Evidence of Dimming/Brightening Over Greece from Long-Term Observations of Sunshine Duration and Cloud Cover

Abstract

Changes in solar radiation reaching the earth and the transition from global dimming to global brightening during 1980s, has been regarded to reflect decadal changes in anthropogenic aerosol loading. Sunshine duration (SDU) has been proven a useful proxy when historical observations of solar radiation are missing. Long-term observations of SDU and total cloud cover (TCC) from 15 sites were used for the detection of fluctuations and dimming/brightening periods over Greece. A prominent spatial and temporal variation of SDU and TCC was found. As expected, SDU is negatively correlated with TCC, with years with positive SDU anomalies corresponding almost always to years with negative TCC anomalies. However, both variables exhibited trends of the same sign during certain periods, indicating a weakening of correlation in low frequencies. Almost all sites in the country experienced a decrease of the annual SDU until mid 1980s, and this is more pronounced in Western Greece. An ongoing increase of SDU since the mid 1980s is then observed, which at certain areas of Southeastern Greece amounts to +20 h/year. This increase is not accompanied with synchronous decrease of TCC, possibly pointing to the impact of aerosols loads on solar radiation and SDU fluctuations.