Cloud cover changes determine solar dimming and brightening

Gerald Stanhill, Ori Achiman, Rafael Rosa and Shabtai Cohen. The cause of solar dimming and brightening at the Earth’s surface during the last half century: evidence from measurements of sunshine duration. Journal of Geophysical Research: Atmospheres. Vol. 119 Issue 14 DOI: 10.1002/2013JD021308

Analysis of the Angstrom-Prescott relationship between normalized values of global radiation and sunshine duration measured during the last 50 years made at five sites with a wide range of climate and aerosol emissions showed few significant differences in atmospheric transmissivity under clear or cloud covered skies between years when global dimming occurred and years when global brightening was measured. Nor in most cases were there any significant change in the parameters or in their relationships to annual rates of fossil fuel combustion in the surrounding 1° cells. It is concluded that at the sites studied changes in cloud cover rather than anthropogenic aerosols emissions played the major role in determining solar dimming and brightening during the last half century and that there are reasons to suppose that these findings may have wider relevance.

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