By Katharine Sanderson

Aerosols' complicated influence on our climate just got more threatening: they could make methane a more potent greenhouse gas than previously realized, say climate modellers.

Drew Shindell, at NASA Goddard Institute for Space Studies, New York, and colleagues ran a range of computerized models to show that methane's global warming potential is greater when combined with aerosols -- atmospheric particles such as dust, sea salt, sulphates and black carbon.

The International Panel on Climate Change (IPCC) and treaties such as the Kyoto Protocol assume methane to be, tonne-for-tonne, 25 times more potent than carbon dioxide at warming the planet. But the interaction with aerosols bumps up methane's relative global warming potential (GWP) to about 33, though there is a lot of uncertainty around the exact figure. In the Kyoto Protocol, GWPs are used to govern emissions trading. "This study is saying those GWPs should be revisited because they're leaving out an important thing," says Shindell.

Shindell also thinks climate policy-makers need to pay much more attention to restricting short-lived pollutants, such as methane, carbon monoxide, volatile organic compounds (VOCs) and aerosols. This could create significant changes in the local and global climate quite quickly, he says, whereas the effects of efforts to reduce emissions of long-lived carbon dioxide will not be seen for many years. "The short-lived things are really powerful," he adds. The work is published in Science.

The increased influence attributed to methane is interesting, says Frank Dentener, from the European Commission's Institute for Environment and Sustainability, Ispra, Italy. He points out that cutting methane emissions often doesn't cost anything and can actually end up making money -- by collecting the gas to sell, or by saving emissions in making a process more efficient and cheaper.

Mixed-up models

Methane, aerosols and other short-lived pollutants have a complicated chemical relationship, only some of which Shindell's models could capture. For example, methane leads to increased formation of ozone in the troposphere, which can reduce agricultural yields. It is also eventually oxidized to carbon dioxide; or by other chemical reactions can form water vapour - also a greenhouse gas - in the stratosphere.

In yet another example, methane's reaction with hydroxyl reduces the amount of that chemical available to create cooling sulphate aerosols.

The models don't yet completely take all the processes into account. But simply trying to couple ozone chemistry and aerosol chemistry like this is an advance, says Dentener. "Instead of looking at single components they look in much greater details at what emissions of these components will do to a whole sequence of things."

Shindell says that future long-term climate models should begin to include more of these local secondary processes. "We know these things happen in the atmosphere", says Shindell. "The current strategy of assuming [their effects] are zero is a poor choice."

Aerosols and short-lived gases aren't totally ignored. "The timing of this paper is excellent", says Greg Carmichael, a climate scientist at the University of Iowa in Iowa City. "There is growing awareness among the policy makers and the scientific communities that the coming period represents a key and important opportunity to link air and climate concerns." Even as recently as last week, scientists and policy makers met in Gothenburg, Sweden, at a workshop to discuss intermediate climate policies.

Dentener cautions that the calculations involved to work out new GWPs are "rather tricky". He would like to see other modellers tackle the same problem and reach a scientific consensus. But he adds that policy makers are starting to realize that there must be a balance between long-term and short-term strategies.

The negative and positive effects of air pollutants must be taken seriously, says Almut Arneth of Lund University in Sweden, who also writes in Science this week. She says most assessments suggest that the cooling effect of sulphates at the moment outweighs the warming effect of black carbon. So legislating for the removal of air pollutants, particularly sulphates, would cause a rapid warming of the climate. "If we want to start developing really successful climate policies we've got to look at air pollution as well", she says.

Shindell calls for the IPCC to look at "not just the climate but the whole ecosystem and health impacts of dealing with these pollutants". This could help in both the near and long term, he says. "I'm hoping the next round of IPCC will be better. I still don't see a lot of emphasis on near-term solutions to climate."