Total ozone trends from quality-controlled ground-based data (1964-1994)

Seasonal and year-round trend analyses of ground-based total ozone data that have been carefully quality controlled are presented for 46 Dobson stations, for four regions of the former USSR and for the two polar regions. The trend model incorporated the 10.7-cm solar flux and equatorial stratospheric wind as indicators for removal of the solar and quasibiennial components in ozone variations. The trends were calculated for two main time intervals: from January 1964 through March 1994, with slope since 1970, and from January 1979 through March 1994. The analyses show continuous year-round ozone decline in middle latitudes (35 degrees-60 degrees) where the estimated values are -4.3 and -4.1% per decade for the period since January 1979 in the northern and southern hemispheres respectively. The northern hemisphere data permit estimation of the trends over vast regions such as North America -4.1+/-1.5%, Europe -5.1+/-2.0%, Siberia and Far East -5.6+/-1.8% (values and two sigma intervals are given in percent per decade for year-round trend). The ozone decline over the Arctic is about -5.6+/-2.0% per decade for year-round trend and -7.5+/-3.8% for the winter-spring season. The most dramatic ozone decline occurs in spring over Antarctica, -22+/-7.2% per decade. The trends estimated for the 1979-1994 period are about 1.5% per decade stronger in northern and southern middle and high latitudes than the trends with slope since 1970, which shows a substantial acceleration of the rates of ozone decline. The sensitivity of the ozone trends to the extremely low ozone during the 1992-1993 period is discussed. Most of the ozone data and calibrations in this paper were the base for the Ozone Trends section of the World Meteorological Organization/United Nations Environment Program Ozone Assessment 1994. The results presented here in general concur with the assessment and provide further estimates of the ozone changes in the northern middle latitudes in view of another extremely low winter-spring season of 1994-1995.