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Two Italian homogenized data sets of sunshine duration (SD) and global radiation (E-g) relative anomalies are used to investigate to what extent these two variables agree with respect to their temporal evolution. They are compared for northern and southern Italy over the period 1959-2013. Both under all-sky and clear-sky conditions, the SD records tend to show a shorter and less intense decrease until the 1980s (global dimming) with respect to the E-g ones, while there is a better agreement in the subsequent period when both variables increase (brightening period). To investigate whether such behavior can be explained by a different sensitivity of SD and E-g to atmospheric turbidity variations, the observed clear-sky trends are compared to those estimated by a model based both on Lambert-Beer's law and on a simple estimation of diffuse radiation. Results show that most of the differences observed in the trends of the clear-sky SD and E-g records can be explained considering a realistic pattern of atmospheric turbidity in the 1959-2013 period. The only exception concerns winter and autumn in northern Italy where clear-sky SD does not decrease in the dimming period as much as it would be expected on the basis of the corresponding increase in atmospheric turbidity. One reason for this discrepancy could be the influence of other variables like relative humidity. This case study highlights that changes in atmospheric turbidity have to be kept in mind when SD is used to investigate the multidecadal evolution of E-g.

Plain Language Summary Two homogenized datasets of sunshine duration and global radiation relative anomalies over Italy are used to investigate to what extent these two variables agree under all-sky and clear-sky conditions in terms of their decadal variations. They are compared for northern and southern Italy over the 1959-2013 common period. Global radiation shows a decreasing tendency until the mid-1980s and an increasing tendency in the following period both under all-sky and clear-sky conditions. Moreover, it shows stronger tendencies than those observed for sunshine duration. So a model has been applied in order to verify if these differences can be explained with a different sensitivity to atmospheric turbidity changes. The results show that most of the differences observed in the trends of the clear-sky sunshine duration and global radiation records can be explained considering the patterns of atmospheric turbidity in the 1959-2013 period. The only exception concerns winter and autumn in northern Italy. The reasons of this discrepancy could be due both to instrumental problems and to the influence of other variables.