HONG KONG • Scientists may have been overestimating China's emissions of carbon dioxide, the main greenhouse gas driving global warming, by more than 10 per cent, because of inaccurate assumptions about the country's coal-burning, according to a study just published.
The study's finding, published on Wednesday in the journal, Nature, does not mean that the total level of greenhouse gases in the atmosphere is any lower than scientists had thought. That accumulation is measured independently. Rather, the finding may affect discussions of how much responsibility China bears for global warming, compared with other nations.
"This doesn't change the fact that China is still the largest emitter in the world," said Professor Dabo Guan, who is one of the paper's two dozen authors, in a telephone interview from Beijing. He is a professor of climate-change economics at the University of East Anglia in England. "But it shows we need to know a more accurate baseline for emissions, not only for China but also for the other emissions giants."
The study looked in detail at the coal used as fuel in China and found that it is generally less rich in carbon and is burned less efficiently than scientists had assumed. That means that burned coal yields less carbon dioxide than had been thought (as well as less energy).
China's proposed commitments to curtail its emissions of greenhouse gases are crucial to a new international agreement on global warming, which governments hope to reach in Paris late this year.
"We measured thousands of samples of coal from mines across China and found that the carbon content of the coal being burned in China is actually much lower than what has been assumed in previous estimates of emissions," said greenhouse-gas scientist Steven Davis, who is from the University of California, and one of the authors.
Estimating a country's carbon dioxide emissions entails some scientific detective work. Researchers start with information about fossil fuel consumption, and then assess how much carbon is contained in those fuels and what fraction of that carbon is actually combusted and ends up in the atmosphere.
China does not publish official data on annual greenhouse-gas emissions, so "international organisations have to make larger assumptions" than are required for other major countries, said another author of the study, senior researcher Glen Peters at the Center for International Climate and Environmental Research-Oslo.
Those assumptions often rely on coal carbon content and combustion data collected in the United States and Europe, said postdoctoral research fellow Zhu Liu at Harvard University, who is another of the paper's authors. "This is the first time we've applied real measurement of the coal quality on a national scale in China," Dr Liu said.
The researchers found that, on average, each lump of coal in China was 40 per cent less potent as a source of carbon dioxide emissions than the default figure used for coal by the United Nations' scientific panel on climate change.
NEW YORK TIMES