China has emitted significantly less carbon since 2000 than previously estimated because of erroneous assumptions about the quality of the country's coal, a study released today claims. That may sound like great news—but it doesn't mean that the world is warming at a slower rate or that the need to reduce emissions has become less urgent, the researchers warn. China remains the world's largest carbon emitter, and the new study doesn't make its target of reversing its growth in emissions by 2030 any easier to attain.

The main benefit of the study, based on new analyses of the carbon content of the country's coal, is that "it provides a baseline for future emission policies," says Dabo Guan, a co-author of the paper and a climate change economist at Tsinghua University in Beijing and the University of East Anglia, in Norwich, U.K.

"There is no doubt that the authors have made a significant step forward" in characterizing China's emissions, and the country's effort to improve the quality of its climate data "is very welcome," says Josep Canadell, an earth system scientist at Australia's Commonwealth Scientific and Industrial Research Organisation in Canberra.

Key to the new estimate are so-called emissions factors, which are derived from the carbon content, heating value, oxidation rate, and other variables that allow carbon emissions to be calculated for the amount of a given fuel consumed. In the absence of better data, emissions factors for China's coal were so far assumed to be similar to those of coal mined in the United States and Europe, Guan says. But the coal actually used in China is of lower quality, with a lower carbon content. 

The team was able to derive new emissions factors based on data from Chinese government programs that analyzed nearly 5000 coal samples taken throughout the country. "It's the first time ever for someone to take into account, systematically and comprehensively, the quality of the coal actually used in China," Guan says. They also made new estimates of emissions from imported coal, oil, and gas, and from cement production. In a paper published online today in  Nature, they conclude that between 2000 and 2013, cumulative carbon emissions for the energy and cement sectors were about 2.9 gigatons—or 13% less than previous estimates, which pegged China’s carbon emissions at something over 22 gigatons. (This paper uses carbon emissions rather than the more frequently cited carbon dioxide emissions.)

It is not all good news, however. Relying on low quality coal "is a bad thing," Guan says. More of it must be burned to produce a given amount of heat, and that increases the release of particulates and other pollutants. And the new number has little effect on the big picture of climate change. The time frame in which China's emissions were overestimated "is too short to have a cumulative impact on climate scenarios," says Zhu Liu, the lead author and a climate change specialist at Harvard University's John F. Kennedy School of Government.

Last November, China pledged to halt the growth in its emissions by 2030. That target has been "applauded by the international community given China's emissions have been growing at rates of 5% to 8% over the past decade and a half," says Canadell, who is also executive director of the Global Carbon Project, an international consortium of scientists studying the global carbon cycle. Not all climate scientists agree, however; the Climate Action Tracker, an alliance of four European research groups, rates the targets "inadequate."

But whatever you think of China's ambitions, the lower estimate of past emissions won't make fulfilling them any easier. "What is important is the rate of present growth and how this growth goes down to zero by 2030 at the latest," Canadell says. In fact, the research team used a new approach to conclude that between 2000 and 2010, China's energy consumption grew by 9.9% annually, faster than the 8.8% indicated by national statistics. This will make getting emissions growth to zero "extra challenging," Canadell says.

Regardless of past totals, the focus should be on reducing emissions, says Wang Yi, deputy director-general of the Institute of Policy and Management at the Chinese Academy of Sciences in Beijing, who was not involved in the study. "This research can help China’s government form policies that more accurately target emission reductions," Wang says. (news.sciencemag.org)