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Air pollution helps plants blunt climate change: study

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.
by Staff Writers
Paris (AFP) April 22, 2009
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 on Wednesday.

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 ten percent increase in the amount of carbon stored by the land," she said in a statement.

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

Without this particle pollution, he said, average global surface temperatures would have increased by 1.0 to 1.1 Celsius (1.8 to 2.0 Fahrenheit) since the start of industrialisation, rather than 0.7 C (1.25 F).

The UN's Intergovernmental Panel on Climate Change (IPCC) has predicted that average global temperatures will rise before 2100 by 1.1 to 6.4 C (2.0 F to 11.5 F), depending on efforts to curb the gases that drive global warming.

Any increase above 2.0 C, the panel said, would unleash a maelstrom of human misery, including drought, famine, disease and forced migration.

To stay below that threshold, carbon dioxide concentrations in the atmosphere must be kept below 450 parts per million (ppm). The current level is about 385 ppm.

"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.

A major scientific review released last week at the United Nations showed that warming is itself limiting the capacity of plants to take up CO2, and that an increase in two degrees Celsius (3.6 degrees Fahrenheit) would transform forests from a sink into a net source of CO2.

When plants die, the carbon they store is released into the air.

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Widespread die-off of pinon pine across the southwestern United States during future droughts will occur at least five times faster if climate warms by 4 degrees Celsius, even if future droughts are no worse than droughts of the past century, scientists have discovered in experiments conducted at the University of Arizona's Biosphere 2.

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