Hazy skies boost plant carbon intake

22 April 2009 by Tom Marshall

Particle pollution in the air is making plants absorb more carbon dioxide from the atmosphere by diffusing sunlight, new research reveals.

Smog over city

Particle pollution in the air causes health problems, but it also makes plants absorb more carbon.

The paper, published in Nature, shows that hazier conditions caused by clouds and particles in the air, or 'aerosols', have caused plants to absorb 25 per cent more carbon between 1960 and 1999.

The increase was partly offset by the fact the same aerosols and clouds also reduced the amount of sunlight reaching the Earth so that the total level of photosynthesis fell. But hazy skies meant that the photosynthesis that did happen was more efficient.

The net effect was a 10 per cent increase in the net amount of carbon stored by the land even after the drop in photosynthesis was taken into account.

"The overall effect was to enhance the land carbon sink even though the total solar radiation has decreased," says lead author Dr Lina Mercado, a vegetation modeller at the Centre for Ecology & Hydrology.

Plants perform better when they are being illuminated from several directions at once by light reflected off clouds and airborne particles, rather than under intense sunlight from a single direction. The more diffuse light means fewer leaves are completely shaded at any given moment.

"Although people normally believe that well-watered plants grow best on a bright sunny day, the reverse is true," says co-author Dr Stephen Sitch from the Met Office Hadley Centre (now at the University of Leeds). "Plants often thrive in hazy conditions such as those that exist during periods of increased atmospheric pollution," he explains.

The effect had been observed before in studies of temperate and tropical forests and croplands, but this is the first time it has been incorporated into a global climate model to estimate the total effect from all vegetation.

Scientists have also known for some time that aerosols help cool the planet by reflecting sunlight and making clouds brighter, reducing the amount of sunlight hitting the earth. Increasing human emissions of aerosols and clouds has caused a reduction in solar radiation between the 1950s and the 1980s - a phenomenon known as 'global dimming'.

Mercado notes that global dimming could only go so far before it reaches a threshold after which the benefits of more diffuse light no longer offset the drawbacks of plants photosynthesising less overall.

The findings suggest a conundrum - efforts are afoot to cut aerosol pollution, which consists primarily of sulfates but also includes particles of dust, sea salt, smoke and soot.

These aerosols can cause lung problems when people breathe them in. But making them less common in the atmosphere will make it harder to meet targets for reducing greenhouse gases like carbon dioxide, by reducing plants' ability to absorb those gases.

"Future policies on emissions reductions should take account of the fact that when we clean the air of aerosol pollution, plant photosynthesis will fall and part of the carbon sink will no longer be there," says Mercado. "So it will be harder to stop climate change and we will need to make bigger reductions in CO2 emissions."

The research team included members from the Centre for Ecology & Hydrology, ETH Zurich - the Swiss Federal Institute of Technology - the Met Office Hadley Centre and the University of Exeter.