Saturday, December 2, 2023

Methane myths come up against textbook science

Avatar photo
Denying the science seems an unpromising way for certain agri sectors to go about convincing urban NZ of the reasonableness of their position, says Canterbury University Professor of Physics David Frame.
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.
Reading Time: 8 minutes

By David Frame

Myths are circulating within the farming community about methane’s role in climate change. Unfortunately, the recent visit to New Zealand of Tom Sheahen, an elderly American physicist with no history of physical climate research, has added noise and confusion to debates about the role methane in climate change. 

Sheahen and Jock Allison, a retired NZ ruminant scientist, wrote a piece in 2018 that also makes a number of erroneous claims. Sheahen’s speaking tour repeated many of these errors.

Some of the claims they make in the paper are not coherent: at times they claim that the mainstream research community fails to account for water vapour’s presence in the atmosphere while later in the same document they concede that the Intergovernmental Panel on Climate Change (IPCC) does include an assessment of the role of water vapour. 

The claim that the IPCC does not account for water vapour is entirely incorrect. Every IPCC report, and pretty much every atmospheric physics textbook, discusses water vapour’s crucial role in Earth’s energy budget. 

In a recent text on planetary atmospheres, David Catling and James Kasting wrote simply and concisely on the point.

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. 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). 

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.” 

Figure 1:Global near-surface temperature changes from instrumental data for 1850-2020 according to four different datasets, as well as decadal averages. Source: IPCC (2021).

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.” 

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, as do the textbooks on atmospheric physics I have readily at hand. And this is without even cracking open the voluminous journal-based scholarship on climate modelling, or the various user manuals for climate models. 

In his recent talks, Sheahen made a fuss about a recent piece written by two physicists, Happer and Van Wijngaarden (2019), which largely recycles old material already familiar to the climate research community. 

Happer and Van Wijngaarden conducted line-by-line calculations using a radiation model. This approach has been a part of the toolkit for climate researchers for a long time. 

Sheahen presents this as though it were groundbreaking, and challenging to mainstream climate science. Nothing could be further from the truth. Their results generally converge with the mainstream. The errors they make are in their interpretation of the results, and in thinking that points that are news to them are news to us. 

For instance, Happer and Van Wijngaarden write: “Methane concentrations may never double, but if they do, WH[1] show that this would only increase the forcing by 0.8 Wm−2.” 

This is probably roughly in the right ballpark for the direct effect of methane, though it may neglect methane’s effects on tropospheric ozone and stratospheric water vapour – the paper is not clear on this point. Either way the result isn’t exactly news. The IPCC forcing plot shows that methane’s direct forcing effect between 1750 and today – that is, between ~700ppb and 1800ppb – is around 0.5Wm-2. 

Having done an okay calculation, Happer and Van Wijngaarden go on to say: “This is a tiny fraction of representative total forcings at midlatitudes of about 140 W m−2 at the tropopause and 120 W m−2 at the top of the atmosphere”, which shows a poor understanding of how budgets work. 

The ongoing residual is the source of the concern. I think every farmer will immediately see what is wrong with “my farm’s turnover is about $1.4 million, so I don’t need to worry about the fact my expenditure exceeds my revenue by $8000 every year”.

The problem isn’t so much with what they’ve done numerically as with how they interpret it and with what they assume others have done. In his recent talks Sheahen made the claim that “everything the IPCC has been doing for 40 years has pretended the air was dry air”. This is completely wrong. 

Among Syukyro Manabe’s seminal contributions to climate science, for which he was awarded the Nobel physics prize, was the development of an early climate model with a hydrological cycle, and subsequently a radiative-convective model that incorporated humidity into the calculation of Earth’s energy budget. Almost 60 years after this work, a huge amount has been done to further develop these ideas and to compare the resulting models with observations. 

The real challenge for climate modelling is that parcels of air vary and move around. Their pressure, temperatures, humidity/cloudiness, and chemical composition vary over time. General circulation models attempt this by applying Newton’s laws on a rotating sphere, while also taking account of the energy fluxes between parcels of air as time goes by. 

Earth System models contain these features, as well as active biospheres and more thorough representations of atmospheric chemistry. Nobody claims they are perfect, but they are pretty good, at least if what you care about is understanding how climate changes as GHGs change. 

Climate change deniers quibble over models’ performance, while ignoring that the deniers’ own long-standing predictions of “no change” have been far further from reality.

Sheahen and Allison make the strange claim that, because water vapour added to the atmosphere is present there for only a few days, it is not incorporated into the models. Instead, the assumptions in the General Circulation Models (GCMs) are that water vapour operates as a “positive feedback”, which amplifies the effects of the GHGs by two to three times.

Again, this is wrong. Water vapour’s effects aren’t assumed – the processes are modelled. All climate and Earth System models have water vapour in them. GCMs and Earth system models – the sorts of models people have in mind when they say “IPCC models” – explicitly resolve water vapour. 

Sheahen and Allison go on to say that this indicates an assumption that H2O does not operate in a direct way as do other GHG molecules in the atmosphere.

This is also wrong. Water vapour also absorbs outgoing longwave radiation (OLR), creating absorption bands in the spectrum, and warming the air where the absorption takes place. Models simulate this. But as Catling and Kasting point out, if CO2 levels increase and warm the Earth, then warmer surface waters produce a larger vapour pressure of H2O, which amplifies the greenhouse effect. 

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. Other things also act as feedbacks on the direct radiative effects of GHG: clouds, albedo effects and changes in the lapse rate, which partially offset the water vapour feedback.

