News‎ > ‎

The GWP* metric and methane mitigation potential [CCAC Carbon Budgeting Fellowship Blogpost 3]

posted 1 Mar 2021, 02:35 by Paul Price   [ updated 3 Mar 2021, 09:06 by Barry McMullin ]
  • Reducing fossil fuel usage and carbon dioxide emissions is the priority for climate action. However, Ireland also has comparatively high emissions of methane and nitrous oxide emissions, so it is essential to understand their effect on national carbon budgets and transition pathways aligned with the Paris Agreement. 

  • A new metric called GWP* enables methane assessment and its inclusion in aggregate greenhouse gas budgets with carbon dioxide and nitrous oxide.

  • Use of the GWP* metric shows that permanently increasing the methane flow of annual emissions by 1 tCH4/yr equates to a substantial warming increase, equivalent to a one-off addition of 2400 tCO2 to the atmosphere. The reverse is true for a decrease in methane flow. A permanent cut by 1tCH4/yr equates to a one-off removal of 2400 tCO2 from the atmosphere and warming reduction (not “cooling”).

  • Therefore, using GWP*, the 86 ktCH4/yr increase in Ireland’s methane annual emissions between 2010 and 2019 (mostly from ruminant agriculture) is an added warming contribution equivalent to a one-off addition of over 200 MtCO2, equal to an additional 17 years of current transport emissions.

Ongoing methane emissions do contribute to global warming

Sustained cuts in total Ireland’s methane emissions would enable considerable warming reductions that could be very important in aligning national carbon budgets of greenhouse gas (GHG) emissions with equitably meeting the Paris Agreement temperature targets as shown in our EPA report (McMullin and Price, 2020). A very useful new method of assessing the global warming impact of methane is called GWP*, pronounced ‘GWP star’ (Lynch et al., 2020). Using this formula enables useful analysis of the effect of methane mitigation in alternative low carbon transition pathways without needing a more detailed climate model to assess warming impact. 

However, the usage and implications of GWP* are commonly being misinterpreted and misrepresented. Many media articles and some experts have been wrongly suggesting that GWP* shows that a constant or a very slowly declining flow of methane is “not contributing to global warming”. This is not correct. 

In reality, methane is a potent greenhouse gas (GHG) and a continued flow of it keeps on ‘topping up’ the raised level of atmospheric methane concentration due to a source, even as methane emitted earlier decays. It is therefore misleading to suggest that the short 10-year [half-]life of methane implies that methane does not require serious mitigation attention. Unless a source stops emitting methane completely, it is of course still “contributing to warming” because the ongoing emissions are continuing to sustain a raised atmospheric methane concentration and resultant raised global temperature. It is very important to understand that the behaviour of a sustained or changing flow of methane is very different from that of a single pulse emission (or year). 

GWP* shows relative change

As presented by the Oxford University team that developed it, the GWP* metric only registers relative change over the past 20 years. After showing an initial 20 years of rapid warming due to a new source, it does not register any change in the raised level of warming from a now stable methane flow. 

This means that the GWP* metric does not show the sustained flow contribution to warming from a source; for each year it only shows the change from 20 years previously. Unfortunately this formula construction means that the presented version of the metric does not immediately reveal the full methane mitigation potential that could be available for climate action assessment in terms of trade-offs between sectors and different GHGs. 

This is important because in low carbon transition policy analysis to assess different pathway options and trade-offs we need to compare the full mitigation potential of different sectors and the different costs of alternatives that can affect the available national carbon quota for energy transition in line with Paris targets. As permanent cuts in methane emission achieves rapid and certain warming reduction equating to negative emissions, GWP* can be used to compare the costs and issues of this option with the slower and less certain warming reduction that can potentially be achieved by carbon dioxide removal through forestry or as yet undeveloped methods such as bioenergy with carbon capture and storage.

Using the appropriate GWP100 values for GWP*

Contrary to many reports, the GWP* metric does not introduce any new scientific understanding of the climate system behaviour of methane. What the Oxford University team and their peer-reviewed GWP* papers have done is to cleverly define (Allen et al., 2018) and then refine (Cain et al., 2019; Lynch et al., 2020) a formula, as shown in Note 1, to approximate the actual atmospheric warming behaviour of methane flows accurately. 

