Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Aviation and the Global Atmosphere
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5.3. Detectability of Changes in Ground-Level Irradiance
Our ability to detect future changes in solar UV irradiance attributable to increases in aircraft
emissions depends on two factors. The first factor is the size of the expected change relative to
natural variability and long-term changes from other human causes. The second factor is the
accuracy and stability with which we are capable of making measurements over long periods of time.
The detection of a long-term change in UVery depends on the size of the expected change, the
length of the data record over which the change occurs, and the variability of UVery. Weatherhead et
al. (1998) show that a data record of at least 15 years is necessary to distinguish a statistically
significant (2-sigma) long-term change of 5% per decade in UVery from the natural variability present
at most sites. A long-term change of 5% over 50 years (1% per decade to 2050) would just be
detectable at the best sites. The other factor is whether existing instrumentation is able to detect
expected long-term changes. Presently, many different instruments devoted to monitoring changes in
solar UV during forthcoming decades are deployed over the globe. These instruments are either
spectroradiometers or broadband and narrowband detectors operating in the UV region.
Spectroradiometers are the most suitable instruments because they provide detailed measurements
of global spectral irradiance, which can be used to assess the importance of various UV-controlling
factors. Because of the weak intensity of UV irradiance relative to other parts of the solar spectrum,
however, it is only in recent years that the quality of such measurements has become sufficient to
detect interannual changes in UV irradiance that are attributable to year-to-year changes in ozone
over relatively short (< 10 years) measurement records (Kerr and McElroy, 1993; McKenzie et al.,
1993; Kerr and McElroy, 1994; Gardiner and Kirsch, 1995; Groebner et al., 1996; Bais et al., 1997;
Gurney, 1998). The quality of other types of broadband instruments cannot yet be considered
sufficient to detect these small long-term UV changes (Leszczynski et al., 1996; Mayer and
Seckmeyer, 1996; Blumthaler, 1997; Weatherhead et al., 1997; WMO, 1997).
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