Cleaning up skies choked with smog and soot would sharply curtail the capacity of plants to absorb carbon dioxide and blunt global warming, according to a study released yesterday.

Plants soak up a quarter of all the CO2 humans spew into the atmosphere, and thus plays a critical role in keeping climate change in check.

Through photosynthesis, vegetation transforms sunlight, CO2 and water into sugar nutrients. Common sense would suggest that air pollution in the form of microscopic particles that obstruct the Sun's rays - a phenomenon called "global dimming" - would hamper this process, but the new study shows the opposite is true.

"Surprisingly, the effects of atmospheric pollution seem to have enhanced global plant productivity by as much as a quarter from 1960 to 1999," said Linda Mercado, a researcher at the Met Office Hadley Centre in Britain, and the study's lead author. "This resulted in a net 10 percent increase in the amount of carbon stored by the land," she said in a statement.

The explanation for this botanical paradox lies in the way particle pollution reflects light. Even if plants receive less direct sunshine, the presence of clouds and pollution scatter the light that does filter through such that fewer leaves - which is where photosynthesis occurs - wind up in total shade.

"Plants often thrive in hazy conditions," said colleague and co-author Stephen Sitch.

The findings underline a cruel dilemma: to the extent we succeed in reducing aerosol pollution in coming decades, we will need to slash global carbon dioxide emissions even more than we would have otherwise.

"Aerosols offset approximately 50 percent of the greenhouse gas warming," Knut Alfsen, research director at the Centre for International Environmental Research in Oslo, Norway, said by phone.

"As we continue to clean up the air - which we must do for the sake of human health - the challenge of avoiding dangerous climate change through reductions in CO2 emissions will be even harder," said Peter Cox, a researcher at Britain's University of Exeter and a co-author of the Nature study.

Bangkok beats London

Residents of the Thai capital produce as much carbon pollution as New Yorkers and more than Londoners, a UN-backed study released yesterday shows.

The report, Bangkok: Assessment Report on Climate Change 2009, underscores the city's carbon-intensive habits but also highlights the threat to Bangkok from rising seas caused by global warming.

"In per-capita terms, Bangkok was responsible for producing 7.1 tons of carbon dioxide per annum in 2007," said the report by the Bangkok Metropolitan Authority and the Bangkok-based Green Leaf Foundation, with support from the United Nations Environment Programme.

That was the same level of emissions produced by New Yorkers in 2007. Londoners produced 5.9 tons per capita.

(China Daily va Agencies April 23, 2009)