ESTIMATED PAST AND FUTURE VARIABILITY IN UV-RADIATION IN SOUTH-AFRICABASED ON TRENDS IN TOTAL COLUMN OZONE

Recently released version 7 total column ozone data from the satellite -borne TOMS (Total Ozone Mapping Spectrometer) instruments have been u sed as input to a radiative transfer model to estimate the past variab ility in noon clear-sky erythemal UV radiation over the mid-latitude S outh African cities of Johannesburg, Bloemfontein, Durban, Cape Town a nd Port Elizabeth, from 1978 to 1994. A linear least squares regressio n model, based on measured geophysical time series, was then used to p arameterise the temporal variability in column ozone. By making assump tions regarding the repeatability of these basic time series, the regr ession model could then be used to estimate future ozone levels and, t hrough use of the radiative transfer model, future erythemal UV levels . The statistically insignificant trends in total column ozone derived from the version 7 TOMS data for these five cities result in only sma ll long-term increases in surface UV radiation. However the relative p hases of the various cyclical signals underlying the variability in to tal column ozone could result in summer erythemal UV levels being 1-2 mu W cm(-2) (2.5% to 7%) higher in 1998, 2000 and 2007 than in interme diate years, within the constraints of the ozone regression model. Fur thermore, differences in surface erythemal irradiance resulting from t he geographical separation between the five cities can be expected to exceed long-term trend effects in the near future.