Cars, trains, planes, power stations and forest fires: along with greenhouse gases they also throw out millions of tiny particles. Some, like the black carbon that makes up soot, have a warming impact by absorbing heat and reducing reflectivity when the particles land on snow or ice. Others, such as sulphate aerosols, cause cooling by reflecting away heat or by encouraging the formation of clouds, which also reflect away heat.

Over the coming decades, stringent emissions scenarios mean that our skies are set to become much cleaner, but ironically the lack of aerosols will enable more heat to reach Earth's surface, further boosting global warming. Some scientists calculate that the excess warming due to cleaner skies could be as much as 1°C by 2030. So should we be easing off on the big clean-up?

Until now most climate models have considered aerosols and greenhouse gases in tandem, so it's been hard to tease out the impact of aerosols alone. Nathan Gillett and Knut Von Salzen, both from the Canadian Centre for Climate Modelling and Analysis, have tackled this issue by running separate simulations for aerosols and greenhouse gases. In their model runs, between 2006 and 2100, they considered a variety of scenarios, including one with very stringent emissions controls.

In the case of the most stringent emissions scenario, the pair found that cleaner skies will add 0.4–0.7°C of warming by 2100. Much of this is during the early decades; between 2000 and 2040 aerosol reductions will account for around one third of global warming under this scenario. But greenhouse gases remain the bigger problem. "In the near term, as in the long term, greenhouse gas increases are the dominant driver of warming," the scientists wrote in their paper, published in Environmental Research Letters (ERL).

But unlike greenhouse gas-induced warming, the warming due to cleaner skies won't be evenly spread. Aerosol-reduction-induced warming is expected to be largest in the regions where aerosols are currently causing the most cooling – in particular over much of Asia.

But even if the clean-up is going to make some countries significantly warmer in the short term, there are still many benefits. "Aside from their effects on climate, many aerosol species have negative effects on human health as well as on the environment," said Gillett. "The costs and benefits of these effects need to be considered to determine the best course of action."

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