Ozone damage to vegetation influences global warming

31 December 2010 by Adele Walker

Pollutants from cars and paints can warm the Earth's atmosphere when they turn into ozone and damage plants, a new study has found.

The results are important, because the role of this type of pollution in climate change is complex and needs to be properly understood for mitigation policies to be effective.

Ozone damaged clover

Ozone damaged clover

These short-lived pollutants - like nitrogen oxides and volatile organic compounds - have little direct impact on temperature, but they react in the atmosphere to form the greenhouse gas ozone. Among other things, too much ozone damages plants, making them less good at photosynthesising carbon dioxide from the atmosphere and storing - or sequestering - it in the ground.

This is the first time researchers have included vegetation damage when calculating the potential of these so-called 'ozone precursor' chemicals to affect temperature.

"It's been well known for a long time that ozone damage plants," says Dr Bill Collins of the Met Office Hadley Centre, who led the research, "but no one has made the connection with ozone damage and the effects on the movement of carbon between the earth and the atmosphere."

The team ran a sequence of models to predict the effects of three pollutants - nitrogen oxides (NOx), carbon monoxide (CO) and volatile organic compounds (VOC) - on the Earth's temperature. They looked at how these chemicals affect vegetation across the northern hemisphere.

First they looked at how the pollution reacts in the atmosphere. To predict the effects on ozone levels they ran a model which simulated a 20 per cent reduction in the emissions of each of the chemicals. The emissions were reduced in the model because these pollutants are increasingly being controlled by air quality regulations, and real-world emissions are expected to drop.

A carbon cycle model then examined how these ozone levels would affect different types of plants, using a range of vegetation with varying ozone sensitivity. The model predicted how the different amounts of vegetation damage would affect the levels of carbon dioxide being taken out of the atmosphere and stored in the soil - this exchange of carbon between the land and air is known as carbon flux.

'This gives us another good reason to control air pollution

- Dr Bill Collins, Met Office Hadley Centre

The third model used carbon flux information to work out the effects on atmospheric carbon dioxide and the temperature of the Earth.

The models followed the effects of a one-year reduction in emissions for several decades afterwards.

"Different pollutants have a different effect over time," explains Collins. "For example CO2 lasts for a long time in the atmosphere, while ozone has a short lifetime; its levels can change very quickly but the effects don't last long."

One of the most surprising effects was for NOx, which scientists usually think of as a cooling pollutant because it reduces the warming effect of methane in the atmosphere. But under some conditions the contribution of plant damage was strong enough to turn this around, and gave the gas an overall warming effect in the atmosphere.

In combination, reducing the emissions of all the pollutants resulted in a clear climate-cooling effect. Significantly, this cooling effect is 30-40 per cent greater than when plant damage is excluded from the calculations.

"This gives us another good reason to control air pollution," comments Collins.

"Plant damage really needs to be included in these calculations in the future," he adds.

The study's results, published in the Journal of Geophysical Research, are for a global average temperature, but this varies around the world so more research is needed to look at regional effects.

'How vegetation impacts affect climate metrics for ozone precursors' - WJ Collins, S Sitch and O Boucher (2010). Journal of Geophysical Research 115 doi:10.1029/2010JD014187