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.

Plant life - especially tropical forests - 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 per cent increase in the amount of carbon stored by the land," she said.

Global dimming was especially strong from the 1950s up through the 1980s, corresponding to the period of enhanced plant growth, notes the study, published in the British journal Nature.

Research published last month found that dimming has since continued almost everywhere in the world except Europe.

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.

"Although many people believe that well-watered plants grow best on a bright sunny day, the reverse is true. Plants often thrive in hazy conditions," said colleague and co-author Stephen Sitch. This process of diffuse radiation is well known. But the new study is the first to use a global model to calculate its impact on the ability of plants to absorb CO2.

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 per cent of the greenhouse gas warming," Knut Alfsen, research director at the Centre for International Environmental Research in Oslo, Norway, said.