Possibilities to detect trends in spectral UV irradiance

It is investigated how long-term UV trends can be assessed by analysing the longest time series of measured spectral UV irradiance in Europe, which have been started in the early 1990s in Thessaloniki, Greece and Sodankyld, Finland. It can be concluded that both time series do not yet show an unambiguous yearly trend in UV irradiance. The regression lines show no uniform behaviour and vary irregularly in strength and from one solar zenith angle to the next if all sky conditions are analysed. It is emphasised that these findings do not disagree with previous studies, that significant changes in UV irradiance have been observed over Europe especially in spring. Our study introduces a new method to estimate the required time series length for trend detection using the measured time series in combination with model calculations. At Sodankyld, a reduction of the total ozone column of -5.7% per decade has been observed from 1979 to 1998. A positive UV trend due to such conditions may be detected after 12 years at the earliest. For Thessalonki, a decrease in total ozone of -4.5% per decade has been observed. A corresponding increase of UV irradiance should be detectable after 15 years. It should be noted that a constant ozone trend over the whole period had to be assumed for this analysis. Since 1990 there has been a considerable variability of total ozone, but no steady decrease could be observed. Consequently, no general UV increase could be expected due to ozone changes. Even if there was a constant ozone trend over that period it is shown that even the longest European time series of UV irradiance are still too short to show distinct trends. However, this does not imply that no changes have occurred, it only shows that the large natural variability of UV irradiance has so far hindered the identification of unambiguous trends. The only way to find significant and consistent UV trends is the continuation of high-quality long-term measurements of spectral UV irradiance.