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 9-10 billion tonnes, while forecasts for 2030 range anywhere between 11 billion and 20 billion tonnes.
"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 organizations like the European Union's Emissions Database for Global Atmospheric Research (EDGAR) have overestimated China's emissions by as much as 14 percent 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 China's 2013 carbon emissions at 9.13 billion tonnes, below the EDGAR figure and 5.6 percent 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 tonnes of carbon for every tonne of coal produced, but the Nature authors, looking at around 600 samples from domestic mines, said the figure in China should be closer to 0.518 tonnes.
The study also estimated China produced 2.9 gigatonnes less carbon dioxide than previous estimates over 2000-2013, although Chinese government researchers said it might have overestimated lower-grade coal consumption over the period.
"More cheaper poor quality coal was supplied in 2013 as the industry was hit by lower demand in China," said Jiang Kejun of the Energy Research Institute, a government think tank. "It is not accurate to use emission factors for a single year to calculate China's emissions."
The last time Beijing gave an official number was for 2005, when emissions stood at "approximately" 7.47 billion tonnes. It is due to submit an updated number for 2010 next year.
While the study might ease some of the pressure on China, it still has a lot to do to rein in its spiraling greenhouse gas emissions, said Guan.
"Our estimates, which use a lower emission factor, don't change the fact that China is still the largest emitter in the world," he said.
"This will give some carbon space for the less-developed regions in China but it is not a game-changer. It won't disrupt China's mitigation efforts."
