"The whole point of trying to put all the greenhouse gases on a common scale (using their global warming potential, which is their impact relative to the impact of carbon dioxide) is to allow for greater economic efficiency by letting the market choose the least expensive way to reduce the total climate impact," researcher Drew Shindell toldenvironmentalresearchweb. "By having the wrong value when comparing methane with carbon dioxide or other gases, we will not get the maximum value for our money."

The IPCC's fourth assessment report did not include the effect of gas–aerosol interactions when assessing the global warming potential of short-lived gas emissions, apart from the indirect effect of NOx emissions on nitrate aerosols.

Shindell and colleagues used NASA's Goddard Institute for Space Studies Model for Physical Understanding of Composition-Climate Interactions and Impacts (G-PUCCINI) model to analyse the effect of gases such as methane, carbon monoxide, volatile organic compounds and ammonia emissions on concentrations of sulphate and nitrate particles.

The gases tend to react with hydroxyl molecules that would otherwise have reacted with NOx and SO2 to form nitrate and sulphate aerosols. While aerosols such as black carbon can have a warming effect, both sulphate and nitrate aerosols tend to cool the Earth, both by scattering incoming light and by prolonging the lifetime of clouds.

"More methane means less hydroxyl, less sulphate and more warming," said Shindell.

According to the model, methane decreased the global burden of hydroxyl by 26% and the global burden of sulphate by 11%, while carbon monoxide decreased hydroxyl by 13% and sulphate by 9%. This means that the global warming potential of carbon monoxide is also significantly larger than previous estimates.

NOx, on the other hand, increased the global burden of hydroxyl by 18% and that of sulphate by 13%, giving the gases a net cooling effect when its aerosol effects are taken into consideration. Previously the gases have been associated with the formation of ozone, a greenhouse gas.

"At present emissions of CO, VOCs, SO2 and NOx are all regulated solely for air-quality purposes, but as concern over climate change increases more and more effort is being put into trying to understand the climate implications of air-quality policies, and so we suggest that such analysis should include the gas–aerosol interactions," said Shindell.

According to Shindell, emissions such as methane, CO and VOCs that lead to both air pollution and warming of the climate are really quite powerful and so offer an opportunity to limit near-term climate change – because they don't last in the atmosphere too long – while at the same time cleaning up the air. "To me, this is a very encouraging result," he said. "It doesn't eliminate the need to deal with carbon dioxide, as that's still the most important in long-term climate, but carbon dioxide's long lifetime and slow response time means we can't do much to change near-term climate via carbon dioxide, so targeting the warming air pollutants is a good parallel path to pursue."

Now the team would like to include more realistic ecosystem models in their analysis to examine the interaction between air quality, ecosystems and climate. Ecosystems can produce secondary organic aerosols and interact with air pollutants in a complex manner.

"We are also examining the climate impact of specific economic activities to see the net effect when multiple gases and aerosols are emitted, as this is the case for many activities (e.g. vehicles, power plants) so that the net effect when cooling and warming agents are both emitted isn't obvious without detailed calculations," said Shindell. "These will hopefully make attribution of climate forcing to emissions even more useful to policy makers."

The last word goes to Shindell's colleague Gavin Schmidt. "The bottom line is that the chemistry of the atmosphere can get hideously complicated," he said. "Sorting out what affects climate and what affects air quality isn't simple, but we're making progress."

The researchers reported their work in Science.