The grey, sulphur-laden skies overlying parts of Asia have a bright side — they reflect sunlight back into space, moderating temperatures on the ground. Scientists are now exploring how and where pollution from power plants could offset, for a time, the greenhouse warming of the carbon dioxide they emit.

A new modelling study doubles as a thought experiment in how pollution controls and global warming could interact in China and India, which are projected to account for 80% of new coal-fired power in the coming years. If new power plants were to operate without controlling pollution such as sulphur dioxide (SO2) and nitrogen oxides (NOX), the study finds, the resulting haze would reflect enough sunlight to overpower the warming effect of CO2 and exert local cooling.

But this effect would not be felt uniformly across the globe and would last only a few decades. In the long run, CO2 would always prevail, and the world could experience a rapid warming effect if the skies were cleaned up decades down the road.

"The paper highlights the fundamental inequity and iniquity of anthropogenic climate change: 'enjoy now and make others pay later'," says Meinrat Andreae, an aerosol expert at the Max Planck Institute for Chemistry in Mainz, Germany, who was not involved in the work. In fact, he says, dirty coal plants could be seen as "a very primitive form of geoengineering".

The study, which is under review at Atmospheric Chemistry and Physics, builds on a well-established idea. Global temperatures were relatively stable in the decades leading up to the 1970s, even as fossil-fuel consumption shot up. Then industrialized countries began curbing SO2 and NOX to reduce acid rain and protect public health — and temperatures increased rapidly. The latest work, led by Drew Shindell at NASA's Goddard Institute for Space Studies in New York, looks at how the climate effects of air pollutants and greenhouse gases could play out over time and geography.

The study analysed a suite of scenarios for the years 2000 to 2080, mixing annual rates of power-plant growth from 5% to 10% with various controls on SO2 and NOX pollution. SO2 is a precursor to sulphate aerosols and dominates the cooling effect, which varies depending on when plants adopt pollution controls. The sooner controls are put in place, the sooner the warming potential of CO2 kicks in.

In one scenario assuming rapid growth in coal power with pollution controls phased in between 2040 and 2060, the effect of aerosols from the plants outweighs the effect of their CO2 until the year 2046, when the CO2 effect catches up and then overtakes aerosols. But the effect isn't uniform: the SO2 emissions produce a net cooling across much of the Northern Hemisphere. The Southern Hemisphere and the Arctic see fewer aerosols and, because CO2 has global effects, they exhibit net warming.

How quickly China and India will move to clean up their coal emissions is unclear. In the past few years China has been aggressively installing SO2 scrubbers on many of its power plants in an attempt to improve air quality and protect public health. But some experts have questioned whether those scrubbers are being used properly — or even turned on.