Is “Global Dimming” Really Global?

The answer according to a new paper is NO.

This paper provides further evidence as to why we need to focus on local and regional scales if we are to better understand climate science. The new paper is

Alpert, P., P. Kishcha, Y. J. Kaufman and R. Schwarzbard, “Global Dimming or Local Dimming? — Effect of Urbanization on Sunlight Availability” Geophysical Research Letters,32, L17802, doi:10.1029/2005GL023320. 2005.

The abstract of the paper is

From the 1950s to the 1980s, a significant decrease of surface solar radiation has been observed at different locations throughout the world. Here we show that this phenomenon, widely termed global dimming, is dominated by the large urban sites. The global-scale analysis of year-to-year variations of solar radiation fluxes shows a decline of 0.41 W/m2/yr for highly populated sites compared to only 0.16 W/m2/yr for sparsely populated sites (<0.1 million). Since most of the globe has sparse population, this suggests that solar dimming is of local or regional nature. The dimming is sharpest for the sites at 10°N to 40°N with great industrial activity. In the equatorial regions even the opposite trend to dimming is observed for sparsely populated sites.

This paper illustrates the importance of spatially heterogeneous diabatic heating, as was discussed, for example, on our weblog of July 28th entitled “What is the Importance to Climate of Heterogeneous Spatial Trends in Tropospheric Temperatures?â€?, and recommended in the 2005 National Research Council report “Radiative forcing of climate change: Expanding the concept and addressing uncertainties”.

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