GLOBAL DIMMING

Why the Sun seems to be 'dimming'
David Sington, January 2005 (BBC News)
- 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...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...overall the decline amounted to one to two per cent globally every decade between the 1950s and the 1990s...
- Dimming appears to be caused by air pollution...invisible carbon dioxide - the principal greenhouse gas responsible for global warming - [and] 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...polluted clouds contain a larger number of droplets than unpolluted clouds...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...
- But perhaps the most alarming aspect of global dimming is that it may have led scientists to underestimate the true power of the greenhouse effect...extra energy is being trapped in the Earth's atmosphere by the extra carbon dioxide...What has been surprising is that this extra energy has so far resulted in a temperature rise of just 0.6 degree Celsius...This has led many scientists to conclude that the present-day climate is less sensitive to the effects of carbon dioxide than it was, say, during the ice age, when a similar rise in CO2 led to a temperature rise of six degrees Celsius...
- But it now appears the warming from greenhouse gases has been offset by a strong cooling effect from dimming...two of our pollutants have been cancelling each other out...the climate may in fact be more sensitive to the greenhouse effect than previously thought...
- As things stand, CO2 levels are projected to rise strongly over coming decades, whereas there are encouraging signs that particle pollution is at last being brought under control...Even the most pessimistic forecasts of global warming may now have to be drastically revised upwards...That is unless we act urgently to curb our emissions of greenhouse gases.

SLIDE SHOW: In the early 21st century, it's become clear that air pollution can significantly reduce the amount of sunlight reaching Earth, lower temperatures, and mask the warming effects of greenhouse gases. Climate researcher James Hansen estimates that "global dimming" is cooling our planet by more than a degree Celsius (1.8°F) and fears that as we cut back on the pollution that contributes to dimming, global warming may escalate to a point of no return. Regrettably, in terms of possibly taking corrective action, our current understanding of global dimming has been a long time in the coming, considering the first hints of the phenomenon date back to 18th-century observations of volcanic eruptions. In this slide show, follow a series of historic events and scientific milestones that built the case for global dimming. (Susan K. Lewis, PBS/Nova)