Global dimming is a decline in the amount of sunlight reaching the Earth's surface. It is caused by atmospheric particulate matter, predominantly sulfate aerosols, which are components of air pollution. Global dimming was observed soon after the first systematic measurements of solar irradiance began in the 1950s. This weakening of visible sunlight proceeded at the rate of 4–5% per decade until the 1980s. During these years, air pollution increased due to post-war industrialization. Solar activity did not vary more than the usual during this period.
The observed trends of global dimming and brightening in four major geographic regions. The dimming was greater on the average cloud-free days (red line) than on the average of all days (purple line), strongly suggesting that sulfate aerosols were the cause.
If smoke from wildfires mixes into clouds, it darkens them, decreasing their albedo. If there are no clouds, then smoke can increase albedo, particularly over oceans.
Sun-blocking aerosols around the world steadily declined (red line) since the 1991 eruption of Mount Pinatubo, according to satellite estimates.
Satellite photo showing a thick pall of smoke and haze from forest fires in Eastern China. Such smoke is full of black carbon, which contributes to dimming trends but has an overall warming effect.
Particulates or atmospheric particulate matter are microscopic particles of solid or liquid matter suspended in the air. The term aerosol commonly refers to the particulate/air mixture, as opposed to the particulate matter alone. Sources of particulate matter can be natural or anthropogenic. They have impacts on climate and precipitation that adversely affect human health, in ways additional to direct inhalation.
PM2.5 and PM10 compared with a human hair in a graphic from the Environmental Protection Agency
Excavator (a type of heavy equipment commonly used at construction sites and roadworks) demolishing the remnants of the pre-war Postal Train 0880Station (Dworzec Pocztowy) at Jerozolimskie Avenue, Poland
Particulates in the air causing shades of orange, yellow, pink, and grey in Mumbai during sunset
Air pollution measurement station in Emden, Germany