Global dimming is a gradual reduction in the amount of sunlight reaching the Earth's surface since the 1950s. The effect varies by location but globally is of the order of a 5% reduction over the three decades 1960-1990. This cooling effect may have led scientists to underestimate the effect of greenhouse gases on global warming.
Cause and effects
It is now thought that the effect is probably due to the increased presence of aerosols and other particulates in the atmosphere. It is thought that the water droplets in clouds coalesce around the particles, resulting in the clouds consisting of a greater number of smaller droplets, which in turn makes them more reflective: 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 and location and height. Usually, during the day 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.
Global dimming may have been reported by Atsumu Ohmura in 1989; it was certainly reported by Stanhill and Moreshet in 1992.
Independent research in Israel and Netherlands in the late 1980s showed an apparent reduction in the amount of sunlight despite wide spread evidence that the climate was actually getting hotter (see global warming).
The rate of dimming varies around the world but is on average estimated at around 23% per decade, with a possibility that the trend reversed in the early 1990s. It is difficult to make an exact measurement because of the difficulty in accurately calibrating the instruments and the problem of spatial coverage. Nonetheless the effect is almost certainly real.
Note that the effect (2-3%, as above) is due to changes within the Earth's atmosphere; the value of the solar radiation at the top of the atmosphere has not changed by more than a fraction of this amount.
The effect varies greatly over the globe, but estimates of the global average value are:
5.3% (9 W/m²) over 1958-85 (Stanhill and Moreshet, 1992)
2%/decade over 196493 (Gilgen et al, 1998)
2.7%/decade (total 20 W/m²) up to 2000 (Stanhill and Cohen, 2001)
4% over 1961-1990 (Liepert 2002)  (http://www.realclimate.org/index.php?p=110)
The largest reductions are found in the northern hemisphere mid-latitudes.
Experiments in the Maldives (comparing the atmosphere over the northern and southern islands) in the 1990s showed that the effect of macroscopic pollutants in the atmosphere at that time (blown south from India) caused about a 10% reduction in sunlight reaching the surface in the area under the pollution cloud a much greater reduction than expected from the presence of the particles themselves
 (http://www.google.co.uk/search?q=cache:uaKeKseXPHcJ:www.unep.org/GC/GC22/Document/Executive%2520Summary.pdf+VEERABHADRAN+RAMANATHAN+maldives&hl=en) .
Prior to the research being undertaken, predictions were of a 0.5% to 1% effect from particulate matter; the variation from prediction is explained by cloud formation with the particles acting as the focus for droplet creation. Clouds are very effective at reflecting light back out into space.
Some climate scientists have theorised that aircraft contrails are implicated in global dimming, but the constant flow of air traffic meant that this could not be tested.
The near-total shutdown of civil air traffic during the three days following the September 11, 2001 attacks afforded a rare opportunity in which to observe the climate of the USA absent from the effect of contrails. During this period an increase in diurnal temperature variation of over 1 C° was observed, i.e. aircraft contrails have been raising nighttime temperatures and/or lowering daytime temperatures by much more than previously thought.
Global dimming may have caused large scale changes in weather patterns. Climate models speculatively suggest that this reduction in sunshine at the surface may have led to the failure of the monsoon in sub-Saharan Africa during the 1970s and 1980s, together with the associated famines, caused by Northern hemisphere pollution cooling the Atlantic. This is not universally accepted and is very difficult to prove.
The solution to global dimming would appear to be to cut atmospheric pollution. This has been taking place in Europe for some years, and there is evidence that global dimming had indeed decreased as a result.
European data from 1990 onwards which has shown that chemical scrubbing of exhaust gases from power plants and catalytic converters on car exhausts has improved air quality but may have accelerated the rate of warming.
However, the rise in average temperatures in Europe in the early years of the 2000s - which is presumed to be a consequence of the reduction in pollution - resulted in forest fires in Portugal, nearly 15,000 heat-related deaths in France, and increased the melting of alpine glaciers during the summer of 2003.