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3.7. Summary

• Statistical models used in the analysis were compared by using 3 test data sets. This comparison revealed only minor differences in trends obtained by the models. Somewhat greater differences were found in the uncertainties estimated for the trends and other variables included in the models. Results are most sensitive to the details of the model for time series with significant missing data.
• Decadal variations are a ubiquitous feature of ozone observations, in addition to QBO and faster time scale dynamical variability. Inclusion of these terms does not have strong influence on the calculated trends for long time series. Much of the observed decadal changes are approximately in phase with the solar cycle for the observational record, suggesting a solar mechanism. There is a strong potential confusion of solar and volcanic signals for the recent record. Although these effects have relatively small impacts on trend estimates, it does limit our ability to interpret decadal variability.
• The upper stratosphere (altitudes between about 30 and 50 km) is a region where changes in ozone were originally predicted to occur. At these altitudes the chemistry should be dominated by gas-phase reactions. When the upper stratospheric data are fit to a standard statistical model, negative trends are found throughout the region with statistically significant peak values of -6 to -8%/decade at 40-45 km altitude. There is good agreement between SAGE I/II and Umkehr. The SBUV/SBUV2 combined record shows less negative trends. We place less confidence in the SBUV/SBUV2 result due to potential problems with the present version (6.1.2) of the NOAA-11 SBUV2 data. There is a factor of two seasonal variation, with a maximum negative trend in winter. There is no significant inter-hemispheric difference in upper stratospheric trends based on SAGE I/II version 5.96 data extended through 1996.
• The lower stratosphere (altitudes between about 10 and 30 km) is the region where we expect much of the trend which has been deduced from column data to occur. The primary trend instruments in this region are sondes (up to 27 km) and SAGE (20-30 km). For sondes, sampling of the data prior to trend analysis has as much or more effect on derived trends than do the details of the statistical model. The optimal selection criteria for the use of sonde data in trend analyses is a subject of debate.
• Trends from 8 individual ozonesonde stations in the northern mid-latitudes are negative throughout the lower stratosphere. For 1970 through 1996 they range from -3 to -10%/decade at 17 km and are statistically significant at all stations. The trends show little seasonal variability above 20 km. The seasonal variability of the trend in the ozone profile occurs mostly in the altitude range of 10-20 km. The exact time dependence of this seasonal variability is uncertain. European stations show a winter-spring maximum, while Canadian stations show a spring-summer maximum. Details of this seasonal maximum were somewhat different in the two analyses of the sonde data.
• There is a reasonable agreement between SAGE I/II trends and sonde trends over the altitude region from 15 to 27 km at northern mid-latitudes (for the time period 1979-1996). This is a significant improvement compared to previous comparisons mainly due to the inclusion of the latitude-dependent altitude correction to the SAGE I data. The agreement in the derived trends from SAGE II only and the sondes is excellent (for the time period 1984-1996). SAGE II trends in the 15-20 km region in the tropics are much more negative than those in northern mid-latitudes but there are insufficient sonde records with which to compare these results.
• It is difficult to make generalisations concerning trends in tropospheric ozone. The only data from which to make conclusions are sonde data from a small number of stations. Trends calculated for Canadian stations are negative or near zero for the period from 1970 through 1996 and also for the period from 1980 through 1996. Trends calculated for 3 European stations are strongly positive for the period 1970 through 1996 but are essentially zero at two of these stations when data from 1980 through 1996 are considered. Trends calculated for the Japanese stations are strongly positive for the period from 1970 through 1996 but are either positive or not significantly different from zero over the period from 1980 through 1996.
• Trends in the column amount of ozone above 20 km deduced from SAGE I/II are much less than the column trends deduced from TOMS. The TOMS/SAGE differences are consistent with the sonde trends below 20 km. There is also a consistent seasonal variation for satellite and sonde data. Both indicate that the primary seasonal variation in mid-latitude ozone trends occurs between 10 and 20 km altitude, with maximum during northern hemisphere winter and spring.
• A first attempt has been made to combine the trends and uncertainties estimated from all available measurement systems. This has been done for northern mid-latitude measurements. The result is a statistically significant (at the 2s level) negative trend at all altitudes between 10 and 45 km. The combined trend has two local extremes, -7.4 ±2.0%/decade at 40 km and -7.6 ±4.6%/decade at 15 km altitude. A minimum trend of -2.0 ±1.8%/decade was deduced at 30 km altitude.

We thank the many contributors to this document who are not listed as Panel Members. In particular we thank the Chicago group, Tiao, Choi, and Zhang, and Inna Megretskaia of Harvard for the extensive calculations that they performed on trends using sonde data. We thank Gordon Labow for his work in preparing data sets to make the calculation of trends much easier. Finally, we thank the members of the Chapter 1 and 2 panels for the extensive work that resulted in improved data sets for analysis and improved ability to estimate uncertainties in the trends.