Aerosols are tiny particles suspended in the air.
Human activities, such as the burning of fossil fuels and the alteration of natural surface cover, also generate aerosols. Averaged over the globe, aerosols made by human activities currently account for about 10 percent of the total amount of aerosols in our atmosphere. Scientists have much to learn about the way aerosols affect regional and global climate. We have yet to accurately quantify the impacts of natural aerosols on climate, relative to impacts of aerosols made by humans. Moreover, we do not know whether the total amount of atmospheric aerosol is roughly constant, is diminishing, or is increasing.