Australian scientists defend GWP 100 metric, warn against ‘distracting’ debates

James Nason, 14/11/2023

Two Australian climate scientists have defended the GWP100 metric as the most appropriate metric for assessing the impact of the livestock sector’s methane emissions on global temperatures.

They have also urged against continued debate over which metric provides the most appropriate and objective basis for setting policy that will impact livestock producers, suggesting the issue is a distraction that risks putting the brake on important action to reduce emissions.

To watch the full Farmers for Climate Action webinar recording click here  or scroll to bottom of article.

The “GWP100” metric (which stands for ‘Global Warming Potential’ measured over a 100 year time scale) is the most widely-adopted and internationally-accepted calculation for comparing different gases, and is central to global policy discussions which aim to meet the Paris Agreement target of limiting global temperature increases to 1.5 degrees Celcius above pre-industrial levels.

However not all scientists agree that GWP100 is the most appropriate metric for measuring the warming effect of methane emissions from livestock.

During a visit to Australia earlier this year Professor Myles Allen from Oxford University said the GWP100 metric expresses methane emissions as CO2 equivalent, which “pretends that methane is a kind of carbon dioxide, and it isn’t”.

He said a recent IPCC report states that GWP100 overstates the effect of constant methane emissions on global temperature by a factor of three to four times, and understates emissions from a growing source of methane by a factor of four to five times, noting that “these are big errors”.

Several years ago Professor Allen and his team helped to develop a metric known as GWP* (Global Warming Potential Star), which shows that in a stable herd gently decreasing in size at a rate of 0.3 percent per year over 20 years the warming effect of emissions would be negated.

Achieving a slight decrease in herd size over time without decreasing production would be achievable through advances in genetic selection, animal health improvements and emissions-inhibiting feed sources and supplements.

Professor Allen advocates for reporting the warming impact of emissions using GWP* alongside reporting with GWP100, and said the UN explicitly allowed for different countries to use all relevant metrics, not just GWP100. He added that 35 scientists who have published on metrics since the Paris Agreement in 2015 have also recently signed a consensus statement saying separate treatment of methane from carbon dioxide is needed in setting climate targets.

In response to claims by some critics that GWP* gives livestock producers a “free pass” on methane emissions, he told the Australian audience such claims were incorrect, because any increase in herd size and resulting emissions are severely penalised under the metric.

Frank Mitloehner from the University of California Davis has also supported the case for using GWP*. He said GWP100 provides carbon emission equivalent numbers, but not warming, while GWP* describes warming, which is the primary reason people are concerned about greenhouse gases.

Australia’s peak cattle producer representative body Cattle Australia has also recently raised questions around the industry’s commitment to using GWP100 metrics to underpin its goal of achieving Carbon Neutrality by 2030, calling for a review into that goal and modernising its own policy based on a Climate Neutrality goal which incorporates GWP* metrics.

Farmers for Climate Action webinar hears metrics debate is ‘a distraction’

Against that backdrop two Australian scientists defended the use of GWP100 and warned the industry against debating its use in a webinar hosted by the Farmers for Climate Action group last week.

The webinar was addressed by Professor Mark Howden, Director of the Institute for Climate, Energy and Disaster Solutions at the Australian National University and Vice Chair of the United Nations’ Intergovernmental Panel on Climate Change, and Professor of Sustainable Agriculture at the University of Melbourne Richard Eckard, and Director of the Primary Industries Climate Challenges Centre.

Prof Howden started the presentation by explaining why methane is a problem, stating that methane concentrations in the atmosphere have tripled since pre-industrial times and now stand at record levels.

Methane was the second largest-human-caused greenhouse gas, he said, generating about a third of the radiative forcing of carbon dioxide.

“Methane by itself is actually a very significant greenhouse gas, it does not cool the climate,” he said.

“So when, if you hear people saying in some way that methane emissions cool the climate, that is just not the case.

“Every kilo of methane that is emitted makes the Earth warmer than it would otherwise be.”

Prof Howden said scientists have “an increasingly good understanding of what is going on”, adding: “It is not complete, but it’s actually more than good enough to make sensible decisions about policies and preferences for emissions reduction.

