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Thursday, September 11, 2008

Blue skies, scorched Earth? What is Global Dimming?


With fierce storm after storm hitting the Caribbean and threatening our coasts this hurricane season seems to illuminate the ever more apparent effects of climate change. While we do not have more storms than usual the intensity appears to be on the rise, i.e. more storms are reaching higher categories thus our extremes are getting more extreme [Science]. This alone calls for our attention and efforts to make changes to decrease the greenhouse effect that is warming the Earth and causing greater highs and lows but there is more to this than meets the eye.

It is clearly necessary to work to decrease emissions that are contributing to climate change but what we may unknowingly be doing is exacerbating the problem by not fully understanding the complexity of the system. Now I am not saying we should allow massive amounts of pollution, I am merely suggesting it is more complicated than the simplified version we get watching TV or the glossy greenwashed sound bytes we are so often exposed to. Even this write up is overly simplified but such is life. The main complication I speak of is the phenomena that is termed “global dimming.” Just like it sounds, global dimming describes the decrease in sunlight that is actually reaching Earth’s surface.

Over the past 4 decades scientists have watched the global incoming solar radiation decrease by 12% [American Geophysical Union]. TWELVE PERCENT. That is huge! Breaking it down for each continent the drops in sunlight recorded between the 1950s and 1990s are staggering. The level dropped 9% in Antarctica, 10% over the U.S., nearly 30% over Russia, and up to 60% in parts of England. With that you would expect that we should be cooling down, right? It is logical to think if less sunlight reaches Earth the temperature would drop, just think of a cloudy day or even just standing in the shade. It makes a significant difference but we are not cooling down, we continue to get warmer.

With this in mind it may be that global dimming is masking or delaying the potentially far more extreme effects of global warming. Particle pollution, like ash, soot, and sulfur dioxide, in the atmosphere is believed to be the culprit for global dimming. Just like sunlight bounces off the top of clouds, so too will it bounce off of polluting particles. The compounding factor though, is that the polluting particles apparently make the clouds themselves far more reflective. This is because there are many more pollution particles in polluted clouds (as opposed to natural particles such as pollen or salt crystals in unpolluted clouds) so water droplets cannot condense to be large enough to fall as rain, rather they remain suspended in the atmosphere and continue to reflect sunlight back out of the atmosphere and drought ensues.

If we were to eliminate global dimming all together without addressing global warming the increase in temperature and extreme weather might be more significant than previously predicted. This has been recorded with one full degree temperature change in just a few days of decreased pollution in the days following the 9/11 attacks [Nature]. This reverse effect of dimming has been has been blamed along with global warming for increasing temperatures, appropriately called global brightening [Geophysical Research Letters]. No one wants smoggy skies but it could be smog and particulates that may be shielding us from the full consequences of the greenhouse gases we pump into our air daily. Clear blue skies are something many folks thinks of when they think of “going green” but those clear blue skies may hold more heat than we can handle.

Original here

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