Time to change our methane metrics? The case for using GWP* when looking at the climate change impact of methane.

The NFU (National Farmers Union) make the case for adopting GWP* as a metric for measuring the impact of ruminant livestock on climate change. But what exactly is GWP* and how does it differ from established emissions accounting processes?
October 4, 2023

Currently GWP is the accepted metric used to measure greenhouse gases 

Global Warming Potential (GWP) is an emission metric that has been in existence for over 30 years.[i] It is used by the UNFCCC common reporting formats, the IPCC inventory guidelines, and the Paris Agreement rulebook. The function of this metric is to establish the warming equivalence of carbon dioxide with other greenhouse gases, for example, methane, and then use this to create cost-effective mitigation targets.

GWP100 is the most common GWP measure and also the internationally accepted standard. Countries will use GWP100 for their emissions accounting. GWP100 means the warming potential of a greenhouse gas over 100 years. To give an example, nitrous oxide has a GWP100 of 273, which means that 1 tonne of nitrous oxide has the same warming potential as 273 tonnes of carbon dioxide over 100 years.[ii]

However, GWP100 is not as accurate for calculating short-lived emissions like methane because it does not take into account the removal of emissions from the atmosphere.[iii] Using GWP100 to calculate the impact of a constant methane source on global warming overstates this impact by a factor of 3 to 4. GWP100 also understates the impact of a new methane source on global warming by a factor of 4 to 5 over 20 years since the new source is emitted.[iv]

This is why GWP* has been proposed as the metric to measure the impact of methane 

GWP* was developed by climate change scientists at the University of Oxford, the University of Reading, Victoria University of Wellington, and the Center for International Climate and Environmental Research (CICERO). Their research was published in the journal of Climate and Atmospheric Science in 2018.[v] 

GWP* uses the CO2 equivalent emissions calculated using GWP100 to calculate CO2 ‘warming equivalent’. Carbon dioxide is a long-lived climate pollutant (LLCP), it accumulates in the climate system. However, natural atmospheric removals limit the indefinite increases of the atmospheric concentrations of short-lived climate pollutants (SLCP) with stable emissions rates. For example, methane is broken down naturally in about 12 years.[vi] So, the amount of warming caused by carbon dioxide is due to cumulative CO2 emissions to date, whereas the level of warming caused by methane is determined by the current rate of emissions at any given date.[vii]  Therefore, the sustained emissions of an SLCP can have a similar impact to a fixed amount of carbon dioxide. In this way you can calculate the carbon dioxide warming equivalent (CO2-w.e.) of an SLCP rather than just a carbon dioxide equivalent (CO2-e).[viii]

The dairy and livestock industries are under pressure to reduce emissions. Previously we discussed how beef and dairy are responsible for a significant proportion of the emissions in the food production sector. 

The NFU (National Farmers’ Union) is arguing for the adoption of GWP* when calculating the contribution of ruminant agriculture to climate change.[ix] Oxford Martin School and the University of Oxford have said that the actual impact of cattle and sheep farming on global temperatures is reflected more accurately by expressing methane emissions as CO2-warming-equivalent.[x] Accurately understanding the amount of warming that agriculture is responsible for is argued to be important for then reducing these emissions as much as possible.[xi]

The NFU is calling for:

1)       A unified approach to dual accounting with government and industry reporting on emissions from agriculture at the national scale using GWP100 and GWP*.

2)   Solutions for incorporating GWP* into on-farm GHG (Greenhouse Gas) calculators so individual producers are not disadvantaged for expanding when national herd/flock emissions remain unchanged or are decreasing due to uptake of new tools and technology.

3)   Consistent and simple messaging to consumers on the temperature impacts from methane and the biogenic carbon cycle, including finding a more suitable way for the carbon footprint of a food product to be measured.

4)   The impact of genetics and selective breeding on reducing methane emissions to be measured with GWP100 and GWP*.

5)   A joined-up approach to GHGs across government departments, e.g. when looking at air quality under Clean Air Strategy and GHG emissions reductions versus the future of ELMs (environmental land management schemes).[xii]

However, GWP* is an inaccurate metric for climate change mitigation

According to Meinshausen and Nicholls (2022), GWP* does not meet four out of five of the key functions of a metric that would be needed to replace GWP100. GWP* can be used as an ‘exchange rate’ for greenhouse gases and then be used to create mitigation targets. However, it falls short at approximating the marginal climate effect of an emission action, enabling control feedback for policy instruments, being a simple and transparent tool for non-specialists, and being consistent with existing climate policy.[xiii] Both the IPCC Assessment Reports and the GHG Protocol use GWP100, so moving away from this would mean a major disconnect from national and industry measures. A lack of consistency would make it harder for businesses to gather data on their scope 3 emissions. Furthermore, GWP* is not a ‘neutral’ metric. ‘Metric neutrality’ is the idea that where the emissions occur (e.g. country, sector or facility) is irrelevant when comparing the climate effect of the same absolute level of emissions. However, GWP* weights emissions differently depending on the emission history of the country, project, or facility. Meinshausen and Nicholls say that GWP* shouldn’t be seen as a ‘metric’ but rather a climate model.[xiv]


[i] Meinshausen and Nicholls- GWP* is a model, not a metric

[ii] AHDB- What is GWP*?

[iii] NFU- NFU calls for new methane metric to be used in GHG calculations

[iv] IPCC- Climate Change 2021: The Physical Science Basis

[v] Allen et al.- A solution to the misrepresentations of CO2-equivalent emissions of short-lived climate pollutants under ambitious mitigation

[vi] Oxford Martin School and University of Oxford- Climate metrics for ruminant livestock

[vii] Ibid

[viii] Lynch et al.- Demonstrating GWP*: a means of reporting warming-equivalent emissions that captures the contrasting impacts of short- and long-lived climate pollutants

[ix] NFU- NFU calls for new methane metric to be used in GHG calculations

[x] Oxford Martin School and University of Oxford- Climate metrics for ruminant livestock

[xi] NFU- NFU calls for new methane metric to be used in GHG calculations

[xii] Ibid

[xiii] Meinshausen and Nicholls- GWP* is a model, not a metric

[xiv] Ibid

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Gemma Drake
Research Analyst

Gemma recently graduated with a degree in International Development. She is currently studying for an MSc in Sustainable Urbanism, which examines urban planning and urban design through a sustainability lens. “I’m passionate about addressing sustainability challenges in a holistic and pragmatic way. Zero Carbon Academy's diverse range of services targets many of the areas that need support if we are to transition to a liveable future. I’m excited to see the impact that the Academy makes.”

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