“So we have got a fairly good science base from which to work from.”

Prof Howden said methane directly affects global temperatures by intercepting outgoing longwave radiation and re-radiating it, in the process keeping the Earth warmer than it would otherwise be.

It also indirectly affected global temperatures through impacting concentrations of greenhouse gases in the atmosphere including ozone, water and carbon dioxide.

“It particularly does that through its interaction with what’s called the hydroxy radical or hydroxy ion which is the active sort of molecule in the atmosphere that destroys methane.

“But it also has implications for ozone and other factors.

“And how this plays out in terms of the direct and indirect influences differs slightly by source.”

He said carbon dioxide and methane have very different characteristics, and a metric that enables the two to be compared was needed.

Carbon dioxide is a very long-lived greenhouse gas which is relatively a lot weaker in terms of it warming potential than methane, he said.

Methane is a shorter-lived greenhouse gas with a stronger direct impact on radiative forcing and climate change.

The case for GWP100

Professor Howden said a metric that enabled different gases to be compared was needed for decision making.

The Paris Agreement had “explicitly adopted” the GWP100 metric as a way of converting methane into carbon dioxide equivalence.

“Having a universally agreed metric is pretty fundamental to easing the wheels or greasing the wheels of decision making.

“That enables us to make the rapid progress in in terms of emission reductions that we need to make if we’re actually to limit the impacts of climate change.

“So arguing for longer and longer about metrics would actually just be put the brakes on that necessary action.”

He said the GWP100 metric acts like a “currency converter”, converting methane into carbon dioxide equivalence in terms of its warming.

He said the metric needed to be simple, transparent, predictable, consistent with the existing policy and management environment and not generate inequities.

The metric that met most criteria most effectively was GWP100, he said..

“The decision has made been to use a 100-year timeframe so that’s the GWP100 metric.

“It simply compares the warming effect of methane in the atmosphere over the same period, so it is a simple ratio of warming between those two different greenhouse gases.”

He said GWP100 took into account all of the various sources and sinks of methane, which led to a slightly lower GWP100 score for methane emissions from livestock compared to methane emissions from fossil fuels.

Fossil fuel sources had an immediate effect in terms of warming when they were emitted, followed by a long-term subsidiary warming period as the methane broke down into carbon dioxide.

Biological sources of methane such as from livestock effectively took a carbon dioxide molecule from the atmosphere, put it into a much more radiatively-active form (methane), which then broke back down into carbon dioxide.

Biological methane went from low radiative forcing to high radiative forcing to low radiative forcing, while fossil-based methane molecule went from nothing to methane to carbon dioxide.

“So it is a different scenario you’re dealing with, and consequently the GWP from a biologically based methane molecule is slightly lower than that of a fossil fuel molecule.

“That biological source is already taken into account in the calculations of GWP100, because there’s two different factors there depending on source.”

Disagreement between scientists over when livestock are greenhouse neutral

Professor Howden disputed the view that livestock herds keep emitting the same amount of methane each year are greenhouse neutral.

“A lot of people talk about the lifetime of methane and what happens if we keep on emitting the same amount of methane each year, doesn’t that make us greenhouse neutral?

“The answer is no.”

Expanding on that, he pointed to a graph produced by NZ climate scientist and IPCC vice-chair Andy Reisinger.

He said the graph showed that when there are constant additions of methane adding onto each other, a levelling out of the concentration of additional methane in the atmosphere does eventually occur, but it happens after about 40 to 50 years.

“So if you’re sort of thinking, ‘oh well, we can just level it out in 12 years’, that’s not the case and that’s because of the lifetime of methane and how that propagates.”

He also said that methane breaks down over time in the atmosphere “but doesn’t go back to zero”, because of the residual dynamics of carbon dioxide in the atmosphere.

“What happens if we add year after year after year of the same pulse in terms of temperature, we see it keep on going up and it keeps on going up.

“So the idea that just leveling off our methane emissions somehow solves the climate change problem simply isn’t true.”

Professor Howden said IPCC reports show the livestock sector will have to reduce emissions by 60 percent to achieve the Paris Agreement Goal.

