Trends in aerosol radiative effects over Europe inferred from observed cloud cover, solar dimming and solar brightening

[ 1] We examine multidecadal changes in surface downward shortwave ( SW) radiation flux, total cloud cover, SW cloud effect, and related parameters over Europe during 1965 2004 using monthly gridded data from the Global Energy Balance Archive (GEBA), synoptic cloud reports, and the International Satellite Cloud Climatology Project (ISCCP). One key issue is distinguishing the effects of natural cloud variability from long-term anthropogenic aerosol influences on surface SW flux. Accordingly, we introduce the concept of cloud cover radiative effect (CCRE), defined as the change in downward SW flux produced by a change in cloud cover. The correlation between pan-European time series of CCRE anomalies and GEBA solar radiation anomalies is 0.88, indicating that cloud cover variability and associated changes in cloud albedo dominate SW radiation variability on monthly to decadal timescales. After these weather-related cloud effects are removed by subtracting CCRE anomalies from GEBA solar radiation anomalies via linear regression, a distinct decreasing trend followed by a distinct increasing trend remain in the residual time series. Depending on the method of trend calculation, pan-European residual flux declined by a statistically significant 2.7 - 3.5 W m(-2) per decade during 1971 - 1986 and rose by a statistically significant 2.0 - 2.3 W m(-2) per decade during 1987 - 2002. The fact that independent grid boxes exhibit mostly negative trends in the earlier period and mostly positive trends in the later period demonstrates that these long-term variations in SW flux are real and widespread over Europe. Changes in cloud cover cannot account for the trends in surface SW flux since cloud cover actually slightly decreased during 1971 - 1986 and slightly increased during 1987 - 2002. The most likely explanation is changes in anthropogenic aerosol emissions that led to more scattering and absorption of SW radiation during the earlier period of solar dimming'' and less scattering and absorption during the later period of solar brightening.''