Total ozone trends
Long term evolution of total ozone
Since the start of the observations in 1971 there have been different periods of stratospheric load with ozone depleting species. During the first years there were almost no man-made ozone depleting molecules in the stratosphere. From about 1980 onwards the concentration gradually increased, to peak in the second half of the 1990's (1997). Now concentrations have stabilized and show a decrease due the ban of CFC's (chlorofluorocarbons) and halons containing Bromine in the Montreal protocol, signed in 1987. Therefore it is common to use the data before 1980 as a kind of reference before anthropogenic influence. To see whether there is a difference in the ozone changes, trends are calculated for two periods (before and after 1997) in the next figure.
Evolution of the ozone column at Uccle as observed with Dobson 40 (1972-1989), Brewer 16 (1990-2001), and Brewer 178 (2002-present). The times of major volcanic erutions affecting the ozone layer are indicated on the time axis.
The trends are -0.25 % per year and +0.19 % per year for the periods 1980-1997 and 1997-2016, respectively. Although this can be interpreted as a first sign of the recovery of the ozone layer, it should however be noted that the last period is too short for final conclusions about the recovery of the ozone layer (note the large year to year variability during the last decade). Information about the state of the ozone layer from RMIB is also available in Dutch in environmental reports of the "Vlaamse Milieumaatschappij". The section on the ozone layer depletion in this report can be found here. More detailed information (in Dutch) is available in the background document.
Trends as function of season
To calculate the total ozone trends as a function of season, the total ozone column time series has been split up into monthly mean ozone values in Dobson Units (DU). Then a least square linear regression is applied to these time series. During a time period of 45 years, there is only a significant negative total ozone column trend present in the data during May and June as shown in the figure below.
Seasonal ozone trends at Uccle in percent per year derived from the ozone observations with Dobson nr. 40 (1971-1989), Brewer nr. 16 (1990-2001), and Brewer nr. 178 (2002-present) between 1971 and 2016. The vertical bars represent the 95 % confidence level of the calculated trends.