“So really important to note a 60 percent reduction is not 100 percent reduction, so under these scenarios you don’t have to go Net Zero for methane, what you have to do though is significant reductions in methane.

“And that’s really an important difference because we don’t have the technologies to go net zero in methane from agriculture, but we do have the technology and management to make significant reduction and that’s a very, very significant difference there.”

Scientists asked if reduction in herd size only answer

The presentation appeared to painted a relatively bleak picture for the future of the livestock sector, prompting a question from during the Q&A session that followed asking if there was anything other than a reduction of the national herd size that will make a difference?

Neither of the two scientists involved provided a definitive ‘yes or no’ answer to that question, but Richard Eckard said that while there was no silver bullet, a number of options were showing promise that could be used in a complementary way to reduce emissions.

“If we go back even 20 years, we didn’t know we could reduce methane by 50pc, there are now technologies in the marketplace coming through like Bovaer that can achieve it.”

This was more easily achieved in intensive systems than extensive rangeland systems, he noted. Other options may include reintroducing legumes and forages with secondary compounds into grazing systems, along with genetic selection for lower emitting livestock.

“In the next generation we’ll have breeding options that can achieve 10 percent we’ll have legume options that can achieve 10 percent.”

Methane reductions seen as a “quick win”

Professor Howden said one of the reasons there was so much focus on reducing methane emissions in policy discussions was because reductions in methane provide relatively “quick wins”.

“It’s necessary if we’re to keep temperatures within 1.5 degrees, you can’t achieve 1.5 degrees unless you have reductions in methane emissions and there’s also a very high social cost of carbon in relation to methane.

“A paper that just came out last week looked at their base case or their median estimate was around $4000 a ton of methane with a significant range around that, so that’s roughly speaking $6000 Australian dollars a ton is your social cost of methane.

“That’s the cost when you emit a ton of methane, that’s the aggregated cost over the lifetime of that methane.”

He said more than 100 countries, including Australia, had signed the Global Methane Pledge to reduce emissions by 30 percent by 2030, and a GWP metric was not required to measure that that 30 percent reduction.

“We don’t want to get distracted by GWP metric arguments because what we need is absolute reductions in methane if we’re to achieve the Paris agreement goals.”

Important to avoid inequities for farmers in developing world

Professor Richard Eckard said it was important to avoid inequities for farmers in the developing world, which provided barriers in international negotiations.

He said one of the problems with the GWP* metric was that it assigns zero warming to constant methane emissions.

He said that meant a Californian confinement dairy with 20,000 cows producing 3000 tons of methane per year with emissions declining by 3 percent per decade would be considered to be climate neutral, while a farmer in Kenya who increased his herd from one cow to two cows would get penalised, because of his herd and overall emissions increased.

“I’d ask you … which one is warming the atmosphere more?”

He said GWP* only suited a mature livestock environment where animal numbers were stable or decreasing, “which is what’s happening with livestock in Australia”.

“But it actively disadvantages emerging economies from achieving their own food security or generating wealth through livestock.”

Major supply chains already onboard with GWP100

He also said major multinational supply chain companies which have set targets for greenhouse gas emission reductions aligned with the IPCC and Paris Climate Agreement had options on the global stage would go elsewhere and buy from countries that do meet the standard.

“We have got to be very careful in making sure that what we’re doing aligns with what our supply chain wants, that’s actually the most critical thing for the future of Agriculture is meeting those supply chain targets.”

GWP* would also penalise industry for doubling value of red meat production

He also said industry’s stated goal of doubling the value of Australian red meat sales by 2030 implied the industry will grow from 25 million cattle to 35 million cattle.

“With GWP* if we adopt that as a metric it means all new 10 million animals added to the national herd now are multiplied by 128 because, there’s no animals from 20 years ago that match them to deduct, which if you work that through it means instead of a global warming of 27 in Australia we would have an average global warming potential of 44.

“I’m not sure that we’ve actually thought that through that that’s actually what we get.”

‘Just moving deck chairs around on Titanic’

Arguments about metrics were “just moving deck chairs around on the Titanic,” he said.

