Global Dimming is the gradual reduction in the amount of global direct irradiance at the Earth's surface that was observed for several decades after the start of systematic measurements in 1950s.
It is thought to have been caused by an increase in particulates such as black carbon in the atmosphere due to human action.
The effect varied by location, but worldwide it was of the order of a 4% reduction over the three decades from 1960-1990.
The trend reversed during the past decade. Global dimming has interfered with the hydrological cycle by reducing evaporation and may have caused droughts in some areas.
Global dimming also creates a cooling effect that may have partially masked the effect of greenhouse gases on global warming........
It is thought that global dimming was probably due to the increased presence of aerosol particles in the atmosphere caused by human action. Aerosols and other particulates absorb solar energy and reflect sunlight back into space. The pollutants can also become nuclei for cloud droplets.
It is also thought that the water droplets in clouds coalesce around the particles. Increased pollution causes more particulates and thereby creates clouds consisting of a greater number of smaller droplets (that is, the same amount of water is spread over more droplets). The smaller droplets make clouds more reflective, so that more incoming sunlight is reflected back into space and less reaches the earth's surface.
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.
Also it has been thought that humans help produce the particles in the earth's atmosphere, resulting in global dimming.
Paralumun New Age Village