Solar radiation (sunlight) at the earth surface is a key determinant of climate and the primary energy source for life on our planet. Variations in this quantity therefore profoundly affect the human and terrestrial environment. Worldwide measurements of surface solar radiation began in the 1960s and are collected in the Global Energy Balance Archive (GEBA) at ETH Zurich. Several studies found a widespread decrease of sunlight over land surfaces based on the data in GEBA covering the period 1960 to 1990, on the order of 4%-6%. This phenomenon has become popular under the expression global dimming and evoked considerable media response.
However, all these studies were restricted to data prior to 1990, since observations after 1990 were not easily accessible. Within the framework of NCCR Project 2.1 (global climate processes) we are undertaking a major effort to update GEBA for the period from 1990 to present. In addition, from 1992 onward, a new worldwide network of high quality radiation measurements has been established with its data Centre at ETH Zurich (Baseline Surface Radiation Network, BSRN).  
In a recent study, we evaluated these newly available surface observational records to investigate the trends in solar radiation in the more recent years (Wild et al. 2005). We found that global dimming is no longer evident in the 1990s. Instead, a brightening is observed since the late 1980s. The trend reversal is in line with  trends in Earth reflectance and cloudiness. In addition, the brightening is also found under cloud-free skies, pointing to a decrease in aerosol loadings during the 1990s. This is confirmed by long term measurements of atmospheric transparency, which indicate that the atmosphere has recently become cleaner in many locations, after decades of increasing pollution (Fig 2). This was favoured by the breakdown of the communist system in Russia and Eastern Europe, leading to strongly reduced pollution, as well as generally more effective air quality measures.
The transition towards more sunlight at the earth’s surface since the late 1980s after decades of decline may significantly affected surface climate. From the 1960s to the 1980s, the dimming may have been large enough to counterbalance the greenhouse-induced increase in downward longwave radiation, so that the available energy at the surface was rather decreasing than increasing (Wild et al. 2004). This is in line with long-term records of  worldwide pan evaporation measurements, which show a decrease of (potential) evaporation over the same period.
Since the mid 1980s, the solar dimming was no longer there to mask the increasing greenhouse effect, so that the greenhouse effect became fully apparent  during the 1990s. This is reflected, e.g., in much stronger temperature increase during the 1990s compared to earlier decades, as well as a substantial retreat of mountain glaciers, which was suppressed during the prior decades of dimming.

Ref.
Wild, M., A. Ohmura, H. Gilgen, Geophys. Res. Lett. 31, 2003GL091188 (2004).
Wild, M. et al. 2005: From dimming to brightening: Decadal changes in solar radiation at the Earth’s surface. Science, 308, 847-850.