Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Aviation and the Global Atmosphere
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3.5. Long-Term Changes in Observed Cloudiness and Cloud-Related Parameters
Other reports in this collection
Contrails have long been considered possible modifiers of regional climate (Murcray, 1970; Changnon, 1981). Contrails may increase total cloud and cirrus cloud
amounts, and consequently change the Earth's radiation balance. As a result, surface and upper tropospheric temperatures may change (Detwiler, 1983; Frankel et
al., 1997). In the following sections, we examine changes in cloudiness and other climate parameters for their possible relationship to aircraft operations. Some data
indicate changes in observed cirrus cloudiness. These observations are used to provide a first estimate of an upper bound on the increase in contrail-cirrus coverage
since the beginning of the jet air traffic era. Limitations in attributing observed trends to aircraft are discussed in Section 3.5.3.
3.5.1. Changes in the Occurrence and Cover of Cirrus Clouds
3.5.1.1. Surface Observations
Although most studies reporting trends in cloud cover have considered total cloud cover (Henderson-Sellers, 1989, 1992; Angell, 1990; Plantico et al., 1990; Karl et al.,
1993), we focus here on studies reporting trends in cirrus cloud cover, which is more relevant to the issue of aviation effects on cloudiness. Observations at
Hohenpeissenberg, Germany, indicate that the frequency of high clouds during sunny hours increased from 45% in 1954 to 70% in 1995 (Vandersee, 1997; Winkler et
al., 1997). Such large changes are not atypical of regional cloud climatologies (e.g., Rebetez and Beniston, 1998). Over the same period, global radiation during
sunshine hours decreased by about 10%, indicating that the observed cloud trend is not an artifact. Similarly, cirrus frequency increased between 1964 and 1990
under cloudy-with-sun conditions by 27% over Hamburg and 15% over Hohenpeissenberg (in northern and southern Germany, respectively) (Liepert et al., 1994;
Liepert, 1997). These changes are not directly attributable to aircraft, however; instead, they might be caused by an increase in tropopause altitude or higher relative
humidity in the upper troposphere (Winkler et al., 1997). Cirrus cover over Salt Lake City and Denver has increased from about 12% annual mean cover to 20% in the
period from the early 1960s (i.e., since the beginning of the jet aircraft era) to the 1980s, possibly because of increased air traffic over those cities. These trends are
significantly lower in the 1990s, and similar observations over Chicago and San Francisco show no obvious trends (Liou et al., 1990; Frankel et al., 1997).
Global trends of observed total and cirrus cover can be deduced from ground-based cloud observations over land and ocean (Warren et al., 1986, 1988; Hahn et al.,
1994, 1996) for 1971-91. For the period 1982-91, mean global trends in cirrus occurrence frequency were 1.7 and 6.2% per decade over land and ocean, respectively
(Boucher, 1999). The decadal trend was 5.6% over North America and 13.3% over the heavy air traffic region located in the northeastern United States of America.
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