As the former chair of the Consultative Group of Experts organized by the United Nations Framework Convention on Climate Change (UNFCCC) to help developing countries to produce carbon-emission inventories, I question the claim that China's emissions from coal have been overestimated (see Nature 524, 276; 2015 and Z. Liu et al. Nature 524, 335–338; 2015).

The accuracy of estimates depends largely on emission-factor estimates for the coal China uses (emission factor is the amount of carbon oxidized per unit of fuel consumed). For example, Liu and colleagues report emission factors that were estimated from the average carbon content of a range of high-quality to low-quality Chinese coal types. They write that these emission factors are 40% below the default values recommended by the 2006 guidelines of the Intergovernmental Panel on Climate Change (IPCC). However, I find their comparison flawed because the IPCC factor they use derives from coking coal, which contains more carbon and so has a higher emission factor than an 'average' coal type.

Furthermore, the conclusion by Liu et al. that China's fossil-fuel use in 2000–12 exceeded official figures by 10% seems incompatible with the authors' estimated emissions being 12% lower than those calculated by the Chinese government. The official team's higher estimate was based on information from China's coal-quality database and from coal-trading contracts.

Such ambiguities call for clear resolution so that estimates of China's emissions are accurately conveyed.