Finally, the really silly stuff. Sheahen and Allison assert that there has been no significant increase in the world’s temperature in the past couple of decades, the well-known and accepted “pause”. The accompanying plot (after figure 2.11 in the most recent Working Group I report of the IPCC) includes global mean near-surface temperatures according to several of the main data centres. They agree extremely well. 

They all show that between the mid-1970s and the mid-2020s there has been about 1degC of warming. There are wiggles and wobbles in the data, but the trend is clear and unmistakeable. So is the cause: it is overwhelmingly due to increases in the concentrations of greenhouse gases. 

Ironically, this topic was also the subject of work by a high-profile climate sceptic. Richard Mueller from Berkeley was convinced that the climate research community had been evaluating temperatures all wrong, so he secured a pile of money and set about doing an independent assessment … and got the same answer as everyone else. His is the pink line.

Many climate sceptics would have you believe there is a vast conspiracy of thousands of scientists working together to suppress the truth. 

Among other flaws, the conspiracy argument misunderstands the incentives in front of scientists. If I could show that anthropogenic climate change was no threat, I would do it in a heartbeat: (1) I would win a Nobel prize; (2) I could sign an eight-figure book deal to tell the story; (3) I would annoy Greenpeace and others who have, in my view, inordinate and misplaced faith in global institutions. Major wins all around and I just don’t see the downside.

There may well be any number of scientists who are deeply committed for political reasons to “the process” around climate change. But there are plenty of us who would be glad to see it gone if it could be shown to be unjustified. And as many would-be censors – from Renaissance Italy to Soviet Russia – have found out, free enquiry tends to win out over attempts to suppress it, especially over timescales longer than a decade or two. 

Farming leaders are free to believe or reject mainstream science. There’s a marketplace of ideas and you are free to set your own position. If you want to bet the farm on fringe science, go for it. I don’t mind – it’s not my farm. 

But be aware that the more you go around claiming the IPCC doesn’t consider water vapour, line overlaps, or the logarithmic dependence of forcing on concentrations, the less anyone will listen to you because none of these things are true. 

Methane is a powerful GHG, partly because there’s so little of it, and it and its consequent products contribute about a third of current warming. 

Farmers have been fortunate to enjoy considerable influence over public policy in many areas, including climate policy. 

Historically, New Zealand’s provinces have been powerful and the experiences of and incentives in front of the farming community have been widely shared. In the context of an urbanising world, where fewer people in 21st Century NZ have a direct affinity with rural communities, your ability to retain your influence depends increasingly on your ability to persuade others of the reasonableness of your position. 

Denying textbook science seems an unpromising way of going about this.


1 DC Catling and JF Kasting, 2017, Atmospheric Evolution on Inhabited and Lifeless Worlds, Cambridge University Press, Chapter 2: Energy and Radiation in Planetary Atmospheres, p35. 

2 On water vapour’s relative contribution to radiative forcing, the Second Assessment Report, p161, Climate Change 1994, p174, Third Assessment Report, p49, p88, Fourth Assessment Report, Working Group I, Chapter 1, p97, p115, Fifth Assessment Report, Working Group I, Chapter 1, p126-7, Chapter 8, p666, Sixth Assessment Report, Working Group I, Chapter 7, p969-70, p975. On overlaps between spectral lines see the Second Assessment Report p122, Climate Change 1994, p18, Third Assessment Report, p356, p359, Fourth Assessment Report, Working Group I, Chapter 2, p143, 144; Fifth Assessment Report, Working Group I, Chapter 8, p675-6, Sixth Assessment Report, Working Group I, Chapter 7, p944. On logarithmic dependence of forcing on CO2 see, especially, Climate Change 1994: Radiative Forcing of Climate Change and An Evaluation of the IPCC IS92 Emission Scenarios, p218; Fourth Assessment Report, Working Group I, Chapter 2, p140, Sixth Assessment Report, Working Group I, Chapter 7, p980. In general, the earlier reports are clearer on these basics than the later reports, which had the explicit brief of providing updates since the last report, rather than a comprehensive coverage of the entire field. Like the writers of a TV series, they assume if you are reading the nth edition you are already familiar with the 1:n-1 editions, that is, the back story. 

3 For instance, see RT Pierrehumbert, Principles of Planetary Climate, Chapters 3 & 4, especially sections 4.4.6-4.4.9; J Marshall and RA Plumb, Atmosphere, Ocean and Climate Dynamics, Chapter 2; and DG Andrews, Introduction to Atmospheric Physics, Chapter 3, Chapter 8. Andrews has a nice figure in section 3.5 (p 71, p73) showing the spectral properties of the real and idealised atmosphere, and the contribution that various lines make to this. 

4 One Wm-2 might not sound like much. It’s one joule, per square metre, per second. There are square metres on the surface of the earth and there are seconds in a year. So Wm-2 = joules; roughly 73,000 times more energy than was released in Tsar Bomba, the largest nuclear weapon ever developed and tested – which in turn was around 1600 times more energy than was released in the Hiroshima and Nagasaki atom bombs together. And that’s one year, and the world is currently at about 2.5Wm-2, every year, for the foreseeable.

5 Very simple models (such as FAIR and MAGICC) may, on the basis of observations, parameterise the impacts of water vapour, as well as the effects of cloud, albedo and lapse rate on surface temperatures. These parameterisations can easily express ranges of observational uncertainties.

6 See Pierrehumbert, Principles of Planetary Climate, p278-9.

People are also reading