Thursday, August 11, 2022

Wrestling with the metrics of methane

In the shouting match over greenhouse gases we often can’t agree even on the terms of engagement.
Two of the main culprits in global warming, carbon dioxide and methane, cannot simply be set side by side and compared with one another, says the writer

In my previous article, I explained that there is much controversy about how methane should be compared with carbon dioxide in terms of global warming. The problem arises because methane is a powerful greenhouse gas but it lasts only a short time in the atmosphere. In contrast, carbon dioxide is a weak greenhouse gas but lasts much longer. Also, there is a lot more carbon dioxide than methane released into the atmosphere.

Big problems arise when methane is shoe-horned into carbon dioxide equivalence. 

First, many people may be surprised to learn that the issue of carbon-dioxide equivalence and the associated controversies are not really about the science. Scientists understand the nonsense of trying to estimate how many apples it takes to equate to one orange, with the answer depending totally on the chosen measures. Similarly, scientists understand that methane has a totally different emission profile to carbon dioxide and there is no simple equivalence measure.

However, the general public, together with policy makers and politicians, like to keep things simple. They want to be able to add the two together regardless of the problems and flawed thinking this creates.

The internationally accepted way for reporting emissions back to the United Nations Framework Convention on Climate Change, which is where climate-change policy issues are decided, is to think of carbon dioxide as the big brother and methane as the little brother. Little brother has to fit in with big brother. Accordingly, methane is typically converted to units of carbon dioxide equivalence (CO₂e) on a 100-year timeframe of global warming, with this denoted as the GWP100, or global-warming potential, effect. 

In doing the calculations, the methodology looks forwards rather than backwards. It doesn’t matter what emissions might have occurred in the past. The only question being answered is how much warming over the next 100 years will be caused by the new emissions.

Given that specific question, and within the current limits of scientific knowledge, the GWP100 metric gives correct answers. If you want another answer, you have to change the question.

When government officials report that agriculture makes up almost half of New Zealand’s emissions, this is based on this 100-year assumption, although that caveat is almost always lost in media reporting. Similarly, if agriculture were to enter the Emission Trading Scheme as currently structured, it would be using the GWP100 equivalence assumption that underpins that system.

I will use two examples to illustrate the importance of this assumption.

Some people are concerned about what will happen to the Greenland and Antarctic glaciers. If these glaciers melt, it will be over a period of many hundreds and possibly thousands of years. If they do melt, sea levels will rise many metres, and mega-cities across the world will drown.   

If this occurs as a consequence of greenhouse gases, it will be almost totally due to carbon dioxide that is piling up in the atmosphere. This is because much of the carbon dioxide released today will still be in the atmosphere in another 500 years. In contrast, the methane that is released in the next 50 years will all be gone, and hence is essentially irrelevant to that long-term situation.

The second example relates to a current focus on short-term temperatures, with 2050 being a particular focus. We read continually in the media about the so-called challenge of keeping the accumulated global increase in temperature to less than 1.5⁰C compared to pre-industrial 1850. 

In a scientific framework, there is nothing special about 2050. But society likes targets that can be enunciated in simple terms. This 2050 target, linked to a pre-industrial 1850 baseline, is what climate-science politicians and lobbyists are focusing on. This is despite the pre-1950 component of the change almost certainly being largely natural and unrelated to human activities. 

Given this target, combined with the scientific fact that it is impossible to reduce the ongoing temperature effects of past emissions of carbon dioxide, it makes sense to place significant emphasis on reducing global methane emissions. It could indeed be an important way to influence temperatures in 2050, albeit by, in all likelihood, less than 0.1⁰C at that time, even if undertaken globally.

The key take-home from these examples is that just like apples do not have an orange equivalence, so too it is important to not conflate issues of short-term versus long-term greenhouse gases.  There is no simple overarching metric for comparison.

