Global dimming

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. The effect varies by location, but worldwide it has been estimated to be of the order of a 4% reduction over the three decades from 1960–1990. However, since 1990, the trend has reversed.
It is thought to have been caused by an increase in particulates such as sulphate aerosols in the atmosphere due to human action. The switch from a "global dimming" trend to a "brightening" trend in 1990 happened just as global aerosol levels started to decline.
Global dimming 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

Causes and effects

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. 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

Relationship to global warming

Some scientists now consider that the effects of global dimming have masked the effect of global warming to some extent and that resolving global dimming may therefore lead to increases in predictions of future temperature rise. According to Beate Liepert, "We lived in a global warming plus a global dimming world and now we are taking out global dimming. So we end up with the global warming world, which will be much worse than we thought it will be, much hotter."The magnitude of this masking effect is one of the central problems in climate change with significant implications for future climate changes and policy responses to global warming.
But it's much more complicated than an either warming or dimming issue. Global warming and global dimming are not mutually exclusive or contradictory. In a paper published on March 8, 2005 in the American Geophysical Union's Geophysical Research Letters, a research team led by Anastasia Romanou of Columbia University's Department of Applied Physics and Mathematics, New York, also showed that the apparently opposing forces of global warming and global dimming can occur at the same time. Global dimming interacts with global warming by blocking sunlight that would otherwise cause evaporation and the particulates bind to water droplets. Water vapor is one of the greenhouse gases. On the other hand, global dimming is affected by evaporation and rain. Rain has the effect of clearing out polluted skies.
Brown clouds have been found to amplify global warming according to V. Ramanathan, an atmospheric chemist at the Scripps Institution of Oceanography in La Jolla, CA. "The conventional thinking is that brown clouds have masked as much as 50 percent of global warming by greenhouse gases through so-called global dimming," ... "While this is true globally, this study reveals that over southern and eastern Asia, the soot particles in the brown clouds are in fact amplifying the atmospheric warming trend caused by greenhouse gases by as much as 50 percent.