"Climate scientists at the Harvard School of Engineering and Applied Sciences (SEAS) have discovered that particulate pollution in the late 20th century created a "warming hole" over the eastern United States-that is, a cold patch where the effects of global warming were temporarily obscured."

"While greenhouse gases like carbon dioxide and methane warm the Earth's surface, tiny particles in the air can have the reverse effect on regional scales.

'What we've shown is that particulate pollution over the eastern United States has delayed the warming that we would expect to see from increasing greenhouse gases,' says lead author Eric Leibensperger (Ph.D. '11), who completed the work as a graduate student in applied physics at SEAS."

SPX had the story April 30, 2012. [2]