Dear EarthTalk: I’ve heard of global warming, of course, but what on Earth is “global dimming?” -- Max S., Seattle, WA
Global dimming is a less well-known but real phenomenon resulting from atmospheric pollution. The burning of fossil fuels by industry and internal combustion engines, in addition to releasing the carbon dioxide that collects and traps the sun’s heat within our atmosphere, causes the emission of so-called particulate pollution—composed primarily of sulphur dioxide, soot and ash. When these particulates enter the atmosphere they absorb solar energy and reflect sunlight otherwise bound for the Earth’s surface back into space. Particulate pollution also changes the properties of clouds—so-called “brown clouds” are more reflective and produce less rainfall than their more pristine counterparts. The reduction in heat reaching the Earth’s surface as a result of both of these processes is what researchers have dubbed global dimming.