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  Today Online 6 Apr 07
Shedding some light on global dimming
Pollution is blocking out sunlight – which is hiding the effects of global warming, warn scientists
Sheralyn Tay

GLOBAL warming is only half the story, there's another phenomenon we're in the dark about — literally. Too many pollutants in the air causes a reduction of sunlight getting through the atmosphere, an effect known as global dimming.

And Singaporeans are already feeling its effect — when the haze comes blowing. "When Indonesia burns, the haze causes the effect of global dimming and blocks out the sunlight," said Dr John Yong, Assistant Professor of Natural Sciences and Science Education at the Nanyang Technological University's National Institute of Education.

This dimming effect reduces the amount of sunlight that reaches the earth, making it seem cooler, but also masking the full effect of global warming.

One area where the impact of global dimming on climate change can easily be seen: Air travel. According to Dr Yong, the "911 effect" was discovered in the days just after the terrorist attacks on the World Trade Center in the United States on Sept 11, 2001. "All air travel stopped, and what happened? The temperature went up!" Dr Yong said.

Climate scientists, who had previously theorised that aircraft vapour trails were related to global dimming, noted that the absence of these trails correlated to an increase of about 1°C in the daily temperature in parts of the United States.

"So, in a way, we have pollutants to thank for protecting us from global warming," said Dr Yong with a laugh.

Between the '50s and the '90s, British scientist Gerry Stanhill found that the amount of sunlight reaching the earth fell by between 10 and 30 per cent in different parts of the world. The rate of dimming varied but scientists believe it to drop by an average of around 2 to 3 per cent each decade.

While there are no estimates for Singapore, statistics from around Asia put the dimming effect to be about 10 to 37 per cent, with Hong Kong facing the worst effects.

Global dimming also has serious repercussions on agriculture. "With less sunlight, there will be less photosynthesis (the manner in which plants make food) and this will affect the food chain," said Dr Yong.

In addition, the particulate matters (PM), those nano-sized particles that cause the dimming effect, have a significant impact on health. PM10 and PM2.5 — particulate matter that is smaller than 10 or 2.5 microns, respectively — are associated with respiratory illness and even cancer.

All this creates a dilemma for scientists trying to deal with global dimming. "If we clean up the air, you'll have a big heat problem," Dr Yong said, citing heat waves in Europe that may be linked to the continent's efforts to cut down on emissions of particulate matters.

"There must be a concerted effort to clean the air and be carbon-neutral. If not, when you just clear the nano-particulates it will be very hot, but if you reduce carbon emissions only, crops will still fail, so both of these must be done in tandem. There's the headache," he said.

Dr John Yong will be speaking about at the Society of Bioscience & Technology's Public Symposium "Managing Global Climate Change through Bioscience" on April 21. For more information, visit www.socbioscience.org
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Related articles on Global: Climate change
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