Co-Authors:
Stanhill, G., Institute of Soil, Water and Environmental Sciences, ARO, Volcani Center, Bet Dagan 50250, Israel
Cohen, S., Institute of Soil, Water and Environmental Sciences, ARO, Volcani Center, Bet Dagan 50250, Israel
Abstract:
A number of studies show that significant reductions in solar radiation reaching the Earth's surface have occurred during the past 50 years. This review analyzes the most accurate measurements, those made with thermopile pyranometers, and concludes that the reduction has globally averaged 0.51 ± 0.05 W m-2 per year, equivalent to a reduction of 2.7% per decade, and now totals 20 W m-2, seven times the errors of measurement. Possible causes of the reductions are considered. Based on current knowledge, the most probable is that increases in man made aerosols and other air pollutants have changed the optical properties of the atmosphere, in particular those of clouds. The effects of the observed solar radiation reductions on plant processes and agricultural productivity are reviewed. While model studies indicate that reductions in productivity and transpiration will be proportional to those in radiation this conclusion is not supported by some of the experimental evidence. This suggests a lesser sensitivity, especially in high-radiation, arid climates, due to the shade tolerance of many crops and anticipated reductions in water stress. Finally the steps needed to strengthen the evidence for global dimming, elucidate its causes and determine its agricultural consequences are outlined. © 2001 Elsevier Science B.V.
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