Let’s go ahead and state the obvious: It will be impossible to hash out any sort of global agreement on climate change if we can’t even agree on how much carbon-dioxide different countries are actually putting into the air.

Yet the data on this can be surprisingly unreliable — particularly in the case of the world’s biggest carbon emitter, China. A new paper in Nature Climate Change finds that there’s a real mystery as to how much carbon China is actually emitting. The national-level statistics say one thing. The provincial-level statistics say another. And the gap between the two numbers came to about 1.4 gigatons in 2010 — a staggering amount, equivalent to all the carbon-dioxide that Japan put into the air that year.

Dashed line represents provincial carbon data.

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.

Alternatively, there’s the possibility that government officials are massaging the data. Local politicians, for instance, are promoted based on their economic performance. So they have an incentive to overstate growth — and then revise the emissions data so that it matches. (The researchers note that the “carbon gap” between national and local data lines up pretty well with a similar “GDP gap” in the statistics.) But that’s not all. Satellites have also turned up evidence that the national government in Beijing may be understating some coal activity — possibly in order to claim success on its environmental laws.

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.

And, of course, it’s far more difficult to agree on a global treaty for emissions cuts — or for China to build its own domestic cap-and-trade system — if the data’s this sloppy.

Related: U.S. cut its carbon emissions in 2011, but China erased the gains.