According to national-level statistics, Chinese carbon emissions grew at a 7.5 percent annual pace between 1997 and 2010, largely from coal use. But according to provincial statistics, emissions grew at an 8.5 percent pace. That’s a puzzling discrepancy, and it’s not clear which figure is actually correct.

The researchers, Dabo Guan, Zhu Liu, Yong Geng, Sören Lindner and Klaus Hubacek, come up with two possible explanations for the gap. The first is that the data is simply messy, due to the fact that many smaller Chinese firms are burning coal without the national government knowing about it. That might be due to shoddy record keeping. Or it might be due to black-market activity — small inefficient coal mines and coal-washing mills that were shuttered by the government and then quietly reopened elsewhere. …

So it’s a mystery. But it’s a critical mystery. As the researchers note, it’s more difficult for scientists to model future climate change if they can’t be sure of how much carbon the world’s largest emitter is actually belching out. Right now, the International Energy Agency thinks we’re on pace to warm the planet by a staggering 6°C by the end of the century. But that’s based on China’s national-level data. What if the provincial-level data is correct and China’s emissions are actually 20 percent higher? Suddenly the picture looks even hotter.