Visibility on clear days has declined in much of the world since the 1970s thanks to a rise in airborne pollutants, according to scientists . They described a “global dimming” in particular over south and east Asia, South America, Australia and Africa, while visibility remained relatively stable over North America and improved over Europe, the researchers said. Aerosols, tiny particles or liquid droplets belched into the air by the burning of fossil fuels and other sources, are responsible for the dimming, the researchers said. “Aerosols are going up over a lot of the world, especially Asia,” Robert Dickinson of the University of Texas, one of the researchers, said in a telephone interview. Dickinson and two University of Maryland researchers tracked measurements of visibility — the distance someone can see on clear days — taken from 1973 to 2007 at 3,250 meteorological stations worldwide. Aerosols like soot, dust and sulfur dioxide particles all harmed visibility, they said in the journal Science. The researchers used recent satellite data to confirm that the visibility measurements from the meteorological stations were a good indicator of aerosol concentrations in the air. The aerosols from burning coal, industrial processes and the burning of tropical forests can influence the climate and be a detriment to health, the researchers said. Other pollutants such as carbon dioxide and other so-called greenhouse gases are transparent and do not affect visibility. The data will help researchers understand long-term changes in air pollution and how these are associated with climate change, said Kaicun Wang of the University of Maryland. “This study provides basic information for future climate studies,” Wang said in a telephone interview. The scientists blamed increased industrial activity in places like China and India for some of the decreased visibility, while they said air quality regulations in Europe helped improve visibility there since the mid-1980s. The aerosols can have variable cooling and heating effects on surface temperatures, reflecting light back into space and reducing solar radiation at the Earth’s surface or absorbing solar radiation and heating the atmosphere, they added.
Global dimming is the gradual reduction in the amount of global direct irradiance at the Earth’s surface that was observed for several decades after the start of systematic measurements in the 1950s. The effect varies by location, but worldwide it has been estimated to be of the order of a 4% reduction over the three decades from 1960–1990. However, after discounting an anomaly caused by the eruption of Mount Pinatubo in 1991, a very slight reversal in the overall trend has been observed.
Global dimming has interfered with the hydrological cycle by reducing evaporation and may have reduced rainfall in some areas. Global dimming also creates a cooling effect that may have partially masked the effect of greenhouse gases on global warming.
Deliberate manipulation of this dimming effect is now being considered as a geoengineering technique to reduce the impact of climate change.