International organisations could be overestimating emissions from China, the world’s biggest producer of greenhouse gas because of problems in the way they calculate their data, said a study published by Nature.

With talks on a new global climate accord set to take place in Paris in December, China, the world’s biggest producer of climate-warming gas, has promised to bring emissions to a peak by around 2030, but it remains unclear how much CO2 China is actually producing and how much it will produce in 15 years.

While there is no official figure for Chinese carbon emissions last year, estimates stand at around nine to 10 billion tons, while forecasts for 2030 range anywhere between 11 billion and 20 billion tons.

“Without an accurate baseline, any target will become a number-crunching game,” said Dabo Guan, chair of Climate Change Economics at the University of East Anglia, and one of the authors of the Nature study. The paper said organisations like the EU’s Emissions Database for Global Atmospheric Research (Edgar) have overestimated China’s emissions by as much as 14 per cent by using default conversion rates that should not apply in China.

“The main difference in our paper is for the first time we have taken fuel quality into consideration, which is missing from other estimates,” said Guan.

Taking into account China’s lower quality coal, the study calculated its 2013 carbon emissions at 9.13 billion tons, below the Edgar figure and 5.6 per cent lower than an estimate in oil major BP’s statistical yearbook.

The Intergovernmental Panel on Climate Change (IPCC) recommends a default ‘emission factor’ of 0.713 tons of carbon for every ton of coal produced, but the Nature authors said the figure in China should be closer to 0.518 tons.