The widening search for reliable economic data on China may soon focus on the country's enormous output of coal.
Since early June, reports have sounded alarms about growing mountains of coal at China's transfer points as stockpiles swell following the sag in economic growth to three-year lows.
Inventories at major power plants hit a record high of 93 million tons after climbing over 48 percent, giving generators enough coal for 28 days of use, the National Development and Reform Commission (NDRC) said on June 14.
The stockpile at Qinhuangdao port, the main transfer center in northern Hebei province, reached 9.46 million tons on June 18, just shy of capacity, the official Xinhua news agency said.
Steam coal prices have dropped 23 percent from a year earlier to an average of 641 yuan ($100.35) per metric ton, the English-language China Daily reported.
The China Coal Transportation and Distribution Association estimated the country's total stocks had risen to 300 million tons by the end of June, forcing many mines in Inner Mongolia to halt production, according to United Press International.
With gross domestic product (GDP) growth in the second quarter dipping to a three-year low of 7.6 percent, the rise in first-half power consumption has weakened to 5.5 percent from 12.2 percent a year before, the State Electricity Regulatory Commission (SERC) said.
That could help explain the growing heaps of coal, as the country's mines failed to slow down in time to keep pace with the trend.
But nearly all of the numbers have come into question amid concerns that the economy may be in worse shape than official statistics suggest.
On June 23, The New York Times raised doubts about the official power figures, citing the rising coal pile as a sign that electricity output may be overstated.
Inflation of the power numbers could mean that GDP was overstated, as well.
Economists have long looked to power use as a reliable indicator of growth, but China's long struggle with statistical fraud has raised suspicions.
Most recently in April, National Bureau of Statistics (NBS) chief Ma Jiantang said local officials were continuing to rig economic data by ordering companies to file exaggerated production and profit reports.
Now, first-half figures from the China National Coal Association (CNCA) may raise more concerns that there may be no way of knowing what is really going on.
The CNCA numbers reported by Platts news agency on July 20 appear to show that coal supplies and consumption are roughly in balance, raising the question of why stockpiles are growing at all.
Domestic coal production rose 5.6 percent to 1.91 billion tons, while net imports stood at 58.54 million tons. Consumption was said to be 1.97 billion tons, or about 1 million tons more than supplies.
The seeming contradiction with the high stockpiles could be explained by increased hydropower generation, which rose 12.7 percent during the period due to heavier rainfall, according to SERC. But coal use by the power sector still grew 2.7 percent to 990 million tons, CNCA said.
The accounting may be difficult, or more simply explained if the numbers are just plain wrong.
Kevin Jianjun Tu, senior associate in the energy and climate program at the Carnegie Endowment for International Peace in Washington, said the coal reports have not been reliable.
In a study of China's coal data over the last decade, Tu has found that provincial totals for consumption have routinely exceeded NBS national figures by 500 million tons or more per year.
While the NBS has taken steps to resolve the discrepancies, "gray market" coal trade in the provinces has represented as much as 39 percent of officially reported production in the past.
"At the end of the year, the NBS always needs to adjust the figures," Tu said in an interview.
Since China burns four times as much coal as the second-place United States, the margin of error in its reporting could be huge, equaling the volume of major producers like Australia or Indonesia.
The gap between official and actual coal data may be so large as to make any explanation of changes in stockpiles unverifiable.
Tu said that some coal producers have strong motivations to keep actual output levels off the books.
"Producers, especially private producers, don't want to pay taxes, or they may not have a government permit to produce," he said. "That's part of the reason, even for state-owned mines, because if they report all of their production, they have to pay more tax."
Tu doesn't believe the central government has any interest in manipulating coal data, but it has been unable to change illicit practices that have taken place for years.
The inaccuracy of the data makes it questionable to cite rising coal stocks as an economic indicator and hard for officials to manage energy policy.
"It's extremely difficult for them to figure out what's going on," said Tu.
Although the NBS has been dealing with issues of data fraud for a decade or more, the problems have drawn increasing attention as China's influence on world markets grows.
One report this month by the CNBC financial news network compared the inaccuracy of China's economic reporting to the scandal over the rigging of "Libor" benchmark interest rates under the headline, "Think Libor's Bad? Fake China GDP Is Worse."
CNBC quoted financial analyst and author Larry McDonald as saying China's real GDP growth was much slower than the second-quarter official figure of 7.6 percent, judging by power use and other "harder-to-fudge" indicators.
"Is Chinese GDP the new Libor?" McDonald asked. "More and more investors are starting to question the Chinese math on GDP."