Many and probably most climate scientists would agree with the above paragraph about not conflating short- and long-life greenhouse gases. But alas, many politicians and lobbyists have no understanding.   

Accordingly, some climate scientists have stepped forward from analysing the climate itself to try to find alternative metrics that can be used in the policy framework where politicians and lobbyists work.

In understanding these alternative methane metrics, I often refer to the invisible methane cloud sourced from NZ’s pastoral animals. It is the current size of this invisible cloud that determines the current temperature effects of NZ’s historical pastoral emissions.

Think of this atmospheric cloud as being the atmospheric equivalent of a bathtub with a tap running and the plug partly removed. Water flows in and water flows out. The amount of water in the bath will depend on how fast the tap is running and how fast the water is leaving down the plughole.

With methane, scientists know that the flow of methane into the atmosphere from NZ ruminant animals is close to what it was 30 years ago.  As a consequence, and linked to the scientific knowledge that about 8% of methane molecules decompose each year, an approximate balance in the atmospheric “bathtub” has been reached and the atmospheric cloud of NZ pastoral-sourced methane is close to stable. 

Hence, this argument goes, NZ’s agriculturally sourced methane is contributing to further global warming in a minimal way. 

In contrast, with carbon dioxide only a small amount of carbon dioxide leaks out through the plughole and the stock of carbon dioxide in the atmosphere keeps increasing each year.  

This insight, variously stated, underpins the GWP-star metric, written GWP*, which some people are now promoting vociferously. Whereas GWP100 looks forward at the effect of this year’s emissions, the GWP* metric focuses on the changes in emissions that have occurred compared to a historical period and hence, in relation to methane, on the change in size of the atmospheric-warming cloud. 

The argument goes that because the NZ-sourced atmospheric methane cloud is essentially stable, NZ farmers should not be charged for doing what they, or more particularly their animals, have been doing for a very long time.

Recall that earlier in this article I said that the GWP100 metric provides an accurate measure of the 100-year warming effect of current emissions, with this effect being measured relative to no emissions at all occurring. In contrast, the GWP* metric answers a different question, which is whether or not the atmospheric methane cloud is increasing. Each is correct for the question that is answered.

Many people in NZ agriculture are very keen on GWP*, for the obvious reason that it leads towards a conclusion that any methane charges incurred by NZ agriculture should be minor.  

However, the GWP* metric gives a very different answer in situations where the atmospheric cloud is increasing from new emitting activities.  Accordingly, using this as a basis for emission charging leads to new emitters being charged heavily. 

The principle of using past emissions to justify ongoing emissions is called “grandfathering”. It is the reverse of a key principle in the Paris Agreement that developed countries with current high emissions must carry the main burden of emission reduction.

Here is just one of many examples of grandfathering effects: 

Some eight years ago, I led a Ministry of Foreign Affairs and Trade-funded dairy-development design team to Colombia. Although close to the equator, Colombia includes temperate lands at altitudes of between 2,000m and more than 3,000m, which is highly suitable for dairying. Also, the Colombian Government sees dairying as a wonderful alternative to growing cocaine. However, if the GWP* metric were to be used to assess greenhouse gas effects, Colombian dairy developments would show up badly because they generate new emissions. 

So, given these ethical issues, how do we move forward here in NZ, where pastoral-sourced food underpins so much of our export-led economy?

Once again, in this article I have only scratched the surface of multiple complex issues. Although I think there is a path forward for both NZ agriculture and NZ society, that must be another article.

So, my final take-home message from this article is that climate-change issues are indeed complex.  Also, much of the debate is ill-informed. It is like two warrior groups standing on either side of a wide gully shouting insults at each other, with each group accusing the other of being ignorant. Each warrior group can hear only itself.

More: Keith Woodford was professor of Farm Management and Agribusiness at Lincoln University for 15 years through to 2015. He is now managing director at AgriFood Systems Ltd. He can be contacted at  Previous articles can be found at

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