Regional Changes

The areal extent of the changes in global radiation and their global impact has been the subject of much debate and some investigation. Significant rates of dimming and brightening have been observed at many sites remote from major sources of air pollution, for example the polar regions [35,36], and the largest trends have been observed in heavily polluted regions (e.g. Hong Kong [39], India [49] and China [50]), suggesting a significant relationship between pollution rates and global radiation trends. Alpert et al. [51] found that dimming from the 1950s to the 1980s averaged 0.41 W m 2 a 2 for highly populated sites while for sparsely populated sites, that is, populations <0.1 x 106 dimming was only 0.16 Wm 2 a 2. In equatorial locations with low population density there were slightly increasing trends. Since most of the globe is sparsely populated this implies that the spatially averaged changes in Eg# are significant, but smaller than those obtained by averaging the data, which may be biased toward population centres. However, to date no model has been developed to integrate population density and its influence on Eg# with the worldwide grid of Eg# in order to update the estimates of dimming and brightening, and current estimates revolve around those given in Table 2. Trends observed from satellites are for wide regions ([47]; Table 2) and it is encouraging that those trends are similar to those computed by averaging data from surface stations.

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