Nova this evening aired a piece titled "Dimming Sun" which is simply remarkable. If you get the opportunity, tune in.
To summarize: atmospheric scientists have observed that the amount of sunlight reaching the ground is declining due to atmospheric pollution. This dimming effect is masking the true magnitude of global warming - suggesting that global warming is twice as pronounced as we have observed up to now.
Here is a very rapid rundown:
This dimming effect has been measured around the world with a global decline of sunlight of about 10% in the last half-century. Locally, the declines have been as much as 30%. The dimming comes from the pollution particles causing water vapor to remain suspended in the atmosphere in clouds rather than condensing and falling as rain. The added cloud cover reflects solar radiation back into space preventing it from reaching the ground, but the cover also serves as a blanket, reducing the temperature difference from day to night.
One specific source of this dimming that was measured is the effect of airplanes leaving vapor contrails in the atmosphere. This is clearly visible from space at times when conditions are favorable. The effect was measured after 9/11 when all aircraft in the US were grounded for 72 hours. During that time, the average high/low temperature difference across the nation increased by 2 degrees F - a huge amount - due solely to aircraft being grounded.
In addition to the relative cooling effect on the planet, the increase in cloud cover has significantly reduced the amount of evaporation taking place. Sunlight is the main driver of evaporation, so even in the face of a warming atmosphere, the decline of sunlight is reducing ocean evaporation which is no suspected to be a major contributer to droughts throughout Africa in the last three decades.
The catch-22 for us is how we deal with global dimming. Here in the US we have made substantial progress toward improving air quality. Anyone living in LA would be hard pressed to remember the last time we had a Stage 1 smog alert (1997). Just 20 years prior there were 121 in one year - and the population has increased substantially over that time. But the reduction of air pollution is causing global warming to accelerate - especially in areas east of the US - such as Greenland. So as we address airborne pollution but not CO2 emission, the effects of global warming will become increasingly pronounced, causing scientists such as James Hansen to warn that global warming will be even more severe than they predicted just a few years ago, simply due to the masking effects of global dimming.
Its a truly sobering view even for those of us that have been concerned about global warming, but weren't aware that there were any measurable global impacts of 10% or more. To say that man is not so powerful as to have a significant impact on the planet's climate is now simple ignorance. This was one of the most powerful illustrations of the need for pollution and climate change to be taken seriously that I have seen on TV. I strongly recommend watching - there was much more than I could summarize here.