How do aerosol histories affect solar “dimming” and “brightening” over Europe?: IPCC-AR4 models versus observations



[1] 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. The trends mainly occur under cloud-free skies, and they are primarily caused by the direct aerosol radiative effect. The present study compares observed cloud-free solar “dimming” and “brightening” trends with corresponding output from IPCC-AR4 20th century simulations and furthermore examines 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 (1) satellite cloud observations, synoptic cloud reports, and surface solar irradiance measurements and (2) 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 for Europe). Considering all models with identical aerosol histories, differences in cloud-free radiation trends are in all but one case less than 0.7 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” is due to incorrect aerosol emission histories rather than other factors.