Aerosol emissions and dimming/brightening in Europe: Sensitivity studies with ECHAM5-HAM

Observational data indicate a decrease of surface solar radiation (SSR) in Europe from about 1950 to the mid-1980s, followed by a renewed increase. Changing aerosol emissions have been suggested as a likely cause for this observed dimming and brightening. To quantify this hypothesis, we performed ensembles of transient sensitivity experiments with the global climate model ECHAM5-HAM, which includes interactive treatment of aerosols. The simulations cover the period 1950-2005 and use transient aerosol emissions (National Institute of Environmental Science, Japan) and prescribed sea surface temperatures (SSTs) from the Hadley Centre. The simulated clear-sky dimming and brightening can be attributed to changing aerosol emissions from fossil fuel combustion. Ensemble means of modeled SSR trends are in agreement with observed values. Dimming ceases too early in the model, around 1970. Potential causes are discussed. Brightening sets in at about the right time. Regional differences of modeled SSR are substantial, with clear-sky dimming trends ranging from -6.1 (eastern Europe) to -0.4 W m(-2) decade(-1) (British Isles) and brightening trends ranging from + 1.3 to +6.3 W m(-2) decade(-1) (Scandinavia and eastern Europe). All-sky conditions show similar trends in the ensemble mean, but the spread among ensemble members is considerable, emphasizing the importance of clouds. Surface temperatures are found to depend mostly on the prescribed SSTs, with an additional aerosol component in some regions like eastern Europe. For precipitation, internal variability is too large to allow for any firm conclusions.