Professor Howden said that he, Richard Eckard and Professor Annette Cowie from the University of New England recently wrote to the National Farmers Federation recently, urging the industry to “move past the argument on metrics”.

“We need to focus on reductions in absolute methane emissions and we need it to reinforce the social license of the industry because there are alternatives coming that don’t have the same emission reduction sort of baggage that traditional agriculture does.

He said the industry “cannot afford to be naïve” about the emerging development of food without agriculture and the need to “maintain social license”.

“That can happen best through effective action. We need to be thinking about investment in cost-effective and safe technologies, we don’t want additives which  actually cause problems with animal health, we need to have clear adoption pathways for our emission reduction activities and we need to be working hard on consumer acceptance so that there’s actually a positive market signal for what we’re doing.”

To view the full webinar click on video below


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  1. mick alexander, 15/11/2023

    A waste of an hour? How wrong can they be? No science, just lots of models, graphs, made up assumptions and trying to scare the industry to agree. There is no scientific validity except if your being paid to push a narrative. What are the testing protocols, the science behind monitoring and measuring methane? How do they measure the cow methane verses fracking methane? No science just no science at all. Seriously these IPCC people blaming cattle for atmospheric methane when they probably know the truth – two years ago, they didn’t believe soils could sequester carbon when we are doing it today and very successfully – Experts without real world knowledge telling us what to do –
    “According to the national greenhouse gas inventory, agriculture contributes 14 per cent of Australia’s total emissions, livestock about 11 per cent and methane specifically from livestock about 9.6 per cent.” Now where do these modelled assumptions come from? Some boffin decided to make up a number to blame cows because farmers have no voice and every IPCC expert copied them in lockstep. The experts know the majority of methane comes from gas wells, mining, fracking (too hard) and energy industries and not agriculture. To make it worse, someone made up a number such as 8 or 12 years for the methane to degrade in the troposphere, About time these experts started putting up. Reality is – if 100 times more hydroxyl is produced in healthy pastures, which degrades 100 times the methane, carbon dioxide and other ghg’s, than those produced on a farm, then who is going to pay the farmer? I have asked MLA to fund research supporting farmers to assess and compare hydroxyl production under various conditions and the professors told them not to do the research. Why do the two professors not want to fund real science?

  2. mick alexander, 15/11/2023

    For the very first time, I actually agree with the two professors of climate change, prof Howden and Prof Eckhardt who argue that discussing the metrics is a distraction. metrics are a distraction of magnitude. Instead we need to conduct some real science to assess which pastures and which environments are the most appropriate to be creating the largest volume of hydroxyl ions (radicals) at ground level that actually degrade the methane the fastest. Lets not talk metrics and modelling that never work in any situation. Modelling is the reason we have so much non scientific information in the world today. Modelling without correct assumptions is the whole reason cows are being blamed for methane and carbon in the troposphere (and poor science). Reality is that methane in the atmosphere comes from many sources and farming with cows has never been proven scientifically to be one of them. Yes, cows emit methane and so do the two professors. Which one shall we link as a causal effect? Now, some real science, the hydroxyl ion that degrades methane and other GHG’s is formed by the photo-oxidation process during daylight hours (from the sunshine) when water is transpiring off pastures and other plants. In a healthy actively growing pasture, this process is maximised to the point that hydroxyl is freely produced constantly during daylight hours (like a solar panel). Now if a cow or any microbial process releases methane or other GHG, then the free radical hydroxyl will immediately attach within seconds to begin the degradation process. I would welcome the two professors to show the world the science that proves the hydroxyl radical does not work on land as it does in the troposphere.

  3. Mal Cock, 15/11/2023

    What is the GWP100 situation with the Indian cattle herd, which I believe is the biggest herd in the world?

  4. Matthew Della Gola, 15/11/2023

    And bud light thought dylan mulvayney was what there consumers wanted. There is no more a convincing argument than a consensus. The livestock producers are the easy targets once again. Just throw this next genius piece of legislation onto the mountain of brain farts our state, national and international governments have come up with. If you think a few cow burps comes close to even what comes from human forms of waste your kidding yourself. Cheers Matthew Della Gola

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