An expert in public policy says the debate over whether coal seam gas is cleaner than coal depends on the timeframe used to measure emissions.

According to a scientific paper by US academic Nathan Hultman, coal seam gas is 56 per cent cleaner than coal over a 100-year time frame, but when compared to coal over 20 years it is less than 20 per cent cleaner.

Professor Hultman, from the school of public policy at the University of Maryland, says using the "most extreme assumptions", coal seam gas is actually dirtier than coal over a 20-year time frame.

He told Radio National's Background Briefing the choice of timescale was a "value judgment" and "depended on when you thought the real problem of climate change was going to bite".

"The reason you would be worried about methane in particular over a 20-year time horizon is if you are thinking we are on the verge of a kind of tipping point in climate change right now," he said.

"If you really think right now we are very close to melting the ice caps or pushing the climate from a moderately steady state into a kind of bad outcome where you've got runaway climate change."

According to the Intergovernmental Panel on Climate Change, the global warming impact of greenhouse gases can be measured across three different times frames: 20 years, 100 years or 500 years.

Each is valid and chosen by atmospheric scientists depending on what scientific point they wish to highlight.

Coal seam gas is mostly methane - a powerful greenhouse gas that has a relatively short life span.

This means that over a 20-year time frame, coal seam gas has a much higher global warming impact than if it is measured over 100 years.

Professor Hultman said he was concerned about climate change "tipping points", but wanted to emphasise the situation over 100 years because of concern about the impact of "gases that go up there and stay up there", like carbon dioxide and sulphur dioxide.

"I worry that we are going to paint ourselves into a corner and get to a point where there is a lot of climate gases up there and we can't get them out very fast," he said.

Fugitive emissions

Professor Hultman's paper was cited by the oil and gas lobby group APPEA in their submission to the Federal Government's review into the way in which fugitive emissions from coal seam gas are measured.

APPEA has consistently said coal seam gas is 70 per cent cleaner than coal.

But in September the Government released a report which found that the absence of published information about fugitive emissions - greenhouse gases that leak into the atmosphere during the extraction process - was a matter of "public policy concern".

The Pitt & Sherry report recommended the Government begin a research process to examine "the relative magnitude of the various individual sources" of fugitive emissions.

"Extraction of [coal seam gas] and shale gas differs from the extraction of conventional natural gas in two important ways," the report said.

"First, many more wells are drilled for a given volume of gas production.

"Second, much more extensive use is made of hydraulic fracturing (fracking) of the gas-bearing rock strata, to release the gas.

"Because of these reasons, it is thought likely that the sources, and perhaps also the volumes, of fugitive methane releases may differ from those associated with conventional gas production, and that consequently different approaches to the measurement of fugitive emissions may be required."

Earlier this week the Department of Climate Change and Energy Efficiency announced it would be working with CSIRO to conduct field research into fugitive emissions from coal seam gas in 2013.

APPEA issued a statement saying it supported "independent research to better inform development of revised emission factors and methodologies that better reflect Australian gas production operations."

Professor Hultman's paper, The greenhouse impact of unconventional gas for electricity generation, was published in Environmental Research Letters (2011).

Wendy Carlisle's full report, The missing emissions, can be downloaded from the Background Briefing website.