|This image of the Earth shows surface air temperature change from an IS92a experiment using a HadCM2 model carried out by the Met Office, it includes the cooling effect of sulphate aerosols. Click on the image for a movie (MPEG Movie, 2.1Mb).|
The 'dimming' effect was first noticed by Gerry Stanhill who published his findings in 2001 to some scepticisim, however the effect has recently been independently confirmed. Although highly variable, the overall globally rate of decline of sunlight levels is one to two per cent every decade from the 1950s until the 1990s.
Dimming appears to be caused by air pollution. While burning fossil fuels produces carbon dioxide, the main gas responsible for global warming, it also produces particles of soot, ash and other pollutants. Airborne, and carried high into the atmosphere, these particles reflect sunlight back into space. The particles also provide seed for rain clouds, and research now shows that polluted clouds contain larger numbers of water droplets than normal clouds, which also increases the reflectivity of the atmosphere to sunlight.
What is worrying of course is that this increased reflectivity has caused us to, dramatically perhaps, underestimate the strength of the greenhouse effect.
Despite the large amounts of extra carbon dioxide we have produced there has been a suprisingly small change in the mean global temperature, just 0.5C to date. A similar rise in carbon dioxide levels, during the Jurassic period, led to a rise ten times this, at around 5.0C. However if the effects of global warming have been offset by dimming, then we may be in trouble, as particle pollution has been brought more firmly under control than carbin dioxide emission. While particulate levels are predicted to fall, the level of carbon dixoide in the atmosphere is set to continue to rise over the next few decades.
This means that the cooling effect will decline, while the warming effect continues to grow. Even the worst predictions for the effects of global warming may now be too optimistic...
The BBC has made a transcript of the Horizon episode on "Global Dimming" available, along with some questions and answers.