Dumb Gas Deeming

The way carbon emissions are calculated involves the use of a lot of assumptions. There are two particularly problematic assumptions used in calculating the emissions associated with gas which I am greatly concerned about. We need to improve our carbon accounting when it comes to gas so that the industry is properly held to account.

Richard Keech



What’s wrong with this picture?  So-called ‘natural gas’ is fossil methane. We need a clear-eyed view of the reality of using methane as a fuel in our buildings.  As I see it, the key issues with using methane here in Australia are that the real-world emissions are much higher than generally supposed because of the impact of un-burned methane released to the atmosphere. Two things – global warming potential, and the amount of methane released to atmosphere – make methane release highly problematic.

Unburned release.  The way we get methane as a fuel involves a pathway of many steps – prospecting, production, processing, (sometimes) storage, transmission and distribution, and end use.  At every point in this complicated system there is a degree of methane, CO2 or other things, that get released to atmosphere before the methane can be fully burned.  Note, I’m careful not to generalise this as ‘leakage’, because in many of the sources of release are, alarmingly, entirely deliberate and part of the designed operation of the system as discussed in this ABC news story featuring Tim Forcey (see also here).

Global Warming Potential

Wrong GWP.  Whenever we reckon the climate impact of a greenhouse gas, we scale it to equivalence with carbon dioxide by multiplying by a number called the global-warming potential (GWP).   The complication with this is that different materials stay in the atmosphere for different times (‘residence times’).  Methane is a more potent greenhouse gas than CO2, but its residence time is shorter – only about twelve years. 

The right GWP. Scientists generally calculate both one-hundred year and twenty-year time horizons when looking at different greenhouse gases, and apply a different GWP to each timeframe. The one-hundred-year GWP of methane is 28, but over twenty years it is a massive 83[1] times more potent than CO2Generally, in climate accounting, the one-hundred-year values are used.  However, I’m of the view that we should use the time horizon that best matches the time frame for meaningful action.   No one who properly understands the climate emergency thinks we have a century left to fix this.  We’ll be lucky if we have twenty years.  So, if we apply a GWP of 83 to methane, then straight away that scales up the measured impact of unburned methane by a factor of three.  An example of a jurisdiction that is already using the twenty-year GWP instead of the one-hundred-year is the state of New York[2].

Methane release

Release rates: official. The total emissive impact of the unburned release is equal to the GWP multiplied by the amount that gets released.  The Government reference document for emission accounting is called the National Greenhouse Accounts Factors, which is updated every year.   According to this document, using methane from a pipeline, the impact of release of unburned methane is only 7% of the total greenhouse impact of using methane as a fuel. By my calculations, this means the government is assuming that the net rate of release of unburned methane is only about 0.3%.

Release rates: realistic.  Looking at the many reports and studies, I’ve formed the view that realistic rates of unburned methane release, in eastern Australia, are more likely to be at least 3%, and possibly much more.  At a net release rate of greater than 3.4%, the emissions from releases exceeds the emissions from burning alone (assuming GWP of 83).  Study after study suggest that we’ve been systematically underestimating the emissions from gas.

Unconventional gas.  As time passes, gas gets progressively worse from an emissions point of view as the best quality gas fields get used up[4].  As the conventional gas fields in eastern Australia decline, we’ll become increasingly dependent upon unconventional gas.  Many new gas fields are on the cusp of being opened up across Australia.  The emissions impact of opening up these carbon bombs needs to be avoided at all costs.

What does this mean?  I like this from Rachel Golden of the Sierra Club:

“There is no pathway to stabilizing the climate without phasing gas out of our homes and buildings.  This is a must-do for the climate and a liveable planet”

Damned Deeming

In accounting for emissions, both factors – the GWP and the leakage rate -need to be applied. The ‘bible’ for emissions accounting in Australia is the ‘National Greenhouse Accounts Factors‘[3]. This tells people doing carbon accounting what assumptions to apply, including the GWP and leakage rate.

Cover image of reference document 'National Greenhouse Accounts Factors'

The use of these factors amounts to using deemed values which, I think, distorts the integrity of the carbon-accounting process.


The mainstream view about emissions from the use of gas is focussed almost entirely on the combustion emissions, and tends to overlook the very consequential emissions from the release of un-burned methane into the air. In other words, it’s been common to overlook direct methane release because of the view that this is a trivial component of the emissions profile.

The consequence of this oversight is that the net effective emissions from gas are almost certainly under-estimated, and have been for a very long time.

What should we be doing?

We need to get our carbon accounting back on track by doing the following:

  1. GWP. All parties doing carbon reporting should adopt increased emissions reporting stringency using the 20-year GWP, in line with the state of New York. This would be most easily done by the Federal Government updating the guidance in the National Greenhouse Accounts Factors to use the 20-year GWP, rather than the 100-year factor;
  2. Update baselines. There should be a much greater requirement on developing accurate and updated baselines of actual emissions at all places and times in the process of gas production and use;
  3. Benefit of the doubt. While the new baselines are being established, the accounting methods should not give the industry the benefit of the doubt when it comes to making assumptions about releases in carbon accounting. Where there is doubt, assumptions should be conservative, i.e., err on the side of the planet;
  4. Measure. In some places where, today, deemed values are used, it should become a requirement to perform actual measurements of emissions. This should apply to significantly emitting projects and, in particular, new gas prospecting, production and distribution.


[1] IPCC, Assessment Report 6, Working Group 1 report.

[2] Howarth, R., 2020-08-25 “Methane emissions from fossil fuels: exploring recent changes in greenhouse-gas reporting requirements for the State of New York”, Journal of Integrative Environmental Sciences, https://www.tandfonline.com/doi/full/10.1080/1943815X.2020.1789666

[3] Government of Australia, “National Greenhouse Accounts Factors”, updated regularly, https://www.industry.gov.au/data-and-publications/national-greenhouse-accounts-factors-2021

[4] Hart, P., 2007-03-20, “Esso/BHP could put carbon under sea”, The Age, https://www.theage.com.au/business/esso-bhp-could-put-carbon-under-sea-20070320-ge4grv.html