Aviation and the Global Atmosphere

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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 http://www.ipcc.ch/ipccreports/sres/aviation/039.htm (3 von 6)08.05.2008 02:42:06
Aviation and the Global Atmosphere those areas and is equivalent to about 0.3% coverage of the Earth's surface, 0.2% more than for line-shaped contrails. If no counteracting process took place that reduced cloudiness over this same period, this value gives an upper bound for total aviation-induced cloudiness. 3.5.2. Changes in Other Climate Parameters 3.5.2.1. Sunshine Duration and Surface Radiation Observations from 100 stations over the United States of America showed that the mean value for total cloud cover over the years 1970-88 increased by 2.0 ± 1.3% relative to the years 1950-68 (Angell, 1990). Sunshine duration decreased by only 0.8 ± 1.2%, presumably because cirrus clouds are often too thin to reduce measured sunshine duration. Sunshine duration decreased by an average of 3.7% per decade over Germany between 1953 and 1989 (Liepert et al., 1994). Contrails were estimated to be insufficient to account for the diminished sunshine. In an analysis of radiation trends in Germany from 1964 to 1990, solar radiation during cloudy periods with sun decreased strongly (by 20 to 80 W m -2 ) (Liepert, 1997). The change might be attributable to aircraft, but the strong reduction in diffuse radiation revealed a more turbid atmosphere, which cannot be explained by increasing cirrus coverage alone. 3.5.2.2. Temperature A substantial decrease in diurnal surface temperature range (DTR) has been observed on all continents (Karl et al., 1984; Rebetez and Beniston, 1998). The reasons for this decrease are not clear; it could be caused by a change in aerosol burden, cloud amount, cloud ceiling height (Hansen et al., 1995), soil moisture, or absorption of solar radiation by increased water vapor in the atmosphere (Roeckner et al., 1998). A weak correlation was found between contrail occurrence data and DTR over the United States of America (DeGrand et al., 1990; Travis and Changnon, 1997). The relationship was strongest during the summer and autumn and most significant over the southwestern United States of America. Rebetez and Beniston (1998) find large DTR trends in the Swiss Alps (a region with heavy air traffic) in correlation with low-level clouds but no reduction of DTR at higher elevation sites, contradicting a potential major impact on DTR from aviation-induced cirrus. From data taken between 1901 and 1977 over the midwestern United States of America, increases of moderated temperatures (below average maximum and above average minimum), decreases in sunshine and clear days, and shifts in cloud cover, occurred after 1960; these changes were concentrated in the main east-west air corridor (Changnon, 1981). There are, however, many reasons for changes in global mean surface temperatures since 1880 (Halpert and Bell, 1997). The maximum increase in tropospheric temperatures occurred at northern mid-latitudes in the lower troposphere (Tett et al., 1996; Parker et al., 1997). However, radiative transfer models predict maximum heating rates right below contrails and cirrus in the upper troposphere (Liou, 1986; Section 3.6). Studies using a full-feedback global circulation model (GCM) indicate a nearly constant increase in temperature with altitude for an increase in thin cirrus cloudiness (Hansen et al., 1997). Thus, existing studies do not support the conclusion that aircraft-induced contrails and cloudiness caused changes in global tropospheric temperatures. 3.5.3. Limitations in Observed Climate Changes The observations cited above suggest that some of the apparent trends in observed cirrus cloudiness and sunshine duration may be caused by air traffic. However, any observed change in cirrus cloudiness or climatic conditions may have other natural or anthropogenic causes (IPCC, 1996). Natural variability includes the El Niño Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO), and the Pacific North America pattern (PNA). For example, the NAO index increased overall from the mid-1960s until 1995 (Hurrell, 1995; Halpert and Bell, 1997). However, there is no evident trend in the NAO index (Hurrell, 1995) and mean position of the Iceland low (Kapala et al., 1998; Mächel et al., 1998) over the period 1982-91 when significant increases in cirrus occurrence and cirrus amount were observed over the North Atlantic Ocean (Boucher, 1998, 1999). Changes in cirrus and subsequent changes in sunshine duration may reflect changes in cyclonic activity (Weber, 1990), ocean surface temperature and related atmospheric dynamics (Wallace et al., 1995), upper troposphere temperature and relative humidity, tropopause altitude (Hoinka, 1998; Steinbrecht et al., 1998), or volcanic effects (Minnis et al., 1993)-or they may be a result of global climate change caused by increased greenhouse gases. Changes in http://www.ipcc.ch/ipccreports/sres/aviation/039.htm (4 von 6)08.05.2008 02:42:06
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