Global dimming is the gradual reduction in the amount of global direct irradiance at the Earth's surface, observed since the beginning of systematic measurements in 1950s. The effect varies by location, but worldwide it is of the order of a 4% reduction over the three decades from 1960–1990. This trend may have reversed during the past decade. Global dimming creates a cooling effect that may have partially masked the effect of greenhouse gases on global warming.
Cause and effects
It is currently thought that the effect of global dimming is probably due to the increased presence of aerosol particles in the atmosphere. Aerosol particles and other particulate pollutants absorb solar energy and reflect sunlight back into space. The pollutants can also become nuclei for cloud droplets. It is thought that the water droplets in clouds coalesce around the particles. Increased pollution, resulting in more particulates, creates clouds consisting of a greater number of smaller droplets, which in turn makes them more reflective, therefore bouncing more sunlight back into space.
Clouds intercept both heat from the sun and heat radiated from the Earth. Their effects are complex and vary in time, location and altitude. Usually during the daytime the interception of sunlight predominates, giving a cooling effect; however, at night the re-radiation of heat to the Earth slows the Earth's heat loss.
BBC Horizon Documentary
- BBC Horizon Global Dimming summary from globalissues.org
- Google Video
- BBC Global Dimming FAQ