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3.1. Introduction

The SPARC/IOC Ozone Trends Panel has been asked to provide an in-depth review of our knowledge of trends in the concentration profiles of ozone. This report will serve as an input to the next WMO/UNEP Scientific Assessment of Ozone Depletion. The present chapter assesses our understanding of the long-term ozone changes that have taken place since 1970 with an emphasis on the profile. As a result of the studies carried out under Chapter 2, data sets with improved long-term calibration and internal consistency are available for this analysis.

This report builds on the chapter on ozone trends of the 1994 WMO/UNEP report (WMO, 1995 and Harris et al., 1997). The emphasis of that report was trends in the total column amount of ozone, although it also contained significant material on trends in the altitude profile. This report will focus on trends in the profile. We will consider trends in the column amount only to check for consistency with column-integrated trends.

The 1994 report concluded that the trends in the ozone profile were generally consistent with the trends in total ozone, but that significant uncertainties remained. One of these was that the trends in the lower stratosphere deduced from SAGE I/II data were significantly larger than those observed by sondes. The introduction of a latitude-dependent correction for the altitude registration of the SAGE I data by Wang et al. (1996) has improved this situation. This correction has also made the upper stratospheric trends deduced from SAGE I/II and those deduced from the SBUV-SBUV/2 instruments more consistent.

Upper stratospheric ozone is only a small portion of the total column. However, trends in this region are important because they should reflect a straightforward response of ozone to the increases in chlorine amount. The chemistry of this region is gas phase and relatively well known, but confirmation through observational results is needed. This report uses SAGE I data as corrected by Wang et al. (1996). The analysis uses version 5.96 SAGE II data as described in Chapter 1. The SBUV/2 data from NOAA-11 used in these calculations is a reprocessed version using an updated calibration as described in Chapter 2.

Chapters 1 and 2 have included an extensive reanalysis of the data from a number of ozone sonde stations. These have included corrections for pump efficiency, solution strength, and other factors. These corrections potentially have a proportionally larger effect on the results of trend analyses in the troposphere. In the troposphere, the sondes remain the only viable measurements with long-term records. An important issue for trends in tropospheric ozone is whether trends have occurred throughout the period since 1970 or have been larger in the 1970s than in the 1980s and 1990s. To examine this question, trends from both 1970 and from 1980 are considered in this chapter.

In the present report we employ statistical analyses to test for significant occurrences of linear trends. This approximates the expected behaviour of ozone due to changes in the stratospheric chlorine amount. The time periods used are either 1970 to the present or 1979-80 to the present. Because of the implementation of the Montreal Protocol, chlorine amounts have reached their maximum in the troposphere and begun to decline. The amount of chlorine in the stratosphere should peak within a year or two and the ozone decline should then begin to reverse itself. This is the last report in which we will be able to address the primary question by searching for a linear trend.