What is global dimming? It is a cooling effect that appears to have partially masked the effects of global warming. Global dimming is caused by a reduction in the amount of sunlight hitting the earth due to the presence of aerosolized particulate pollution, such as jet contrails. These airborne particles reflect sunlight back into space before it hits the surface of the earth, reducing warming and thereby masking the effects of global warming.
This streaming video (below the fold) by Nova shows how the average temperature range in the United States jumped by more than one degree celsius (two degrees fahrenheit) in those three days after 9-11, when all commercial airliners were grounded. This was caused by the loss of only one form of pollution; jet contrails. Not only were these results interesting, but surprisingly, the data also revealed an immediate response to the loss of this one form of pollution; a significant and large increase in temperature range. [6:51]
Conclusion: the effects of global warming are much more serious than previously thought because global dimming partially masks the effects of global warming.