Global dimming is a less well-known but real and very important phenomenon.
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 the 1950s. |  |
The effect depends on locations, but it has been world-widely estimated to be about 4% reductions over the three decades from 1960-1990. Global dimming is considered to have been caused by an increase in particulates such as sulfate aerosols in the atmosphere due to human action such as industry or combustion of fossil fuels. 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 sulfur 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.
It has interfered with the hydrological cycle by reducing evaporation and may have reduced rainfall in some areas. Global dimming also creates a cooling effect that may have partially masked the effect of greenhouse gases on global warming. Deliberate manipulation of this dimming effect is being considered as a geoengineering technique to reduce the impact of global warming.Trend of Global Dimming
These graphs are results from modeled all sky annual mean solar surface irradiance and associated trends over land for (a) global, (b) Europe, and (c) South East Asia. The dotted lines in Figures a–c show 10 year running means derived from the annual mean data. [11]
The amount of sunlight reaching Earth's surface had been steadily declining in recent decades, began to reverse around 1990. This switch from a "global dimming" trend to a "brightening" trend happened just as global aerosol levels started to decline.
It is likely that at least some of this change, particularly over Europe, is due to decreases in airborne pollution. Most governments of developed nations have taken steps to reduce aerosols released into the atmosphere, which helps reduce global dimming.
Sulfate aerosols have declined significantly since 1970 with the Clean Air Act in the U.S and similar policies in Europe. The Clean Air Act was strengthened in 1977 and 1990. According to the EPA, from 1970 to 2005, total emissions of the six principal air pollutants dropped by 53%. In 1975, the masked effects of trapped greenhouse gases finally started to emerge and have dominated ever since.
