The environmental sustainability of our food production methods, and what kinds of agricultural systems might be compatible with keeping global warming below internationally agreed upon limits, are key topics for sustainable food systems research and policy.
Since the food system is an important emitter of three different greenhouse gases; carbon dioxide, methane and nitrous oxide; greater clarity as to their warming impacts and their consequent contribution to climate change is needed.
The climate impacts of different greenhouse gas emissions are often expressed using shorthand emission metrics. However, because these metrics reduce the full physical complexities of different emissions to a simple comparison, they can sometimes fail to represent certain dynamics, and there can be considerable variation in how we report and think about different gases relative to each other using different metric concepts. These issues can in turn cause confusion and lend themselves to misinterpretations.
These observations are particularly true in the case of methane, the valuation of which differs significantly according to the metric used. Recently, this has become a notable topic in discussions over food system sustainability, since agriculture is a major source of human-generated methane emissions. How to think about methane features especially prominently in debates about whether ruminant production can be sustainable, and if so, what types of system should be preferred.
In this piece, we introduce the fundamental climate science, highlight some policy and practical considerations relating to different ways of thinking about emissions, and finally situate the discussion within the context of wider concerns about livestock production and sustainability. We cover these topics in the following sections:
Our understanding of climate change is driven by Earth system science and observation, which essentially tracks the results of our uncontrolled multi-century experiment in increasing greenhouse gas concentrations in the atmosphere. Scientists have developed computer simulation models that try to capture the dynamics of how the Earth responds to greenhouse gases, and can be used to make predictions based on projections of future greenhouse gas emissions.
These models are of varying levels of complexity, from complete Earth system simulations requiring several days’ worth of supercomputer power to run a single scenario; to reduced complexity versions that just capture the key dynamics highlighted by the full models. But as not everyone is familiar with climate models and their use, and for many purposes the full complexities are not always relevant or straightforward to incorporate, we also have even simpler ways of capturing potential effects of emissions of diverse GHGs on the climate using emission metrics.
GHG emission metrics are reporting measures that can be used to express, typically as a single number, a simplified representation of these more complex impacts on the global climate. To inform climate policy it was deemed useful to have a simple metric that can directly relate emissions of different gases to each other. This would result in a universal ‘currency’ of greenhouse gas emissions that could be used to help policy makers design emission reduction strategies and report progress across multiple GHGs.
A way to achieve this was presented in the first IPCC Assessment Report1 with the Global Warming Potential (GWP). The GWP scales emissions of different gases according to the total amount that a one-off emission ‘pulse’ of a given gas changes the atmospheric energy balance (‘radiative forcing’ – which leads to global warming) over a specified time-horizon following the emission. As the main cause of anthropogenic global warming, CO2 was set as the baseline against which other emissions are compared, and so the GWP ‘carbon-dioxide equivalent’ (CO2-e) became the basic currency for most GHG emissions. If CO2 is assigned a value of 1, then all the other GHGs are assigned a number in relation to that 1.
While the GWP, and specifically, the 100-year variant (GWP100), has subsequently become the de facto ‘standard’ metric for reporting and comparing the impacts of different GHG emissions, some climate scientists have continued to highlight its limitations and have discussed alternative or complementary approaches to better capture the impacts different GHGs have on the climate2,3. The IPCC have consistently acknowledged the limitations of emission metrics, including the following summary in the most recent synthesis report: “The choice of emission metric and time horizon depends on type of application and policy context; hence, no single metric is optimal for all policy goals. All metrics have shortcomings, and choices contain value judgments”4. Recently, concerns over what the GWP100 can or cannot tell us have been raised in the context of agricultural sustainability, and whether we might need to rethink some assumptions about the climate impacts of agriculture, and especially of ruminant livestocki.
We should note here that concepts of ‘equivalent emissions,’ while derived from physical climate science, are not generally used in physical climate science itself. The climate models mentioned above which are used to study and predict climate change are based either on emissions of individual gases with individual, distinct properties, or changes to total radiative forcing inferred from emission pathways.
