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Global Dimming
Jamais Cascio, 19 Dec 03

This is not so much a resource as a mystery. Why has the amount of sunlight reaching the ground declined by about .25% per year since 1958? Not the amount of light emitted by the Sun, but the amount actually getting to the lower parts of Earth's atmosphere. Is it pollution, or a complex feedback effect associated with climate change?

Global Dimming may be one of the biggest bits of climate news to show up in quite awhile, and up until quite recently, it's been largely ignored. The notion that the average amount of light hitting the ground has been falling -- and by such a great amount, more than 10% in about three decades -- seemed so odd and unexpected that many climatologists simply couldn't accept it. But in the past year, experiments have proven it -- the skies are getting darker.

The discovery of Global Dimming will help to make climate change models more accurate, as well as solve some mysteries:

But Farquhar had realised that the idea of global dimming could explain one of the most puzzling mysteries of climate science. As the Earth warms, you would expect the rate at which water evaporates to increase. But in fact, study after study using metal pans filled with water has shown that the rate of evaporation has gone down in recent years. When Farquhar compared evaporation data with the global dimming records he got a perfect match. The reduced evaporation was down to less sunlight shining on the water surface.

The causes of the reduction in light remain uncertain. Most researchers think that it's a result of atmospheric pollutants triggering more persistent cloud formation. Others argue that Global Warming-caused evaporation could be leading to more clouds; in that scenario, a continued rise in global temperatures would lead to greater average reductions in light. This is a critical question -- if the Global Dimming has been caused by pollution, efforts to clean up the atmosphere may actually speed up Global Warming.

Right now, there are far more questions than answers about Global Dimming. It will almost certainly be a key area of study in the coming years... as well as a welcome reminder not to discount the unexpected.

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Comments

The global dimming that has been slowly growing for decades has been greatly exacerbated by the Wars in Iraq and Afghanistan. For those of you who recall the effect the eruption of Mt. St. Helens on our atmosphere, just try to imagine the effects of bombing runs for hundreds of days in a row. Yet another secret our government is trying to keep from us.


Posted by: Deborah on 18 Apr 04



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