The metric conveniently utilises the already-existing time series of methane emissions as submitted by nations to the UNFCCC as part of global emissions accounting. The accounted values for GWP100 (Global Warming Potential over 100 years) in these national submissions use a factor of 25 to indicate that 1 tonne of methane per year has the equivalent to 25 tonnes of carbon dioxide (so using GWP100 1tCH4 = 25 tCO2eq). Unhelpfully for scientific use the GWP100 values used in emissions reporting are outdated and updated science has established a GWP100 value of 32 for biogenic methane, with a slightly higher value for fossil methane. The Oxford group use a GWP100 value of 32 for methane in GWP* calculation of a CO2 warming equivalent (CO2we), so the CO2eq values given in national accounts need to be multiplied by 32/25 before use for GWP* calculations.

Methane’s warming impacts: stock effect and rate of flow effect

Methane emission has two distinct global heating effects, a long-term stock effect and substantial and relatively fast action rate of flow effect (fractionally denoted in the GWP* formula by the s and r values, respectively). 

The stock effect can be considered to be cumulative on the time scale of climate action, adding up to more warming year on year. (This effect is due to the slow ocean response to the large initial atmospheric warming impact from an emission, so contrary to some reports, this methane stock effect not due to residual CO2 from methane breakdown.) The stock component of methane emissions is estimated by GWP* at one quarter of GWP100 methane emissions. Therefore given Ireland’s 2019 methane emissions of 13.7 MtCO2eq (93% from ruminant agriculture), the corresponding stock value in GWP* terms is 3.7 MtCO2we, which is still substantial. For the long-term warming effect of agriculture this is additional to 7 MtCO2we in current annual nitrous oxide emissions. 

The rate of flow effect is very different and can have very substantial impacts on the overall methane warming impact from a source: if annual methane emissions are increasing then the resultant warming increases in addition to the stock effect; if methane emissions are decreasing slowly, by 0.32%, per year then the rate effect’s warming reduction exactly cancels out the stock effect warming increase, resulting in no change in warming; and if the flow emissions are decreasing more quickly than this then there is a warming reduction. Warming due to a source only stops if the source stops emitting altogether.

Changes in methane emission have a big warming impact

Examining the GWP* formula shows that the rate of flow effect is determined by a product of [GWPH x r x H] multiplied by the methane emissions in tonnes. Using r = 0.75, the time horizon H of 100 years for the GWP100 value of 32, this equates to 32 x 0.75 x 100 = 2400 CO2we/tCH4. (As presented by the Oxford team, the formula divides the change by a Δt of 20 years to average out the warming impact of a new flow as occurs in reality.) 

This means that permanently increasing methane flow by 1 tCH4/yr equates to a warming increase equivalent to a one-off addition of 2400 tCO2 to the atmosphere. 

The reverse is true for a decrease in methane flow, a permanent cut by 1tCH4/yr equates to a one-off removal of 2400 tCO2 from the atmosphere.

This very large CO2 warming equivalence of a small sustained change in methane flow shows that changes in methane emissions can have a very large effect on aggravating or mitigating climate change. GWP* correctly shows sustained reductions greater than 0.32% per year as having negative CO2we emissions. This is a great improvement on GWP100 which still shows positive CO2eq emissions for methane even if the rate is decreasing, so GWP100 fails to show the warming reduction related to reducing methane flows. 

It is important to note that short time GWP* time series of annual CO2we values can be highly variable and easily misinterpreted because GWP* incorporates a 20 year lag to reflect the incremental delayed warming impact of a year’s emissions (peak warming is 12 years after emission). Annual methane emissions in Ireland, shown by GWP100, decreased from 1998 until 2011 and increased due to dairy expansion thereafter, and, as a result, GWP* annual values went negative due to the decrease and have only become positive since 2015. Our recent working paper comparing recent agricultural methane increases with a mitigation alternative clearly shows the effect of this lag (Price and McMullin, 2020).

