Total solar radiation reaching the surface of the earth depends on sunlight reaching the top of the atmosphere and the transparency of the atmosphere. Recent research has indicated solar radiation reaching the surface of the earth has decreased globally by 4% - 6% during the period of 1960 -1990 (Wild et al., 2005) and 10% over the past 50 years (Stanhill and Cohen, 2001). This phenomenon has been termed "global dimming." However, since about 1990, this trend at the global scale has reversed and "global brightening" has been measured at a rate averaging 0.10% per year (Pinker et al., 2005; Wild et al., 2005; Norris and Wild, 2007). These multi-year trends suggest that the total radiation reaching the earth's surface, as affected by changes in atmospheric transparency, can fluctuate over periods of 20 - 50 years (Roderick, 2006).

Sunlight reaching the top of the atmosphere is affected by solar output and earth orientation, while cloudiness and both natural and anthropogenic sources of particulate and aerosol material affect atmospheric transparency. Declines in atmospheric transparency have been linked to volcanic activity (Roderick et al., 2001; Lohmann et al., 2006), increased cloud cover (Cutforth and Judiesch, 2007) and air pollution (Jauregui and Luyando, 1999; Alpert et al., 2005; Ramana and Ramanathan, 2006), while decreased cloudiness related to global warming (Nazarenko and Menon, 2005) and improved air quality (Schiermeier, 2005; Black et al., 2006; Norris and Wild, 2007) have been touted as increasing atmospheric transparency. Potential ecological effects of changes in solar radiation reaching the surface of the earth include changes in plant productivity (Roderick et al., 2001; Stanhill and Cohen, 2001; Black et al., 2006) and alteration of the hydrological cycle (Cohen et al., 2002; Roderick and Farquhar, 2002; Liepert and Romanou, 2005; Brutsaert, 2006; Robock and Li, 2006; Li et al., 2007).

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