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Total ozone trends
Below the long term evolution of ozone is illustrated by a graph of the running yearly mean values of total ozone and one of the monthly trends of ozone.
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 the 1990's. Now concentrations have stabilized are show a small decrease due the the ban of CFC's in the montreal protocol. 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 in the next figure.
Evolution of the ozone column at Uccle as observed with Dobson 40 and Brewer 16. The times of major volcanic erutions affecting the ozone layer are indicated on the time axis.
The trends are -0.3 % per year and +0.4 % per year for the periods 1981-1996 and 1997-2007, 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 (the period is shorter than one solar cycle of 11 years). Information about the state of the ozone layer from RMIB is also available in Dutch in environmental reports of the "Vlaamse Milieumaatschappij". This report can be downloaded from the website of VMM in the chapter on ozone depletion. More detailed information (in Dutch) is available in the background document.
Trends as funtion 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 33 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 between 1971 and 2004. The vertical bars represent the 95 % confidence level of the calculated trends.