Calculating how much climate-changing carbon dioxide a country pours into the atmosphere is not an exact science. It is especially so for China, the world's biggest emitter and a nation where such statistics are considered by outsiders as unreliable. A foreign study of coal used for electricity and industry has revealed that levels have been overestimated by up to 14 per cent, giving reason for reassessment and causing uncertainty about data from other big polluters like India and Indonesia. But the finding does not mean Beijing can lessen efforts to produce cleaner energy; the world continues to be falling far short of targets to stop temperatures from rising.

China regularly produces energy statistics, but it is less forthcoming with figures on carbon emissions and there are concerns about accuracy. International calculations for China have until now been based on the average for the type of coal used in the EU and US. Chinese-mined coal was found by the team of international researchers to be of a lower grade, burning not as efficiently and as a result, causing less pollution. But the nation uses far more than any other country and has been the world's biggest emitter of greenhouse gas since 2007.

A commitment has been made to help the world attain the target of preventing the average global surface temperature from rising more than 2 degrees Celsius. As the producer of one-third of the world's greenhouse gases, China has a vital role to play in landmark UN climate talks in Paris in December. It has pledged to cut emissions per unit of GDP by between 60 and 65 per cent from 2005 levels and aims for the cuts to peak by 2030. But such promises have limited meaning unless calculations are accurate and transparent.