EarthTalk: What is Global Dimming?
Global dimming is a less well-known but real phenomenon resulting from atmospheric pollution. The burning of fossil fuels by industry and internal combustion engines, in addition to releasing the carbon dioxide that collects and traps the sun's heat within our atmosphere, causes the emission of so-called particulate pollution—composed primarily of sulphur dioxide, soot and ash. When these particulates enter the atmosphere they absorb solar energy and reflect sunlight otherwise bound for the Earth's surface back into space.
Particulate pollution also changes the properties of clouds—so-called "brown clouds" are more reflective and produce less rainfall than their more pristine counterparts. The reduction in heat reaching the Earth's surface as a result of both of these processes is what researchers have dubbed global dimming.
"At first, it sounds like an ironic savior to climate change problems," reports Anup Shah of the website GlobalIssues.org. "However, it is believed that global dimming caused the droughts in Ethiopia in the 1970s and 80s where millions died, because the northern hemisphere oceans were not warm enough to allow rain formation." He adds that global dimming is also hiding the true power of global warming: "By cleaning up global dimming-causing pollutants without tackling greenhouse gas emissions, rapid warming has been observed, and various human health and ecological disasters have resulted, as witnessed during the European heat wave in 2003, which saw thousands of people die."
Just how big an issue is global dimming? Columbia University climatologist Beate Liepert notes a reduction by some four percent of the amount of solar radiation reaching the Earth's surface between 1961 and 1990, a time when particulate emissions began to skyrocket around the world. But a 2007 study by the National Aeronautics and Space Administration (NASA) found an overall reversal of global dimming since 1990, probably due to stricter pollution standards adopted by the U.S. and Europe around that time.