• 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. 

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: 

References

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. https://doi.org/10.1038/s41612-018-0026-8

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. https://doi.org/10.1038/s41612-019-0086-4

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. https://doi.org/10.1088/1748-9326/ab6d7e

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.

• The Ag Climatise roadmap, Government’s plan to reduce agricultural emissions, is premised on a “stable herd”, but no reference year or level has yet been stated. 

• Crucially, the total herd number of beef and dairy animals does not provide meaningful information for climate action policy as dairy cows have much greater greenhouse gas emissions than beef animals. Also, within the total, the ratio of dairy to beef numbers changes through time. 

• A more useful gauge is to state the reference year and GHG target level, then assess relative dairy and beef cattle numbers based on their annual per head emissions.

• A straightforward calculation, based on the EPA 2020 National Inventory Report (NIR) data, indicates that annual total cattle methane has risen by 1.15 MtCO2eq since 2005, which is the reference year for mitigation of non-traded emissions. In terms of methane, for a stable herd relative to 2005, a reduction of 362,000 dairy cows or 940,000 beef cattle would be necessary to compensate for the rise in cattle methane emissions. 

• To maintain a “stable herd” of cattle in Ireland by stabilising methane emissions from 2018, every addition of 10 dairy cows would equate to requiring a reduction in the beef numbers of 26 animals.

• The new 2021 Inventory available in April will update dairy cow and beef per head figures for methane emissions and nitrogen excretion, so this blogpost will need to be updated to allow for the EPA recalculation.

Climate action and agriculture

Ireland has a comparatively large fraction of greenhouse gases (GHGs) from agriculture, particularly the potent global warming gases nitrous oxide (N2O) and methane (CH4). Predominantly this climate pollution is related to the number of dairy and beef cattle, which are significant sources of methane and nitrous oxide. These emissions are correlated with the level of nutrient inputs, particularly reactive nitrogen from chemical fertiliser used to boost grass growth and in concentrate feeds. The Irish government’s recently published Ag Climatise ‘roadmap’ states:

“It is well understood that emissions in agriculture have two key drivers – livestock numbers and fertiliser use.”

Although not directly referenced in the Ag Climatise document itself, the Government has strongly asserted that “the Ag Climatise roadmap is based on the premise of a stable herd”. By contrast the Climate Change Advisory Council has advised that “[a] reduction in the national herd is necessary to reduce absolute greenhouse gas emissions” (CCAC, 2019). The idea of a stable herd has now become a contentious topic in agricultural policy discussion in regard to the possible need for cap or reductions in cattle numbers. 

This blogpost (to be updated based on 2021 EPA data to be released in March) takes a preliminary look in methane-only terms at how we could calculate a "stable cattle herd" in a ‘climate smart’ way relative to a given baseline year.

At least, three important questions arise in regard to the assertion of a stable herd as a premise for climate action: 

1. What year are we using as a benchmark for stable herd in respect to climate action?

• 2005 is the EU reference year basis for non-traded emissions including agriculture. Given the Ag Climatise emphasis is on reducing GHGs in accord with EU policy it makes sense to use 2005 as the basis for assessing Irish agricultural emissions.
  
  

2. Given the big difference in GHG emissions between beef and dairy cattle, how can we judge the herd size in GHG terms if the beef:dairy ratio changes through time?

• Below, this blogpost outlines a straightforward calculation based on emission totals by animal type and the ratio of dairy to beef per head annual methane, based on animal numbers and per head methane emissions given in the EPA National Inventory Report 2020.
  
  

3. What “stable herd” target level might be consistent with societal pathways aligned with Ireland’s ‘fair share’ of a carbon budget for Paris Agreement’s temperature targets?

