Methane: myths & misrepresentations

by Barry Brill

May 18, 2011

The New Zealand Government is the first to legislate financial penalties on natural gas (methane or CH4) emanating from ruminant animals.

The claimed justification is that the ETS levy is an “insurance policy” against the possibility that methane might contribute to a future of dangerous global warming, as the IPCC has theorized.

But, as with any insurance, it should not be taken out unless the premium is reasonably commensurate with the risk being hedged. The obvious problem is that nobody can quantify the value at risk, or the cost of the premium, or the the scope of the coverage, or the likelihood of the event. This article deals only with the last of these.

The IPCC believes dangerous global warming might occur if the atmospheric concentration of greenhouse gases, expressed in carbon-dioxide-equivalents (CO2e), were to exceed 450 parts per million. CO2 volumes are currently about 390ppm.

There is no question that methane is a greenhouse gas, and that its atmospheric concentration owes much to wetlands, peat bogs, permafrost thaws, pipeline leaks, termites, ruminant livestock, landfills, rice-growing and other sources. As at 2010, the volume in the atmosphere is 1.8 parts per million, and New Zealand’s NIWA calculates that livestock accounts for approximately 15% of that total.

Applying the above figures, the atmospheric concentration of CH4 is only 0.7% that of CO2. This would be so trivial as to be ignored, but for the fact that the IPCC contends that CH4 has a “global warming potential” (GWP) which is about 21 times greater than CO2.

The GWP of methane is described by Australia’s CSIRO:

A kilogram of methane released into the air today, for example, will lead to about 20 times more atmospheric warming over the next century than a kilogram of carbon dioxide.

Kilograms? What does weight have to do with atmospheric concentrations? What do kilograms tell us about parts-per-million? Why has the conversation side-slipped from volume to weight?

Comparisons by volume (or molecule)

In a recent peer-reviewed paper* Wilson Flood reminds us that “the greenhouse effect” deals with the behaviour of gases at the molecular level. GHG molecules absorb and re-radiate energy at a rate dependent on their plenitude. Dr Flood observes that to make any sensible comparison of greenhouse gases:

... we must deal with equal numbers of molecules of the two gases. A way of doing this is by means of chemistry’s Avgadro’s Law which states that equal volumes of gases under the same conditions of temperature and pressure contain equal numbers of molecules. However, the mass of a gas molecule depends on the masses of the atoms that make up the molecule so equal volumes of gases will have different masses if the gas molecules are of different mass.

CH4 has an atomic mass of 16 (C=12, H=1), whilst CO2 (O=16) has a mass of 44. So one kilogram of CH4 has 2.75 times the number of molecules in an equal weight of CO2.

So if equal volumes of the two gases are compared, rather than equal weights, the IPCC’s GWP of methane is seen to be grossly over-stated. Molecule for molecule, the warming ability of methane compared to carbon dioxide is 21/2.75 times - i.e. a much more modest multiplier of 7.5.

Recall that the current atmospheric concentration of methane is only 0.7% that of CO2. Its tiny relative quantity mainly explains why it has greater warming ability, given that absorption of energy diminishes logarithmically as concentration increases.

Comparisons by weight

If weight measures are to be used, Dr Flood’s paper points out that the mass of CO2 currently in the atmosphere is 3000 gigatonnes, as compared to CH4 at only 5 gigatonnes. By weight, atmospheric methane is a mere one six-hundredth (0.0016%).

It is widely accepted, by the IPCC and sceptics alike, that doubling the weight of CO2 in the atmosphere is likely to cause a direct temperature rise (without feedbacks) of about 1.0°C. 

Doubling the weight of CH4 in the atmosphere adds only one six-hundredth as much greenhouse gas as doubling CO2. Even when multiplied by a GWP of 21, this would produce warming of only (21/600 x 1°) or 0.035°C (without feedbacks).

If the IPCC is correct in its highly controversial claim that net feedbacks treble the initial forcing, then the total effect of doubling atmospheric methane (by weight) would be a global temperature increase of 1.05°C over the course of a century.

Comparisons by time

Whereas CO2 is said by the IPCC to remain in the atmosphere for hundreds of years, CH4 has an atmospheric half-life of approximately 8 years. Each year, large swathes of CH4 are washed out of the air by rainwater or converted to CO2. In the result, the total quantity builds slowly, if at all.

Dr Flood cites the NOAA graph which shows that methane has increased in concentration by about 0.1ppm in the period from 1988 to 2008. The quantity is increasing at a mere 0.014 gigatonnes per annum. At this rate of increase the doubling of atmospheric CH4 from its present value will take about 360 years.

Compare this with CO2, which scenarios assume will double during this century on a business-as-usual basis.


The available data suggests that total warming from methane increases will be no more that 1.05°C over the next 360 years. If 15% of this total is to be laid at the door of livestock farming, it would amount to 0.15°C – about 0.04° per century.

However, this calculation assumes that all of the world’s livestock are slaughtered to extinction. If the target is merely to cut livestock methane by (say) 10%, then the risk avoided might be 0.015°C after 360 years. And that’s a high estimate, which only applies if all the IPCC theories turn out to be correct.

What sane person would voluntarily pay any premium to insure against this risk?



* Wilson Flood, “The Methane Misconceptions” in Energy and Environment, Vol. 22, No. 3, 2011

I am indebted to Dr Flood for much of the material in this article.