GWP* enables the methane to be included in aggregate all-GHG budgets based on cumulative CO2we emissions from a defined year to assess warming. This allows whole economy societal carbon budgeting to assess the warming impact of different policy options in terms of GHG and costs between different sectors.

Using GWP* to show the full (technical) methane mitigation potential

Informatively, we can use the above GWP* flow rate equivalent of 1 tCH4/yr = 2400 tCO2we to estimate a stable flow equivalent for each year. This is shown in the chart below, which excludes the stock effect and the smoothing achieved by GWP* value but helpfully reveals an approximation of the full amount of warming sustained by methane emissions rate of flow. For each time series year, the tonnes of methane emissions (tCH4) is multiplied by 2400 to get a value for each year reflecting the hypothetical case of warming being sustained at this level of flow. 

Chart, line chart  Description automatically generated

This shows that the level of warming being sustained by Ireland’s methane emissions is very substantial, approximately equating to a one-off addition of 1400 MtCO2we, over 1300 MtCO2we of which is from cattle and sheep. Compared to the rise in agricultural methane of over 200 MtCO2we from 2010 to 2020 using the GWP* factor to show a stable flow impact, looking at the standard GWP100 reporting of agricultural methane shows only a rise of about 2 MtCO2eq. 

In Ireland, 1 tCH4/yr is equivalent to the annual methane emissions of more than 7 dairy cows; since dairy cow numbers have increased by over 400,000 head over the past decade the resultant warming equates to a one-off addition of about 140 MtCO2, indicating that the past decade of dairy expansion has resulted in a warming equivalent to an additional 12 years of transport CO2 emissions. Adding a single dairy cow has the same equivalent impact on global temperature as a one-off release of 320 tonnes of CO2, the same as driving a new car 2.8 million kilometres. Permanently removing a single dairy cow from production has the reverse, warming reduction impact.

Conclusion: methane mitigation has substantial impact on carbon budgets

As shown in our previous research, using GWP* can aggregate all-GHGs into cumulative CO2 warming equivalent values to enable assessment of the warming due to alternative policy pathways. Changing methane emissions has a very substantial effect on Ireland’s rapidly depleting remaining carbon budget in terms of equitably aligning climate action with the Paris Agreement:

Scenarios with higher sustained CH4 reduction rates [than -1% per year] would greatly ease the required net CO2 mitigation rate and limit overshoot of near-term climate targets. (McMullin and Price, 2020)

Note 1: Extract below from Cain et al. , showing the GWP* formula used to calculate the “CO2 warming equivalent” emissions for a single year: 

Text  Description automatically generated


Allen, M.R., Shine, K.P., Fuglestvedt, J.S., Millar, R.J., Cain, M., Frame, D.J., Macey, A.H., 2018. A solution to the misrepresentations of CO₂-equivalent emissions of short-lived climate pollutants under ambitious mitigation. Npj Clim. Atmospheric Sci. 1, 16.

Cain, M., Lynch, J., Allen, M.R., Fuglestvedt, J.S., Frame, D.J., Macey, A.H., 2019. Improved calculation of warming-equivalent emissions for short-lived climate pollutants. NPJ Clim. Atmospheric Sci. 2, 1–7.

Lynch, J., Cain, M., Pierrehumbert, R., Allen, M., 2020. Demonstrating GWP*: a means of reporting warming-equivalent emissions that captures the contrasting impacts of short- and long-lived climate pollutants. Environ. Res. Lett. 15, 044023.

McMullin, B., Price, P., 2020. Synthesis of Literature and Preliminary Modelling Relevant to Society-wide Scenarios for Effective Climate Change Mitigation in Ireland  2016-CCRP-MS.36 (EPA Research Report No. 352). Environmental Protection Agency.

Price, P.R., McMullin, B., 2020. Assessing methane (CH4) from Irish agriculture in climate policy 2005–2020 using the GWP100 and GWP* greenhouse gas (GHG) equivalence metrics 2.