Dimming/brightening in Athens: Trends in Sunshine Duration, Cloud Cover and Reference Evapotranspiration

Evidence of global dimming/brightening is mainly based upon few measurements of solar radiation. A need for more research supported and extended with the use of other climatic variables, such as sunshine duration, recorded for a longer time period and successfully used as a proxy for solar radiation over the past 80 years, has already been urged. Thus, in this study, residual sunshine duration series computed after removal of the cloudiness-related variability, from daily sunshine duration and cloudiness data measured at the National Observatory of Athens are used for highlighting global dimming or brightening periods in Athens, during the period 1951–2001. Furthermore, the consistency of trends in radiation records and their implications for the hydrological cycle and especially the trends in reference evapotranspiration are examined, during the period 1951–2001. The analysis focuses on the seasonal decadal variations, determines and explains the causes of the seasonal trends from the inter-annual and decadal variability of the sunshine duration during the last half past century, with a special emphasis in detecting possible sub-periods in Athens. The signs of trends in the modeled annual and seasonal reference evapotranspiration are estimated, according to the indicated global dimming/brightening periods.