Dust and other tiny particles suspended in Earth’s atmosphere function like a global sunscreen, helping to counteract global warming by blocking sunlight. But the cooling effect of these aerosols has been diminishing since the early 1990’s, according to a new NASA study.

“When more sunlight can get through the atmosphere and warm Earth’s surface, you’re going to have an effect on climate and temperature,” said lead author Michael Mishchenko of NASA’s Goddard Institute for Space Studies.

Aerosol consist of natural particles such as dust and volcanic ash, as well as manmade pollution. The NASA scientists examined satellite aerosol data from 1978 to the present, and found several large, short-lived spikes caused by major volcanic eruptions in 1982 and 1991. Overall they found global aerosols have been declining since about 1990.

By 2005, aerosol levels had dropped by as much as 20 percent from levels found between 1986 and 1990. The finding is detailed in the March 15 issue of the journal Science.

The drop in atmospheric aerosols could help explain recent observations that the amount of sunlight reaching the Earth’s surface has increased in the past decade—a phenomenon known as “global brightening,” since the two trends began around the same time.

However, changes in cloud cover, which also affects the amount of sunlight reaching our planet’s surface, can’t be ruled out just yet.

The scientists are also unsure whether aerosol levels will continue to decline because the particles are not uniformly distributed around the globe and come from many different sources, both natural and man-made.