Global dimming is a term describing the gradual reduction in the amount of sunlight observed 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; the trend has reversed during the past decade. Global dimming creates a cooling effect that 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.
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 2–3% 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 1964–93 (Gilgen et al, 1998)
- 2.7%/decade (total 20 W/m²) up to 2000 (Stanhill and Cohen, 2001)
- 4% over 1961-1990 (Liepert 2002) 
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  . Prior to the research being undertaken, predictions were of a 0.5% to 1% effect from particulate matter; the variation from prediction may be 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 may have been raising nighttime temperatures and/or lowering daytime temperatures by much more than previously thought.
In 2005 Wild et al. and Pinker et al. found that the "dimming" trend had reversed since about 1990 . It is likely that at least some of this change, particularly over Europe, is due to decreases in pollution; most governments have done more to reduce aerosols released into the atmosphere that help global dimming instead of reducing CO2 emissions.
The Baseline Surface Radiation Network (BSRN) has been collecting surface measurements. BSRN didn't get started until the early 1990s and updated the archives. Analysis of recent data reveals the planet's surface has brightened by about 4 percent the past decade. The brightening trend is corroborated by other data, including satellite analyses.
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.
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.
The phenomenon underlying global dimming may also have regional effects. While most of the earth has warmed, the regions that are downwind from major sources of air pollution (specifically sulfur dioxide emissions) have generally cooled. This may explain the cooling of the eastern United States relative to the warming western part .
- Stanhill, G. & Cohen, S. 2001. Global Dimming: a review of the evidence for a widespread and significant reduction in global radiation with discussion of its probable causes and possible agricultural consequences. Agricultural and Forest Meteorology 107, 255-278.
- Roderick, Michael L. and Graham D. Farquhar, 2002. The cause of decreased pan evaporation over the past 50 years. Science Vol. 298, No 5597, pp. 1410-1411, November 15, 2002.
- BBC Horizon TV broadcast of January 13, 2005 BBC Horizon
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- Wild, M et al.; 2005, Science 6 May 2005; 308: 847-850
- Pinker, et al.; 2005, Science 308: 850-854
- The Guardian, 18 December 2003, "Goodbye sunshine"
- Abstract: "Variability in solar radiation over Germany and an assessment of the role of aerosols."
- Liepert, B. G., 2002: Observed Reductions in Surface Solar Radiation in the United States and Worldwide from 1961 to 1990. Geophy. Res. Lett. 29/12, 10.1029/2002GL014910
- BBC Global Dimming Q&A
- Why the Sun seems to be 'dimming'
- Climate Change Chronicles The latest news and information on climate change, alternative energy, global warming and energy efficiency from around the internet
- RealClimate on this issue: part 1, part 2, Global Dimming may have a brighter future
- BBC Horizon Global Dimming summary from globalissues.org
- Watch this documentary free @ documentary-film.net