How do Aerosol Histories Affect Solar Dimming and Brightening Over Europe? - IPCC- AR4 Models vs. Observations

A multidecadal decrease in downward surface solar radiation (solar dimming) followed by a multidecadal increase in surface radiation (solar brightening) have been reported over Europe. Aerosol emissions and aerosol optical depth measurements show concurrent trends of opposite sign. Because solar irradiance trends mainly occur under cloud-free skies they are primarily caused by the direct radiative effect of aerosols. We now compare observed cloud-free solar dimming and brightening trends with corresponding output from IPCC-AR4 20th century simulations and furthermore examine how sulfate and black carbon aerosol histories, used as model input, affect simulated surface radiation trends. Outputs from 14 models are compared to observed cloud-free surface radiation fluxes derived from a combination of a) satellite cloud observations, synoptic cloud reports, and surface solar irradiance measurements, and b) sunshine duration measurements and variability of the atmospheric transmittance derived from solar irradiance measurements. Most models display a transition from decreasing to increasing solar irradiance, but the timing of the reversal varies by about 25 years. Consequently, large discrepancies in sign and magnitude occur between modeled and observed dimming and brightening trends (up to 4.5 Wm-2 per decade between models and observations for Europe). Considering all models with identical aerosol histories, differences in cloud-free radiation trends are less than 0.7 Wm-2 per decade, except for one outlier with a difference of 2.5 Wm-2 per decade. Thirteen of the fourteen models produce a transition from dimming to brightening that is consistent with the timing of the reversal from increasing to decreasing aerosol emissions in the input aerosol history. Consequently, the poor agreement between modeled and observed dimming and brightening over Europe is due to incorrect aerosol emission histories rather than other factors. Observed multidecadal variations in downward solar irradiance at the surface can provide a useful constraint on the time history and on regional patterns of past aerosol forcing.