Background
Several researchers have reported a widespread decrease in the amount of solar radiation received at the earth's surface during the second half of the 20th century, a phenomenon that has come to be known as solar dimming (Gilgen et al., 1998; Liepert, 2002; Stanhill and Cohen, 2001; Cohen et al., 2004).  These findings have long intrigued us, and we have written previously on the subject, most recently in an editorial in which we note that the since the late 1980s solar dimming may have reversed direction to become solar brightening (Pinker et al., 2005; Wild et al., 2005).  In the present study, we report on the work of Alpert et al., who hypothesized that the dimming of the 1950s through 1980s could be explained by the anthropogenic release of pollutants, such as sulfates, nitrates and black carbon, which acted to reduce the amount of solar radiation received at the earth's surface.

What was done
For the 25-year period 1964-1989, the authors examined trends in the surface receipt of solar radiation at 144 urban sites (population greater than 100,000 persons) and 174 rural sites (population less than 100,000 persons) for various latitudinal bands as well as for the entire globe.

What was learned
For the globe as a whole, both urban and rural locations showed a decline in surface solar radiation over the period of study; however, the rate of decline at the urban locations was approximately 2.6 times larger than that observed at the rural locations (-0.41 Wm^-2yr^-1 vs. -0.16 Wm^-2yr^-1).  In addition, a sharper decline in surface solar radiation receipt was found between 10°N and 40°N, which latitudinal zone is nearly coincident with the zone of maximum fossil fuel emissions in the Northern Hemisphere reported by Stanhill and Cohen (2001) for the period 1960-1990.  On the other hand, the latitude band from 15°S to 15°N displayed the opposite trend.  Instead of dimming, this region experienced a brightening of 0.58 Wm^-2yr^-1, which is consistent with the findings of Pinker et al. (2005), who reported a persistent increase in surface solar radiation over the latitudes 20°S to 20°N based on satellite data.

What it means
According to Alpert et al., their findings suggest that solar dimming is "significantly dominated by large cities' contributions to the atmospheric pollution," and that it is "essentially a local phenomenon, observed only in a limited part of the total land area."  In addition, we note that the magnitudes of the original dimming and the more recent brightening are much greater than the magnitude of the concomitant increase in total greenhouse-gas-induced radiative forcing, so much so, in fact, as to make the latter phenomenon one of relatively minor significance compared to the dimming and brightening phenomena, particularly in the places where they are most strongly expressed.  Hence, there is yet another good reason to suggest that the global instrumental temperature record may be significantly "polluted," and in a way that has heretofore not been adequately appreciated.

References
Cohen, S., Liepert, B. and Stanhill, G.  2004.  Global dimming comes of age.  EOS: Transactions, American Geophysical Union 38: 362-363

Gilgen, H.M., Wild, H.M. and Ohmura, A.  1998.  Means and trends of shortwave irradiance at the surface estimated from global energy balance archive data.  Journal of Climate 11: 2042-2061.

Liepert, B.  2002.  Observed reductions of surface solar radiation at sites in the United States and worldwide from 1961 to 1990.  Geophysical Research Letters 29: 1421, doi:10.1029/2002GL014910.

Pinker, R.T., Zhang, B. and Dutton, E.G.  2005.  Do satellites detect trends in surface solar radiation?  Science 308: 850-854.

Stanhill, G. and Cohen, S.  2001.  Global dimming: a review of the evidence for a widespread and significant reduction in global radiation with discussion of its probable causes and possible agricultural consequences.  Agricultural and Forest Meteorology 107: 255-278.

Wild, M., Gilgen, H., Roesch, A., Ohmura, A., Long, C.N., Dutton, E.G., Forgan, B., Kallis, A., Russak, V. and Tsvetkov, A.  2005.  From dimming to brightening: Decadal changes in solar radiation at earth's surface.  Science 308: 847-850.