Confusion over the kinds of coal being burned in China's power plants may have led to the country's carbon emission levels being overestimated by as much as 14 percent, researchers say.

Scientists testing the carbon emissions of the kinds of coals being burned in China found them to be 40 percent less than what was used to make existing calculations of the country's overall carbon emissions, based on a globally averaged formula, a study says.

That was the finding after analyzing the carbon content of coal from more than 4,000 coal mines in China and laboratory tests of hundreds of samples, the researchers report in the journal Nature.

Rapid expansion of coal-fired power plants has been a driving factor in China's economic expansion in the last 15 years.

However, not all coal is the same all over the world, the international research team points out.

"For most of the developed countries, coal has been comprehensively washed but in China the process is not so comprehensive," says study lead author Zhu Liu of Harvard University.

"Basically, the coal contains higher ash; more ash means there is less carbon," he explains. "If we convert the same amount of coal, we get fewer carbon emissions. That's why we get a lower level than previous estimations."

It marks the first time that fuel quality – in this case, the carbon content of coal used – has been taken into account in estimating total emission figures, the researchers say.

"China is the largest coal consumer in the world, but it burns much lower quality coal, such as brown coal, which has a lower heat value and carbon content compared to the [higher grade bituminous] coal burned in the U.S. and Europe," says study researcher Dabo Guan of the University of East Anglia in England.

There remains a lot of work to do, the researchers admit, so that estimates of carbon emission can be refined to give an accurate picture of carbon emission levels and improve climate predictions based on it.

However, even the initial improvement in the estimates of carbon emissions in China – one of the world's biggest users of coal – is important, the researchers say.

"The findings do have very significant global implications as China accounts for one-third of global total emissions," Zhu notes. "If we reduce China's emissions [estimates] by 15 percent, we get a 5 percent less global total."