Direct aerosol effects during periods of solar dimming and brightening hidden in the regression residuals: Evidence from Potsdam measurements

Abstract

A recent empirical study of Stanhill et al. (2014), which was based on the Angstrom-Prescott relationship between global radiation and sunshine duration, was evaluated. The parameters of this relationship seemed to be rather stable across the dimming and brightening periods. Thus, the authors concluded that the variation in global radiation is more influenced by changes in cloud cover and sunshine duration than by the direct aerosol effects. In our study, done for the Potsdam station (one of six globally distributed stations, the source of one of the longest observational records and closely located to former hot spots of aerosol emission), we tested and rejected the hypothesis that the dimming of global radiation directly caused by aerosols is negligible. The residuals of the Angstrom-Prescott regression reveal a statistically significant positive temporal trend and a temporal level segmentation. The latter was consistent with the temporal emission patterns around Potsdam. The trend in the residuals only disappeared when the model intercept varied according to the temporal level segmentation. The magnitude of the direct aerosol effect on the level changes in global radiation derived from the modified Angstrom-Prescott relationship was in the range indicated in previous studies. Thus, from here, a specific request cannot be made for a revision of current climate models state-of-the-art representation of both the cooling effect directly caused by aerosols and the temperature sensitivity to the increase of greenhouse gases.