Global dimming and urbanization: did stronger negative SSR trends collocate with regions of population growth?

Abstract

Abstract. Global dimming refers to the decrease in surface solar radiation (SSR) observed from the 1960s to the 1980s at different measurement sites all around the world. It is under debate whether anthropogenic aerosols emitted from urban areas close to the measurement sites are mainly responsible for the dimming. In order to assess this urbanization impact on SSR, we use spatially explicit population density data of 0.08° resolution to construct population indices (PI) at 157 high data quality sites. Our study extends previous population-based studies by incorporating distance-weighting as a simple aerosol diffusion model. We measured urbanization in the surrounding of a site as the PI change form 1960 to 1990 and found no negative correlation with the corresponding SSR trends from 1964 to 1989 for the 92 sites in Europe and Japan. For the 39 sites in China the correlation coefficients are significant at the 5 % level and reach around −0.35, while for the 26 remaining Asian, mostly Russian sites the correlation coefficients reach around −0.55 at the 1 % significance level. Results are similar, when the absolute levels of PIs are taken as an indicator for urbanization. Our findings call into question the existence of an urbanization effect for the sites in Europe and Japan, while such an effect cannot be ruled out for the sites in Asia, especially in Russia.

Urbanization effect on sunshine duration during global dimming and brightening periods in China

“…This effect saturates when urbanization reaches certain level (ULE > 50%/UP > 20 million persons/PD > 250 person km −2 ), above which a further increase in urbanization won't generate a further alignment of urban and rural dimming, as well as a further strengthening of these trends. Similarly, Imamovic et al (2015) questioned the existence of an urbanization effect on surface solar radiation in the well urbanized regions of Europe and Japan but suggested that such an effect cannot be ruled out in Russia and China. It is assumable that the government interference with regulations on environmental pollution might start first in relatively urbanized/developed regions.…”

“…To ensure that only urban and rural sites under similar climate conditions are compared, Wang et al (2014a) restricted the analysis to urban-rural station pairs within 2° × 2° areas and noted that the average urbanization effect on the mean and the trend of surface solar radiation over Europe, China, and Japan from 1961 to 1990 was small. Questioning the existence of an urbanization effect on surface solar radiation, Imamovic et al (2015) further examined the population indices (PI) at 157 worldwide sites for the 1960s-1980s and concluded that the urbanization effect based on PI is insignificant in Europe and Japan but cannot be ruled out in China and Russia. A similar conclusion was made in China by Wang et al (2012b) using diurnal temperature range (DTR) as a proxy for solar radiation, namely that urbanization effects exist not only during the global dimming period but also in the brightening period.…”

“…Population density (PD) is the most widely used index to quantify the impact of urbanization on solar radiation in recent studies (Alpert et al, 2005;Alpert and Kishcha, 2008;Stanhill and Cohen, 2009;Imamovic et al, 2015). Besides PD, this research introduces three other indices to represent the magnitude and process of urbanization: urbanization level (ULE), urbanization speed (USP) and urban population (UP).…”