Dear EarthTalk: I've heard of global warming, of course, but what on Earth 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.
Whether or not to try to reduce global dimming in a fast-warming world is a conundrum. Most climate scientists believe global dimming is serving to counteract some of the warming effects brought on by increased carbon emissions. "The conventional thinking is that brown clouds have masked as much as 50 percent of global warming by greenhouse gases through so-called global dimming," reports Veerabhadran Ramanathan, an atmospheric chemist at California's Scripps Institution of Oceanography. He adds, however, that brown clouds have been known to amplify warming as a result of various environmental factors, especially in regions of southern and eastern Asia.
Some scientists have gone so far as to propose deliberate manipulation of the dimming effect to reduce the impact of global warming, in other words increasing particulate emissions. But Gavin Schmidt, an atmospheric scientist and one of the voices behind the RealClimate blog, argues that such a scheme would hardly provide a long term fix to our environmental excesses and ills and amount to a Faustian bargain, bringing with it "ever increasing monetary and health costs."
CONTACTS: Global Issues Blog, www.globalissues.org; Scripps Institution of Oceanography, www.sio.ucsd.edu; RealClimate Blog, www.realclimate.org.
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