Jeff Swensen

A team of Harvard researchers recently found, based on new analysis, that China is putting out roughly 14% less carbon than previous estimates suggest.

The discrepancy comes from a consideration of not just the amount, but the type of coal China burns. The researchers found that China burns a lot of low-quality coal, which emits less CO2. The study marks the first time that coal quality is considered in calculating emissions. In the paper, published in Nature, authors Zhu Liu, Dabo Guan and others explain their methodology:

We adopt the ‘apparent-consumption’ approach, which does not depend upon energy consumption data (that previous studies have shown to be not very reliable). Instead, apparent energy consumption is calculated from a mass balance of domestic fuel production, international trade, international fuelling, and changes in stocks, data about which are less subject to ‘adjustment’ by reporting bodies and accounting errors… furthermore, this approach allows imported and domestically produced fuels to be tracked separately so that appropriate emission factors can be applied to these fuels.

By teasing out the different production components and evaluating local coal from China’s largest coal-mining regions, the authors learned that earlier estimates overstated China’s carbon output by 2.9 billion tons, or around 14%, from 2000 to 2013. The revision could change global emissions estimates, which are already tough to measure, by around five percent, Liu said in an email. 

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In some ways, that difference doesn’t really matter. Nature points out that the lower estimates don’t alter China’s position as the world’s greatest carbon polluter, by far. In a phone interview with Fusion, Guan emphasized that the findings shouldn’t take China off the hook.

“Really,” he explained, “our scientific contribution is trying to provide a more accurate estimate for Chinese emission trends. A more robust, more accurate baseline for climate science, from the policy perspective.”

Guan adds that he hopes his study will make China a precedent-setter in terms of accuracy in tracking emissions. But others think Guan’s findings may not be the final world on Chinese emissions. Nature reports:

Scientists are still waiting for the government to release revised estimates of energy production, including imports and exports, over the past decade. Liu says that his team’s estimates are unlikely to change when the latest data are released later this year, but [climate policy researcher Glen] Peters says that the figures may need to rise by as much as 7%.

Tracking carbon emissions is key to keeping tabs on the Earth’s carbon intake — which in turns affects our understanding of climate change. Nature explains that as the emissions output change, scientists' understanding of the carbon cycle falls apart. If China is emitting less carbon than we thought, that means carbon intake models—which look at how much carbon is being absorbed by oceans and forests—are wrong. From Nature:

For comparison, the cumulative reduction in Chinese emissions outlined in the study — roughly 2.9 billion tonnes from 2000 to 2013 — is larger than the estimated amount of carbon that the world’s forests pulled out of the atmosphere from 1990 to 2007. That presents a problem for researchers who study the carbon cycle. “We can easily go back and retroactively adjust Chinese carbon-emission estimates,” says Ashley Ballantyne, a climate scientist at the University of Montana in Missoula. “Unfortunately, we cannot go back and adjust all the previous studies on the global carbon cycle and their conclusions based on the previously biased emission estimates.”

Andy Jacobson, CarbonTracker project lead research at University of Colorado Boulder and NOAA Earth System Research Laboratory, told Fusion in an email that the emissions revision will affect carbon intake modeling. "It will have a ripple effect, causing a revision in our estimates of carbon sinks on land and in the ocean," he said. Carbon sinks are areas that absorb more carbon than they emit, like oceans and forests. Jacobson continued:

We rely very heavily on self-reported economic activity accounts like those being scrutinized in this study, in large part because there aren't enough direct observations of atmospheric composition to unambiguously quantify a given country's emissions.

Jacobson explained that people who track carbon use historical data to figure out how the Earth reacts to carbon pollution.

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"Our historical analysis of atmospheric CO2 provides an important observational constraint on how terrestrial and marine ecosystems have reacted to elevated CO2, ocean warming and acidification, drought, and other weather anomalies," Jacobson said. "This constraint is used to develop and tune carbon cycle models used in climate predictions."

So, he says, revisions affect climate change predictions, indirectly. "Will the oceans and land continue to remove half of our fossil fuel emissions from the atmosphere, or are these sinks going to saturate?"

We do know for certain that China's burning less coal in 2015 than in recent years. So that's something.

Danielle Wiener-Bronner is a news reporter.