To get a sense of how hard it is to measure greenhouse gas emissions in China, it pays to visit the Deqingyuan poultry farm on the outskirts of Beijing where streams of chicken manure are piped from wooden sheds to an industrial gas digester that rises above the ground like a tethered balloon.
Turning waste into kilowatts qualifies Deqingyuan for valuable carbon credits under a UN-backed scheme known as the Clean Development Mechanism. The digester turns all that chicken slurry into natural gas, powering a nearby electricity station and supplying fuel to 39 surrounding villages.
Yet calculating those emissions requires a 54-page, UN-certified rulebook, a methodology that factors everything from the amount of methane removed from the manure to local temperatures and animal weight to come up with a figure.
And that cumbersome process can mean Deqingyuan's emissions savings vary wildly - sometimes by as much as 20pc.
"I don't know how they calculate the figure but there were many researchers from universities who came to assess it," said Vincent Wei, a marketing manager at Helee Bio-Energy Technology, which built the plant.
Precise data collection is a tricky business, as the Volkswagen scandal over discrepancies between the company's emissions claims and the real world performance of its engines has shown.
But getting accurate data is crucial for governments seeking a global climate accord in Paris this December. Negotiators say that, to succeed, any agreement must be built upon "measurable, reportable and verifiable" statistics in order to assess whether countries are on track to meet emissions targets.
And getting a better grasp of the right numbers is particularly crucial in the case of China, which is widely assumed to be the world's largest carbon emitter. China's energy use is so great that even minute errors in data can translate into a difference of millions of tonnes of emissions.
No one currently knows how many tonnes of carbon China emits each year. Its emissions are estimates based on how much raw energy is consumed, and calculations are derived from proxy data consisting mostly of energy consumption as well as industry, agriculture, land use changes and waste.
Many outside observers view the accuracy of those figures with scepticism.
"China's contribution (to the global climate plan in Paris) is based on CO2 emissions but China doesn't publish CO2 emissions," said Glen Peters, senior researcher at the Centre for International Climate and Environmental Research in Oslo. "You're left in the wilderness, really."
Demands for better data played a major role in the failure of the 2009 Copenhagen conference, when China and several developing nations baulked at providing the rest of the world with detailed data, claiming it would be an intrusion on their sovereignty.
The last time Beijing produced an official figure was in 2005, when it said emissions stood at "approximately" 7.47bn tonnes. And while it has promised that emissions will peak by 2030 at the latest, experts say the statistical uncertainty is so great that forecasts on what that peak means can vary from 11bn to 20bn tonnes a year.
That margin is greater than the entire annual carbon footprint of Europe. (Reuters)