This is important to keep in mind through the following discussion, as aspects of the debate suggest that some stakeholders conflate emission metrics with climate science. Some may mistakenly believe that new metrics completely overturn our understanding of atmospheric physics. They do not. Any confusion or disagreement over the role of different gases relates to how they are communicated and the implications for policy – the topic does not represent contention or uncertainty in the physical science of climate change or over the impacts of individual gases like methane.
Because the conventional metrics (e.g. GWP) are based on ‘equivalent’ individual emissions – a direct scaling of different gases relative to CO2 – some may assume that this is exactly how the emissions affect the temperature: that, for example, methane is the same as CO2 except 28 times stronger (28 being the default 100-year global warming potential for methane from the IPCC 5th Assessment Report5). But methane does not contribute to climate change in a directly analogous way to CO2.
Compared to CO2, methane is a much more potent warming gas (per molecule in the atmosphere); but is also much shorter-lived. The figure below shows the warming contribution over time if we emit a one-off pulse emission of 1 Mt of CO2, as compared with a 1Mt CO2-equivalent amount, as defined using the GWP100, of methane (≈ 0.036 Mt CH4) or nitrous oxide (≈ 0.0038 Mt N2O).
The methane emission initially causes a large increase in warming, but this is automatically undone over the next few decades as this methane breaks down (primarily into CO2 – see footnoteii). The CO2 emission has a much smaller warming effect over the first few decades, but it is only removed very slowly from the atmosphere (with a portion of any emissions persisting for millennia), and so its warming impact continues over the 250 years shown here, and well beyond. The nitrous oxide emission shows similar behaviour to CO2 over the first 100 years before then also decaying more rapidly than CO2. As its behaviour is sufficiently similar to CO2 over immediately policy-relevant periods (at least the first century after an emission occurred), N2O can be treated as more directly equivalent to CO2 without running into the complications outlined below for methane, and so we do not elaborate further on the dynamics of N2O emissions in this piece.
These temporal differences between the gases mean that we cannot fully capture or anticipate their contribution to global temperatures simply by using a single metric (such as the GWP100) to compare them.
This is a challenge even for individual, one-off emissions as illustrated above. So what happens if we keep emitting these gases year-on-year over many decades?
Given the uniquely long atmospheric lifespan of CO2, to know how it has contributed to global temperature changes, it is necessary to consider the total amount of CO2 emitted to date (the accumulating ‘stock’). Because of its longevity, even stable CO2 emissions will lead to ever-increasing CO2 concentrations, and so to ever-increasing temperatures (see Fig 4 in our earlier Building Block on agricultural methane). This means that to stop ever-increasing CO2-induced warming, CO2 emissions must be brought down to net-zero (CO2 emissions must cease or be balanced by CO2 removals). Meanwhile, the short lifespan of methane means that its climate impacts are primarily dependent on the ongoing emissions rate (or ‘flow’), that is, the extent to which one pulse of methane is replaced by another because, as illustrated above, the main impacts of a methane emission occur within the first few decades after its release. A constant rate of methane emissions will eventually contribute to the maintenance of an elevated concentration of methane in the atmosphere. Increases in the rate of methane emissions will lead to an increasing concentration of methane and contribute to further warming; reductions in emission rates will reduce methane concentrations and result in the temperature declining again from its existing elevated level - undoing some of the warming we already experience. For further illustration and explanation of these points, see our previous explainer, ‘agricultural methane and its role as a greenhouse gas’.
To capture these dynamics, an alternative application of the Global Warming Potential has recently been proposed, termed GWP*6,7. The calculation underpinning GWP* primarily expresses the warming effects of changes in the emissions rate of methane as equivalent to a large pulse emission or removal of CO2, rather than providing a snapshot of methane emissions at a single point in time (without reference to whether they are rising, falling or staying constant).
