Barry Brill: Do atmospheric methane molecules live alone?

Or does cohabiting natural water vapour mask their warming efforts?

A rare public debate has broken out on a climate science question that really matters – the warming effects of methane molecules in the atmosphere.

The argument was kicked off by Allison & Sheahen (2018) (‘AS18’) which discussed the major greenhouse  gases (water vapour, carbon dioxide, methane and nitrous oxide) and their respective theoretical  contributions to planetary warming.

After Dr Thomas Sheahen spoke on this subject at the 2023 Waikato Fieldays, Professor David Frame in a Farmers Weekly column, expressed disappointment that so many farmers were listening to this argument. Prof Frame is the leading expert adviser to farm leaders who have been engaged in the HWEN negotiations with the outgoing government.

I then commented on Frame’s column in a “Greenwashed” programme on Reality Check Radio^[1]. And the good professor has now written a further Farmers Weekly column, evidently in response.

This back-and-forth is a very welcome process and exemplifies the way science is intended to work. We always need to peel the onion until the very nexus of the disagreement is clearly exposed. Only then can interested parties focus closely on that nub and weigh the available evidence from either side. 

The Sheahen Case

AS18 noted that methane (CH4) molecules are few and far between in the atmosphere (1.8 parts-per-million) and re-radiate outgoing energy photons in an extremely narrow band (7-8 microns) on the infra-red spectrum, so the paper concluded that the warming potential of this gas was small, even in “dry air”.

The paper’s main point, however, was that real-world air is humid, so that CH4 molecules need to compete directly with H2O molecules (which are also reactive in the 7-8 micron band) – and are hopelessly out-numbered.

In a previous paper on this site, I described the argument as follows:   

The level of water vapour in the atmosphere (ie humidity) varies widely in time and space. AS18 takes the average level of water vapour as a conservative 15,000 ppm, or some 8,800 times the volume of atmospheric methane. Talk about unfair competition!

Common sense tells us the chances of a methane molecule colliding with a 7.5-micron photon must be very small.  But now we don’t need to rely on common sense. A hugely complex study which has been published by world-leading researchers, van Wijngaarden & Happer (2022) (‘WH22’), and summarised here and here, has actually measured (at 4.2W/m2) the radiative forcing^[2] attributable to total atmospheric methane. Even if the volume of methane were to double (a highly unlikely eventuality, which could take 400 years) methane’s total  forcing would increase by only 0.6%.

Dr Sheahen points out that laboratories often carry out their experiments using “dry air” – with water vapour first extracted by desiccants. They need to do this because water vapour is so variable that it would confound their results. But then their experiments unfortunately tell us nothing about the real-world methane greenhouse effect which occurs in the presence of water vapour. 

The Frame Case

In “Methane myths come up against textbook science”, Professor Frame over-relies upon ad hominem jibes and appeals to authority (and the odd straw man). Thankfully, he also acknowledges some agreement with WH22 while responding vigorously to aspects of AS18 in the following passages:

1.     In Earth’s modern atmosphere, the two most important greenhouse gases are H2O and CO2. In clear skies, H2O is responsible for ~2/3 of the 33K warming, although it acts in a different manner than CO2 because it is near its condensation temperature in the troposphere.

2.     The claim that the IPCC does not account for water vapour is entirely incorrect. Every IPCC report .. discusses water vapour’s crucial role in Earth’s energy budget.

3.     In just the second paragraph of the very First Assessment Report, the IPCC said: “The main greenhouse gas, water vapour, will increase in response to global warming and further enhance it.” A few pages later it expands slightly on this point: “Water vapour has the largest greenhouse effect, but its concentration in the troposphere is determined internally within the climate system, and, on a global scale, is not affected by human sources and sinks. Water vapour will increase in response to global warming and further enhance it, this process is included in climate models.”              [emphasis added]

4.     This also gives the lie to the claim that climate models don’t include water vapour or clouds. Of course they do, and they have for decades. All six assessment reports cover these points [in a footnote, the author helpfully provides citations for all 6 ARs]

5.     Effectively, H2O is a slave to CO2 and other greenhouse gases: if CO2 levels increase and warm the Earth, then warmer surface waters produce a larger vapour pressure of H2O, which amplifies the greenhouse effect. This neatly encapsulates both the relative magnitude of the effects due to the two species, and the relationship between them: the dependence of atmospheric H2O on temperatures is why we treat it as a feedback, amplifying the direct effects of other GHG, rather than an independent driver (because humans don’t directly change H2O levels). 

6.     Water vapour’s effects aren’t assumed [as was suggested by AS18] – the processes are modelled. All climate and Earth System models have water vapour in them…..Enhanced concentrations of water vapour are not independent of GHG; they occur as a result of changes to GHG. This is why we treat water vapour as a feedback or amplifier of climate change.

The Issues

The Frame article serves its clarifying purpose well, and what began as a whole fistful of arguments can now be narrowed down considerably by these points of agreement:

a.     In addition to methane and carbon dioxide, water vapour (H2O) also absorbs outgoing long-wave radiation (OLR), creating absorption bands in the spectrum, and warming the air where the absorption takes place.

b.     The largest of the greenhouse gases, H2O accounts for more than 65% of the Earth’s total greenhouse warming of about 35°C.

c.     However, GCMs do not take account of its role as an independent driver of greenhouse warming  and instead treat water vapour as a feedback or amplifier of climate change (because humans don’t directly change H2O levels).

d.     Water vapour is ubiquitous and is present in varying percentages throughout the real-world atmosphere. Therefore, the ‘dry air’ created in some laboratories has no role in climate science modelling.

e.     The IPCC says that the forcing effect of adding 1.1ppm of atmospheric methane (the increase since 1750) has been about 0.5W/m2. If the current volume (1.8ppm) were to double, the IPCC and WH22 agree that the increased forcing would be about 0.8W/m2. 

