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Written by Miranda Huey   
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Tuesday, 09 December 2008

Global Dimming: Another Pollution Side-Effect

Everyone's heard about global warming, but relatively few people have heard about global dimming. What is global dimming? Substantial parts of the world are actually dimmer, i.e., less light, darker, than they were several decades ago due to air pollution.1 The United States, for example, is about 10% dimmer, while parts of the former Soviet Union are actually about 30% dimmer. Even Antarctica has dimmed 9%.2 The culprit is two-fold. Burning coal, oil, and wood creates tiny visible particles, called aerosols, which seep into the atmosphere. Firstly, these reflect sunlight directly back into space. Secondly, polluted clouds contain more droplets than unpolluted clouds, leaving the earth with denser clouds which block off more sunlight.3 Because these pollutants tend to diffuse widely in the world's atmosphere, local pollution has become global dimming.4

One scientist, David Travis, verified the phenomenon in the days after September 11, 2001. Because of the terrorist attack, all airplanes in the U.S. were grounded for three days. Airplanes normally leave contrails, which can cover over half of the sky in certain areas. Travis observed that, after those three days, there were warmer days and cooler nights, exactly what one would expect without the insulating effect of aerosols. The temperature range increased over 1 degree Celsius, making it the largest temperature swing in the last thirty years. If this is the effect over the course of removing only one source of pollutant over the course of three days, global dimming's effect on the world could be greater than anyone ever realized.5

Global dimming may have already had devastating consequences. One of the first such consequences of global dimming may have been the 1984 Ethiopian famine. Before the 1970's, the Sahel had always had summer monsoons provide most of the water in the region. The sun would heat northern oceans, and a tropical rain belt would shift north to the Sahel. Starting in the 70's, the monsoons suddenly disappeared. A model which takes into account global dimming concludes that polluted clouds reflected enough sunshine back into space that the oceans were cooled, preventing the tropical rain belt from moving northward. As a result, 50 million people were affected. Now scientists are starting to fear that the same thing will happen in Asia, where billions of people's livelihoods depend on rainfall.6

On the other hand, for the past decade, scientists have also found evidence of the opposite phenomenon, global brightening. This happens when industrialized nations cut down their smog pollution, reducing aerosol emission and solar dimming. Unfortunately, while this alleviates the effects of global dimming, it accelerates global warming.7

However, global dimming is no solution to global warming. Firstly, the root causes of both global warming and global dimming are the same, burning coal, oil, and gas, causing them to happen simultaneously.8 Moreover, in spite of global dimming, the Earth's surface is still getting warmer.9 Additionally, carbon dioxide stays in the atmosphere for at least a century, while aerosols dissipate within a few days. Any global dimming solution to global warming would require exponentially more pollution to counteract the previous effects of greenhouse gases.10

Perhaps worst of all, global dimming is actually masking the true effects of global warming, which means that climate change is actually going to be much more drastic than reports previously predicted. While global warming has so far only seen a 0.6 degree rise, they predict that, if patterns keep going the way they have been, temperatures might just rise 10 degrees by 2100, making many parts of the world uninhabitable.11 Fortunately, if we curb our greenhouse gas emissions now, we may be able to curb the effects of both global dimming and global warming.

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1 http://news.bbc.co.uk/2/hi/science/nature/4171591.stm.
2 http://www.informationclearinghouse.info/article15809.htm
3 http://news.bbc.co.uk/2/hi/science/nature/4171591.stm.
4 http://www.informationclearinghouse.info/article15809.htm.
5 Id.
6 Id.
7 http://www.newscientist.com/article/dn7346-clearing-smog-has-led-to-global-brightening.html
8 http://www.informationclearinghouse.info/article15809.htm
9 http://www.scienceagogo.com/news/20040420233456data_trunc_sys.shtml.
10 http://www.newscientist.com/article/dn7346-clearing-smog-has-led-to-global-brightening.html
11http://news.bbc.co.uk/2/hi/science/nature/4171591.stm.




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