaken to isolate and quantify the effects of contrails (Minnis et al., 1998b). The apparent
trends were calculated separately for regions having a mean value of contrail coverage less than and greater than 0.5% as computed by Sausen et al. (1998) for 1992
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