It is easy to accept Frame’s assertion that the IPCC and all climate science texts are fully informed about the hydrological cycle and the radiative forcing properties of water vapour, and I have little doubt that the GCMs would be quite capable of modelling them. The question is whether the IPCC actually does so. Or does the UN Panel instead take account of only some portion of the total volume of H2O that is in the atmosphere?

Natural water vapour is excluded

Many people seem unaware that all IPCC Assessment Reports are confined to human-caused climate change and that the Panel has no mandate or funding to research or report upon natural influences on the Earth’s temperature.  See Wikipedia :

“The Intergovernmental Panel on Climate Change (IPCC) is an intergovernmental body of the United Nations. Its job is to advance scientific knowledge about climate change caused by human activities."

Whilst WG1 scientists are presumably well aware that real-world greenhouse warming is driven by the total atmospheric volume of the major gases (H2O, CO2, CH4), their own remit is solely concerned with such additions/reductions as may be made by humans.

In the case of H2O, the IPCC says in 2AR  that only the “enhancement by humans” is included in climate models (see above). This is why Frame calls H2O “a slave to CO2 and other greenhouse gases” – because they cause warming; which causes evaporation of seawater; and that “feedback” is the only anthropogenic water vapour (AWV) that is taken into account in the models’ calculations of present or future anthropogenic climate change

It follows that, of the 15,000ppm of atmospheric H2O described in AS18, only a tiny fraction (I’ll generously call it 1%, but it is much less) is AWV and is actually modelled. So, while Sheahen’s description of it as “dry air” might have been a very slight exaggeration, the principle remains sound.

It seems quite clear that all the AR6 climate models (the broad parameters of which are mandated by the IPCC) exclude 99% of the H2O molecules that compete with CH4 to react with outgoing long-wave radiation (OLR) 

Objectives matter

If the objective is to identify climate changes caused by human emissions of greenhouse gases then it is probably very sensible to exclude the effects of natural water vapour from that project

But if the objective is to identify the number of reactions that will occur within the 7-8 micron band of OLR, then it is essential for that project to know the number of competing GHG molecules (anthropogenic or not) that reside within that band.

Radiative forcing is subject to a “law of diminishing returns” so that radiative forcing reduces on a logarithmic scale in inverse proportion to the number of reactive molecules in the relevant band. [See Figure 2, AS18].

If the GHG concentration in the 7-8 micron band were about 8,000 times greater than the previously assumed 1.8ppm, then the band would be saturated – and the impact of incremental methane molecules would be close to zero.

This really matters. If H2O absorbs OLR across the methane band it would mean that the entire global effort to suppress methane emissions (including ‘the war on meat’) has been based on a scientific misunderstanding.

Few papers in the scientific literature deal with the overlap of greenhouse gases. One exception is a journal paper by Gavin Schmidt et al (2010): “Attribution of the present-day total greenhouse effect”. The abstract includes:

“We review the existing literature and use the Goddard Institute for Space Studies ModelE radiation module to provide an overview of the role of each absorber at the present-day and under doubled CO2. With a straightforward scheme for allocating overlaps, we find that water vapour is the dominant contributor (∼50% of the effect), followed by clouds (∼25%) and then CO2 with ∼20%. All other absorbers play only minor roles. In a doubled CO2 scenario, this allocation is essentially unchanged…”

Significantly, this research found that methane and nitrous oxide “play only minor roles” once overlaps with water vapour are taken into account.

The sole outstanding issue

The plea of AS18 was that “the generally accepted effects of CH4 and N2O as infrared- absorbing GHGs, causing about 50% of the total New Zealand emissions, must be urgently reassessed .. because their contribution to global warming is negligible”.

The paper’s case, in brief, was that CH4 and N2O can never cause warming except in specified narrow bands of OLR that are already saturated by GHGs in the form of H2O.

Although the onus of proof of warming clearly lies with the IPCC, as proponents of the hypothesis, AS18 volunteered an explanation that the GCMs must be modelling ‘dry air’. Professor Frame has rebutted that precise explanation – but has effectively conceded that over 99% of total atmospheric water vapour is in fact excluded from calculations of the Earth’s energy budget.

Neither Professor Frame nor any other climate scientist has yet stepped up to challenge the claim that H2O reacts with OLR in the same band as methane. Four years have passed since AS18 was published.

Over the past three years, Professors van Wijngaarden and Happer have published two papers (one with clear skies) that include extremely detailed measurements of the OLR bands absorbed by each greenhouse gas.

While these papers have excited huge interest (and criticism) within the climate science community in both EU and USA, I am unaware of any research that challenges or attempts to rebut these measurements. They must now surely be accepted as uncontroversial.        

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^[1] As I recall, my criticism was directed to the ad hominem stuff. I recommended that Professor Frame avoid getting into a ‘pissing contest’ against a scientific legend with the global stature of  Professor William Happer.

^[2] When a GHG molecule re-radiates (or absorbs) an infrared photon, the reaction “forces” an increase in ambient heat – measured in Watts per square metre

Barry Brill OBE JP LL.M(Hons) M.ComLaw is a former MP and Minister of Energy, Petrocorp director, and chair of the Gas Council, Power NZ, ESANZ, and EMCO. He is presently the Chairman of the New Zealand Climate Science Coalition.