International ozone treaty needed, scientists warn

15 October 2008 by Tom Marshall

Pollution controls aren't cutting levels of ground-level ozone enough to protect human health and the environment, according to a report published last week by the Royal Society.

Smog over city

At present ozone pollution is controlled by a patchwork of local and regional laws; the paper's authors argue this isn't enough, as ozone can persist in the atmosphere for weeks and doesn't respect national boundaries. Climate change will only make cutting this form of pollution harder, they add.

The paper's authors warn that in the UK and other Northern Hemisphere countries, background concentrations of ozone have grown by six per cent, or two parts per billion, a decade since the 1980s. Levels of the pollutant have doubled since the mid-nineteenth century.

Ozone is a powerful oxidant; it irritates the lungs and causes more people to die from respiratory complaints. Those who already suffer from problems like asthma are particularly vulnerable.

Professor David Fowler of the Centre for Ecology & Hydrology (CEH) chaired the Royal Society working group that produced the research.

"Ozone is a global traveller and one of the most pervasive air pollutants," he says. "Weather systems and jet streams transport ozone, and the pollutants that lead to its formation, often far from their point of origin."

Fowler notes that the UK receives most of its ozone pollution from outside Europe, with North America, China and India accounting for particularly large proportions of ozone's precursor gases.

Ozone monitoring equipment

The Centre for Ecology & Hydrology's long term ozone flux measurement facility at Auchencorth Moss in the Scottish Borders

Countries and regions have varying targets for reducing emissions, and varying timescales for achieving them. While many of these sets of rules are steps in the right direction, they are far less effective than a unified framework for all countries would be.

Ozone forms when pollutants from sources like car exhausts and forest fires break down in the atmosphere as sunlight hits them.

Once created, it can hang around for a long time - its average lifetime throughout the lower atmosphere is 22 days, giving it time to move a long way from its source. The two key ingredients are nitrogen oxides and volatile organic compounds, or VOCs, and both need tighter controls.

"Until we have a globally coordinated approach that addresses the international nature of the problem, national and even regional level controls are unlikely to deliver the kind of reductions that are necessary to protect human health and the environment," Fowler adds. At the least, he believes, such a unified framework is needed for the whole Northern Hemisphere.

Setting this up will be an extremely challenging task. Environmental policy-makers are already focused on dealing with other problems such as carbon dioxide emissions.

The issue of ozone is recognised by some in governments, but there isn't yet any comprehensive international policy framework around it - for example, it is not covered by the UN Framework Convention on Climate Change.

Ozone is a global traveller and one of the most pervasive air pollutants

Fowler notes that the measures needed to combat ozone pollution are less drastic than those required to address climate change - many are relatively low-cost steps like making car manufacturers put catalytic converters in new vehicles.

And the costs of acting now are dwarfed by the risk of ignoring the problem. "At the moment these controls would be fairly easy to implement," Fowler explains. "But if we wait until the middle of the century when the problem will be much more serious it will be very expensive indeed to solve."

High levels of ozone already impose significant costs. Scientists estimate that in 2000, ozone pollution caused the loss of €6·7bn of crops in Europe. In North America in the late 1980s, losses were between $2bn and $4bn.

"If we let average ozone levels continue to grow, those numbers will get very large indeed by mid-century - and they will do this just as food shortages begin to be a major problem in themselves," says Fowler.

Choosing the right trees

VOCs are emitted by vegetation as well as human activities, and Fowler suggests planners should pay more attention to planting the kinds of tree with lower emissions.

Research carried out in 2002 by CEH and the University of Lancaster led to the publication of a brochure called Trees & Sustainable Urban Air Quality, which identified the best trees to plant to reduce urban pollution, as well as those that are less beneficial to air quality.

Maples, larches and alder trees fell into the former category, while willows, poplars and oaks could even make pollution worse.

Existing controls mostly apply only to land-based emissions; they need to be extended to cover other areas such as aviation. The shipping industry is a major source of ozone pollution, and presents a particular challenge to regulate because ships sail in international waters and can be registered almost anywhere in the world.

At present the industry is very lightly regulated; ships have long lifetimes and burn particularly polluting low-grade fuel. Fowler says this needs to change. If it doesn't, by 2020 ships will become a bigger source of ozone pollution in Europe than all land-based sources combined.

Policies to limit increases in near-surface ozone must be seen as an even higher priority

Tackling the problem of low-level ozone will be harder in a world affected by further climate change, as such changes will increase ozone production in the most polluted areas of the world.

And in a vicious circle, more ozone also means more climate change, because ozone is in itself a powerful greenhouse gas - scientists estimate it's third behind carbon dioxide and methane.

Ozone also harms plants' ability to absorb carbon dioxide, which could also lead to more CO2 in the atmosphere. This was the conclusion of another piece of research, produced by CEH, the Met Office and the University of Exeter earlier in the summer.

"We estimate that ozone effects on plants could double the importance of ozone increases in the lower atmosphere as a driver of climate change, so policies to limit increases in near-surface ozone must be seen as an even higher priority," said Professor Peter Cox of the University of Exeter, co-author of the earlier paper.