Since peaking in 2005, US domestic energy CO2 emissions have fallen by 8.6 percent. A new report asserts that up to half of this reduction may be down to "energy switching," as generators switch from coal to shale gas (partly on cost grounds), which emits about half the CO2 when burned. Yet the same report, from the Tyndall Centre for Climate Change Research, questions the wisdom of touting shale gas as a low-carbon technology, with its authors actually asserting that "the exploitation of shale gas reserves is likely to increase total emissions." How so?

While it's tempting to look at the reduction in America's domestic energy CO2 output in isolation, the atmosphere doesn't care where CO2 comes from. It's all the same to it. In the US, a reduction in domestic coal consumption in the energy sector has coincided with an increase in the country's coal exports. US coal usage may be down domestically, but that is not to say that that same coal isn't being put to use elsewhere.

In fact, the report's authors calculate that of the total of 650 million tonnes of potential CO2 emissions avoided by reducing coal consumption to date, 340 million have been emitted in other countries (mainly in Europe or Asian) that have imported the coal instead.

The problem is that though shale gas may emit less CO2 at the power station than coal, it appears this is insufficient to offset the emissions from the exported coal. "Despite lower-carbon rhetoric, shale gas is still a carbon intensive energy source," said Dr. John Broderick, lead author of the report Has US Shale Gas Reduced CO2 Emissions?.

The net effect of the rise in shale gas consumption and coal exports could well be an overall increase in CO2 emissions, the authors suggest. "We must seriously consider whether a so-called 'golden age' would be little more than a gilded cage, locking us into a high-carbon future," Broderick said.

The report concludes that without a "meaningful" cap on global CO2 emissions (which is to say that the contribution made by shale gas must eat into, not add to, that of coal), shale gas is likely to do more harm than good. And the report is not optimistic. "Were an abrupt, internationally simultaneous, fuel switch from coal to gas to occur, the remaining safe carbon budget may be consumed less quickly," it concludes. "In the ‘real world’ these conditions are unlikely to coincide."