In recent years, there's been a fair bit of optimism on the topic of China and global warming. Yes, China is the world's largest emitter of carbon-dioxide. But the country has also pledged to curtail its coal use in an attempt to wrestle down its air pollution. So China should get greener as its citizens demand cleaner air, right?

Not necessarily. A striking piece in this week's issue of Nature Climate Change points out that China is currently building a series of big new synthetic gas plants to turn coal into natural gas. The idea is that natural gas will burn more cleanly in urban power plants and help detoxify the air in major cities like Beijing.

But there's an unexpected side effect here: The synthetic gas plants themselves are extremely energy-intensive and could lead China to produce far more carbon-dioxide emissions in the long run — heating the planet even more drastically.

Turning coal into natural gas

The idea of converting coal into natural gas was pursued by the United States during the oil crisis of the 1970s, when the Department of Energy was subsidizing various technologies to achieve energy independence.

Only one large plant ever got built in North Dakota, but now China is grabbing the torch. The Chinese government has already approved nine massive synthetic natural gas plants that will, if finished, produce some 37.1 billion cubic meters of natural gas each year.

In their essay for Nature Climate Change, Chi-Jen Yang and Robert B. Jackson of Duke University point out that even if only some of these plants came online, the environmental consequences could be large. Because the plants themselves are so energy-intensive, using synthetic natural gas to create electricity actually leads to 36 percent to 82 percent more greenhouse-gas emissions than simply burning coal.

If all nine synthetic gas plants come online, Yang and Jackson estimate, they would produce 21 billion tons of carbon-dioxide over 40 years just by themselves. (To put that in perspective, the entire nation of China produced 7.7 billion tons of carbon-dioxide in 2011.) And, while it's possible that these plants could eventually try to capture and sequester the carbon they produce, that technology is still in its infancy.

And China ultimately has plans for 40 synthetic gas plants in all. "Under such a scenario," the authors write, "China will inevitably struggle to reduce its future GHG emissions."

Now, the synthetic gas plants themselves would largely be located in inner Mongolia or Xinjiang, far out west and away from the major population centers. So the power plants near China's cities really could curtail local air pollution like sulfur dioxide and soot — natural gas does burn far more cleanly than coal in this regard. But it would come at the cost of vastly more emissions out west and accelerated global warming.

A fracking alternative?

So is there another option here? Well, there's always low-carbon energy. But China also has massive reserves of natural gas locked away in shale rock across the country. And natural gas companies have been trying to figure out how to harness fracking techniques and unlock that gas.

As Yang and Jackson show, fracking for shale gas in China would be much less drastic from a climate-change perspective than building those big new synthetic-natural gas (SNG) plants: Fewer carbon emissions, fewer toxins, less water needed:

But, so far, China has been struggling to tap that shale gas. As my colleague Steven Mufson reported earlier this year, China has yet to produce any gas from shale despite dozens of exploratory wells. Unlike in the United States, oil companies can't just buy up mineral rights from private individuals — there's a byzantine government bureaucracy thwarting development. The rather unique geology of China's shale and dearth of water in places has also posed a challenge for drilling projects.

As a result, natural gas currently provides just 4 percent of China's total energy, compared with 25 percent in the United States. The Chinese government has said it would like to boost that ratio in order to tamp down on coal pollution that's shaving years off people's lives and sparking protests in the cities. But if fracking develops too slowly (or not at all), the government may turn to dirtier synthetic gas. And once those syngas plants are built, they're almost certain to operate for decades.

"At a minimum, Chinese policymakers should delay implementing their [synthetic natural gas] plan to avoid a potentially costly and  environmentally damaging outcome," write Yang and Jackson. "An even better decision would be to cancel the program entirely."