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.

Pollution dims sunlight in two ways, scientists theorize. Some light bounces off soot particles in the air. The pollution also causes more water droplets to condense out of air, leading to thicker, darker clouds, which also block more light.