Global Dimming?

In the second half of the 20th century, the world became, quite literally, a darker place.

Defying expectation and easy explanation, hundreds of instruments around the world recorded a drop in sunshine reaching the surface of Earth, as much as 10 percent from the late 1950's to the early 90's, or 2 percent to 3 percent a decade. In some regions like Asia, the United States and Europe, the drop was even steeper. In Hong Kong, sunlight decreased 37 percent.

Satellite measurements show that the sun remains as bright as ever, but that less and less sunlight has been making it through the atmosphere to the ground.

Pollution dims sunlight in two ways, scientists theorize. Some light bounces off soot particles in the air and goes back into outer space. The pollution also causes more water droplets to condense out of air, leading to thicker, darker clouds, which also block more light. For that reason, the dimming appears to be more pronounced on cloudy days than sunny ones. Some less polluted regions have had little or no dimming.

NYT article