Urban activity reduces the intensity of solar radiation

The researchers Prof. Pinchas Alpert and Dr. Pavel Kishcha from the Department of Geophysics and Planetary Sciences at Tel Aviv University found that over a period of 25 years (1964-1989), the dimming of the sunlight was a local phenomenon originating from human activity

A new study at Tel Aviv University recently published in GEOPHYSICAL RESEARCH LETTERS (VOL. 35, L08801, doi:10.1029/2007GL033012, 2008) reveals that urban activity reduces the intensity of solar radiation.

The researchers, Prof. Pinchas Alpert and Dr. Pavel Kischa from the Department of Geophysics and Planetary Sciences at Tel Aviv University found that over a period of 25 years (1964-1989), the dimming of sunlight was a local phenomenon originating from human activity. In particular, it was found that the dimming was observed only in limited areas (~30%) of the earth's surface, and was limited to densely populated areas of over 10 people per square meter.

In a previous work (Alpert et al., Geophys. Res. Lett., 2005) the researchers showed that the dimming of sunlight between the years 1964 and 1989 was felt mainly in urban areas with a population greater than 100,000 people. In the present work, the effect of urbanization on the amount of sunlight was investigated by finding the quantitative effect of the effect of the growing population on the dimming of sunlight, referring to all radiation measurements found in the world - 317 measurement points, including sparsely populated areas.

Figure 1 shows changes from year to year (1964-1989) of the average annual radiation fluxes for each year in selected groups according to population density, of different regions of the world. The colored lines represent the linear trend for each group. What emerges from Figure 1 is that the dimming of sunlight is mainly affected by anthropogenic particle emissions: the decrease in solar radiation reaching the surface increases up to 0.32 W/m2/yr, when the population density increases from 10 to 200 persons per square meter.

In areas where the population density is greater than 200 persons per square meter, a saturation effect is observed: the trend of decreasing radiation is felt much less than in areas with a smaller population density. The concentration of particles in mega-cities is much higher than in remote areas. Hence, in the huge cities the direct and indirect effects of the anthropogenic particles on the reduction of solar radiation are much stronger. And this leads, apparently, to saturation in the effects of the particles on the clouds.

Annual average solar radiation intensity (watts per square meter) During the study period, urban areas received less solar radiation compared to remote areas by an amount of ~12 W/m2 which is equivalent to an 8% decrease in radiation.
Year

Figure 1: Changes from year to year (1964-1989) of the average annual radiation fluxes for each year in selected groups of different regions of the world according to population density as follows. The markings of black stars are for very sparsely populated areas with a density of less than 10 persons per square meter, blue circles - 100 >density ≥10 persons per square meter, green triangles 200 >density ≥100 persons per square meter, red triangles 400 >Density≥200 people per square meter and purple triangles - density≥400 people per square meter. The colored lines represent the linear trend. It can be seen that at the end of the period (1989) in densely populated areas (red line) the solar radiation is less by about 10-15 watts per square meter. A decrease of about 8-10% compared to the average power of about 150 watts per square meter.

The research was carried out by Prof. Pinchas Alpert and Dr. Pavel Kischa from the Department of Geophysics and Planetary Sciences, Tel Aviv University and was published in: Geophysical Research Letters (accepted, March 3, 2008)

on the researchers

Prof. Pinchas Alpert: Head of the Department of Geophysics and Planetary Sciences, in the Faculty of Exact Sciences, at Tel Aviv University. He develops the innovative method of separating factors in numerical models (Stein and Alpert, JAS 1993). This method has been adopted by many groups in the world. Prof. Alpert is a co-author of more than 150 peer-reviewed papers, mostly in the field of mesoscale atmospheric dynamics and climate. His research focuses on climate, atmospheric dynamics, numerical methods, models in defined areas and climatic changes. The basic atmospheric model built by him in his PhD thesis in 1980 has been extended and is in use in Belgium (LLN) and Finland (UH). Prof. Alpert founded and heads Israel's National Center for Earth Data in collaboration with NASA.

Dr. Pavel Kischa: Department of Geophysics and Planetary Sciences, Tel Aviv University. Dr. Kischa has extensive experience in the field of atmospheric particle modeling and prediction. He is a co-author of over 30 peer-reviewed articles, mainly in aspects of modeling and forecasting.