• In a recently published EPA Report, Barry McMullin and I looked at the trade-offs between carbon dioxide (CO2) and non-CO2 GHGs for Ireland in society-wide scenarios for effective climate change mitigation (McMullin and Price, 2020). The report’s indicative scenarios make it clear that substantial reductions in methane, 93% from agriculture, are essential to limiting overshoot of Ireland’s all-GHG carbon budget (in addition to the primary climate action focus on reducing fossil fuel usage rapidly to cut CO2 emissions). From this top-down carbon budget basis, it is apparent that a target “stable herd” would need to be much smaller than it is currently – as much as 50% reduction by 2050.

Adjusting herd size for dairy vs. beef differences is essential

Over time, the total numbers and inputs for beef animals and dairy cows are very different and the dairy to beef numbers ratio changes over time. Therefore, the total herd number (simply adding up all cattle including beef and dairy animals), or citing changes in this total, does not provide meaningful information to guide decision-makers in limiting cattle-related GHG emissions. A different measure is needed.

Using the 2020 National Inventory Report data (EPA, 2020a, Annex 3.3), Table 1 shows derived total methane emissions and numbers for all cattle, and sub-totals for dairy and beef cattle, for the 2005–2018 period. Dairy cattle methane increased by 45% and Other cattle methane decreased by 3%, resulting in an overall increase in cattle methane of 11% (46ktCH4/yr) from 418 ktCH4 in 2005 to 463 ktCH4 in 2018.

Table 1. Methane emissions for dairy and beef cattle based on 2020 NIR figures. This table will need to be updated based on revised data for the 2021 NIR that will significantly change the total methane emission and per head intensity values.

As shown in Table 1, dividing total methane by the number of animals gives the average annual per head methane intensity: for 2005 and 2018, for dairy, 121.6 rising nearly 4% to 126.5 kgCH4/head; for beef, 49.4 falling 1.6% to 48.6 kgCH4/head. As shown graphically in Figure 1, this means that a dairy cow emits 2.6 times the average methane emissions of other cattle (beef cattle including sucklers, with dairy heifers). 

Figure 1. As of 2018, dairy cows have 2.6 times the annual methane emissions of beef cattle.

In other words, if further dairy expansion is to occur from 2018 onward then every addition of 10 dairy cows would equate to a reduction in the beef herd of 26 beef animals to prevent a rise in methane emissions. Relative to 2018, the 2027 Sectoral Roadmap: Dairy (Teagasc, 2020) targets an additional 225,000 cows, with 40,000 already added in 2019. 

These values reflect the 2020 NIR, but toward the 2021 NIR the EPA has undertaken a significant recalculation of annual per head cattle methane emissions and excreted nitrogen data as shown in presentation of provisional 2019 emissions (EPA, 2020b), based on updated research (O’Brien and Shalloo, 2019). The provisional data for agriculture, resulting in 2018 total sectoral emissions rising by over 1 MtCO2eq will be finalised for submission by 15 March 2021. Once the 2021 NIR data is updated this blogpost will be revised and a Working Paper will set out the issues raised here in more detail.

Table 2. Reduction in Other cattle required to stabilise cattle methane emissions due to increased dairy cow numbers targeted by the Teagasc 2027 Sectoral Roadmap Dairy, relative to 2018 and 2005.

Teagasc (2017) have stated that stabilising methane emissions is particularly important. If so then beef cattle numbers must fall by the number of added dairy cows multiplied by the dairy to beef ratio of per head annual emissions:

·       Relative to 2018, stabilising methane based on the Teagasc roadmap dairy cow numbers would require a reduction in Other cattle of 104,000 in 2019 and 585,000 by 2027.

·       Relative to 2005, stabilising methane based on the roadmap numbers would require a reduction in Other cattle of 585,179 in 2019 and 1,525,506 by 2027.

Equivalent reductions in Other cattle would be greater than these values if annual per head emissions continue to increase for dairy cows and decrease for beef cattle. Total dairy methane is rising more quickly than total cow numbers because annual methane (and milk) per head is rising. Therefore, total dairy methane emissions are more linearly related to total milk production than to dairy cow numbers.

