20 April 2006
Why the Sun seems to be 'dimming'
By David Sington
Thursday, 13 January 2005, 14:10 GMT
We are all seeing rather less of the Sun, according to scientists who have been looking at five decades of sunlight measurements.
They have reached the disturbing conclusion that the amount of solar energy reaching the Earth's surface has been gradually falling. Paradoxically, the decline in sunlight may mean that global warming is a far greater threat to society than previously thought. The effect was first spotted by Gerry Stanhill, an English scientist working in Israel.
Comparing Israeli sunlight records from the 1950s with current ones, Dr Stanhill was astonished to find a large fall in solar radiation. "There was a staggering 22% drop in the sunlight, and that really amazed me." Intrigued, he searched records from all around the world, and found the same story almost everywhere he looked. Sunlight was falling by 10% over the USA, nearly 30% in parts of the former Soviet Union, and even by 16% in parts of the British Isles.
Although the effect varied greatly from place to place, overall the decline amounted to one to two per cent globally every decade between the 1950s and the 1990s. Dr Stanhill called it "global dimming", but his research, published in 2001, met a sceptical response from other scientists.
It was only recently, when his conclusions were confirmed by Australian scientists using a completely different method to estimate solar radiation, that climate scientists at last woke up to the reality of global dimming.
Dimming appears to be caused by air pollution. Burning coal, oil and wood, whether in cars, power stations or cooking fires, produces not only invisible carbon dioxide - the principal greenhouse gas responsible for global warming - but also tiny airborne particles of soot, ash, sulphur compounds and other pollutants.
This visible air pollution reflects sunlight back into space, preventing it reaching the surface. But the pollution also changes the optical properties of clouds. Because the particles seed the formation of water droplets, polluted clouds contain a larger number of droplets than unpolluted clouds.
Recent research shows that this makes them more reflective than they would otherwise be, again reflecting the Sun's rays back into space. Scientists are now worried that dimming, by shielding the oceans from the full power of the Sun, may be disrupting the pattern of the world's rainfall.
There are suggestions that dimming was behind the droughts in sub-Saharan Africa which claimed hundreds of thousands of lives in the 1970s and 80s. There are disturbing hints the same thing may be happening today in Asia, home to half the world's population. "My main concern is global dimming is also having a detrimental impact on the Asian monsoon," says Professor Veerhabhadran Ramanathan, professor of climate and atmospheric sciences at the University of California, San Diego. "We are talking about billions of people."
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