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Abstract

We study long-term changes in surface radiative fluxes and their causes using surface radiation observations from the German Weather Service.Time series of solar radiation observations initiated at Potsdam in 1937 demonstrate the decrease in global radiation from the 1950s caused by an increase in the atmospheric aerosol load due to anthropogenic activities. This dimming period is followed by a brightening from the middle of the 1980s, i.e. an increase in global radiation caused by a decrease in the aerosol load as a consequence of the implementation of mechanism of air poullution reduction. The decrease in the Aerosol Optical Depth (AOD) is well documented by sunphotometer observations, which were initiated in the mid-eighties.While the aerosol load has stabilized at low levels after the turn of the century and thus the direct aerosol effect has decreased, the global radiation continues to increase. It is likely that other atmospheric phenomena such as cloudiness contribute to the recent increase in global radiation and hence in the incoming radiative energy as the incoming longwave radiation has been also increasing, mainly due to raising air temperature and greenhouse gas concentrations, which is in line with the output of GCM’s. We study changes in cloudiness using the concept of the cloud radiative effect (CRE). Preliminary results indicate that the magnitude of the CRE has decreased which implies a decrease in cloud cover, shift towards a different cloud type and/or changes in microphysical cloud properties.