THE amount of greenhouse gases released by unconventional gas drilling ''exceeds that of oil or coal'', an American study says in contradiction of some claims made by Australia's growing coal-seam gas industry.
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Cornell University researchers analysed the volume of methane leaking from shale-gas wells in the US and concluded that using more gas would make climate change worse, rather than better.
The credibility of the study, to be published in the peer-reviewed Climatic Change, has been questioned by some American researchers and the Australian Petroleum Production and Exploration Association (APPEA), the industry body for coal-seam gas companies.
The authors said their findings undercut the idea that gas was a suitable low-emission ''bridging fuel'' between coal-fired electricity generation and large-scale renewable energy.
''We believe the preponderance of evidence indicates shale gas has a larger greenhouse-gas footprint than conventional gas, considered over any time scale,'' two of the authors, Robert Howarth and David Atkinson, said.
''The greenhouse-gas footprint of shale gas also exceeds that of oil or coal when considered at decadal time scales, no matter how the gas is used.''
They said the ''footprint of shale gas undercuts the logic of its use as a bridging fuel over coming decades, if the goal is to reduce global warming."
Part of the problem, they argue, is that methane is a far more potent heat-trapping gas than carbon dioxide, even though it does not last for as long in the atmosphere.
Over 20 years, the damage from methane thoroughly outstrips that of carbon dioxide. The United Nations considers the next two decades as the crucial period for reducing greenhouse gas emissions.
US shale gas is different from Australian coal-seam gas, being extracted from much shallower rock strata, although many of the technical challenges to preventing leakages are similar here.
The study follows similar findings by the same researchers last April. However, another Cornell University researcher, Lawrence Cathles, said the estimates for the amount of methane leaking from shale-gas wells was ''way too high''.
The APPEA's chief operating officer for eastern Australia, Rick Wilkinson, said the data was wrong. Emissions from coal-seam gas were up to 70 per cent lower than from brown coal.
''I expect that just as Howarth's last work in this field was discredited and disregarded by his peers and experts in this field … this, too, will come to be seen as a piece of activist science from an academic whose biography states, 'My training was in oceanography, and much of my research still focuses on coastal marine ecosystems,''' Mr Wilkinson said.
''Even if it was credible research, it has little or no relevance to the Australian CSG [coal-seam gas] sector due to the operational, geological and regulatory differences which exist.''
The APPEA said that coal-seam gas exported to China reduced emissions by up to 87 per cent compared to a similar amount of coal. It said it no longer regarded gas as a transitional fuel, useful until large-scale renewable power was cheap enough to fully replace coal.
''We see gas as an enduring, not a transitional, source of energy," Mr Wilkinson said.
The Department of Climate Change and Energy Efficiency did not respond to the Herald's questions about its methodology for measuring emissions from coal seam gas projects.
The federal government has previously told the newspaper that its parameters for measuring greenhouse emissions from gas drilling operations were updated every year, in consultation with the industry.
New US guidelines used in the latest Cornell study, and adopted by the US Environmental Protection Agency, are still being considered by the UN Framework Convention on Climate Change, which is expected to make a decision this year on how greenhouse emissions from gas drilling should be measured.