Coherent agricultural and climate policy requires both dairy and beef numbers based on a clearly stated climate action target

If policy-relevant statements regarding 'stable herd' cattle numbers are to be coherent with climate mitigation policy then the reference year, the climate target (stabilising methane or reducing by an amount and date) and the target rationale (EU or Paris-aligned or other) need to be clearly specified. On this basis alternative corresponding dairy and beef numbers to stay within a stable or declining methane limit can be calculated using the annual per head emission data. Adopting such a straightforward approach to coherent agricultural and climate policy would ease carbon budgeting analysis in assessing alternative transition pathways for agriculture and society.  

The calculations above do not imply that any recent-basis estimates of stable cattle numbers, adjusted for methane or not, are aligned with Ireland’s fair share in meeting the Paris Agreement targets. Our recent EPA Report, including preliminary societal mitigation pathways to meet a Paris aligned carbon budget indicates that substantial and sustained reductions in ruminant production level are required to avoid very substantial overshoot of Ireland’s all-GHG carbon budget. Policy-relevant statements could also reflect this understanding.

As noted above, this blogpost is only preliminary. Once the 2021 NIR data is updated this blogpost will be revised and a Working Paper will set out the issues raised here in more detail.

References

CCAC, 2019. Annual Review 2019 (Annual report). Climate Change Advisory Council [Ireland].

EPA, 2020a. IRELAND NATIONAL INVENTORY REPORT 2020. Environmental Protection Agency (Ireland).

EPA, 2020b. EPA’s James Murphy presents “Improvements to the Agriculture Inventory.” https://youtu.be/_5i1nwYTVL8

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.

O’Brien, D., Shalloo, L., 2019. A Review of Livestock Methane Emission Factors (Research report No. 288), EPA Research. Teagasc.

Teagasc, 2020. 2027 SECTORAL ROAD MAP: DAIRY.

Teagasc, 2017. 2017 - Reducing Greenhouse Gas Emissions from Agriculture - Teagasc | Agriculture and Food Development Authority.

Carbon budget research has profound implications for the national priority of achieving a cost-effective, low carbon transition pathway to 2050, aligned with policy accepting Ireland does its ‘fair-share’ to meet the Paris Agreement and the SDGs. 

Beginning in January 2021, working through Dublin City University, I will be researching and advising on Carbon Budgeting as a Fellow to the Climate Change Advisory Council (CCAC) as funded for a two-year period by EPA Research. For the CCAC, its Secretariat will channel requests to Fellows and provide Steering Groups to support them.  

The DCU Supervisors for my Fellowship research are Prof. Barry McMullin and Dr. Aideen O'Dochartaigh so this will also be very much a team effort. Barry and I have worked on two EPA reports on societal climate mitigation for Ireland, one looking at the potential for negative emissions technologies in the context of CO2-only carbon budgets, and a second report extending our literature review and carbon budget modelling to non-CO2 emissions. Aideen is an Assistant Professor in Accounting at DCU Business School with particular expertise in business-sector sustainability accounting and bioeconomy research.

The exact focus of the Fellowship research over the two years will likely depend on the exact content of the upcoming revision of the 2015 Climate Action and Low Carbon Development Act and the new requirements placed on the CCAC as a result of the legislation and developing Government climate change policies. To inform its advice on climate action the CCAC will be looking to the academically independent advice of the new Fellows to provide evidence-based inputs.

For now my work will focus on the Work Packages set out in the Fellowship as approved for funding under the title Carbon budgets to inform climate action: A society-wide, integrated GHG quota and accounting perspective. The research is divided into four work packages, each covering about six months work: 

1.  Agriculture and land use pathways within society-wide transition;

2.  Integrated carbon budget assessment of existing policy emissions scenarios;

3.  Design and assessment of alternative additional integrated emissions scenarios (including negative emissions and methane mitigation);

4.  Integrating national and business-sector carbon budget accounting.

Extending previous carbon budget, mitigation pathway and carbon accounting research the work packages aims to provide finer grained, sectoral-level research and modelling to help the Council provide stakeholders with essential carbon budget context for effective mitigation action.