Scattering aerosols such as sulfate cool the climate, while absorbing aerosols such as black carbon have a warming effect. Little is known about global trends of aerosol properties or composition. Aerosol single scattering albedo (SSA) retrievals from AERONET sun photometer measurements for the period from 1998 to 2010 at four wavelengths reveal that SSA decreased over North America, South America and Pacific Islands, and increased slightly over Europe and East Asia. For most stations, the trend of SSA decrease with wavelength is spectrally flat, pointing to changes in the fraction of black carbon. A decrease in SSA tends to shift aerosol forcing from negative to positive. It is also very likely that the AERONET observed trends in SSA will affect aerosol trend estimates from satellite retrievals.