Conventional application of the GWP acts as a single per-emission exchange rate by which different greenhouse gases are weighted in relation to CO2. GWP* aims to report equivalent emissions based on whether sustained changes in the emission rates of short-lived gases would result in a similar warming contribution to an individual, one-off CO2 emissioniii.
It is not, therefore, that GWP* can now provide a new, ‘correct’ valuation of how many tonnes of CO2 each individual tonne of methane is worth. The aim of GWP* is instead to provide an alternative conceptual framework for representing how methane and CO2 emission scenarios affect global temperatures. By choosing to undertake an activity that emits 1 tonne of methane, we are adding an amount of otherwise avoidable radiative forcing equivalent to 28 tonnes of CO2 over the first 100 years after its emission, as represented by the GWP100 of methane. But while CO2 emissions must reach net-zero to stop temperature increasing, methane emissions can be sustained indefinitely (at some level below today’s emissions) without temperatures increasing further. This is not conceptually possible if we imagine each individual emission of methane and CO2 to be directly equivalent to each other as framed by the GWP100. That is why GWP* takes such a different approach. GWP* is based on the observation that the overall dynamics of how sustained emissions of different gases contribute to global temperature change cannot be represented by any simple means of weighting individual emissions. On the other hand, the design of GWP* also makes it very difficult to apply as a single, straightforward climate indicator (for example, as a carbon footprint label), in the way that GWP100 is commonly used, since the GWP*-based value of today’s methane emissions would depend on the rate of methane emissions from that same entity or activity at some prior point in time.
Debates over the relative merits and appropriate uses of either metric are still playing out. Here, we instead discuss some of the bigger questions that are raised. What might thinking differently about the role of different gases – as could be revealed through climate modelling or GWP*, but is less apparent in how the GWP100 has been used - tell us about how food systems contribute to climate change, and about what types and systems of production could be climatically sustainable?
All anthropogenic greenhouse gas emissions contribute to climate change. Any emitters – whether producing methane, CO2, or other gases – thus play a role in our elevated temperatures. Reducing emissions of any greenhouse gas is beneficial, and all emitters have a responsibility to try and reduce their impacts. To have any chance of limiting warming to 1.5-2 °C above pre-industrial levels, we need to rapidly decrease emissions of all gases, and from all sectors10. This does not however mean that emission reductions of different gases play the same role in meeting these targets.
A halt as of today in CO2 emissions would lead to no further increases in CO2-induced warming, but stable elevated temperatures would persist for many centuries11. For methane, we could stabilise temperatures with some ongoing emissions (if emission rates gradually declined from today’s levels). A halt on methane emissions would lead to a one-off decline in temperature. Stopping methane emissions entirely would reverse most of the warming that methane is currently responsible for, resulting in a one-off decline in global temperatures back down to a new baseline level (which would continue to increase back up again if CO2 emissions were not stopped at the same time). So ‘zero emission’ targets for two different sectors, one emitting only carbon dioxide and one emitting only methane, would result in different contributions to climate change mitigation.
Because CO2 acts cumulatively, reaching ‘net zero’ emissions is the ultimate requirement, but the speed with which we get there is also key. The sooner we start reducing CO2 emissions, and the faster we reach net zero, the lesser the long-term warming we will have from CO2 that has accumulated in the atmosphere. In principle, this means that escalating our efforts and getting to net zero CO2 a few years earlier would permit a larger annual quantity of methane to be emitted indefinitely with the same consequences for global temperature, if that is what we want. Or put another way, for a given temperature cap, the faster we stop CO2 emissions, the higher the rate of methane we are ‘allowed’ to continue emitting. Conversely, reducing methane emissions will ‘allow’ only a fixed, one-off quantity of extra CO2 to be released. This does not avoid the need to bring CO2 emissions to zero, but it can delay the point by which net-zero has to be achieved (because an extra quantity of permissible CO2 translates to additional years of continued emissions).
But while some ongoing methane emissions may be able to give no further temperature increases from those emissions, maintaining these emissions into the future means they will continue to contribute to our elevated temperatures, and the resulting climate damages we will experience. And we are not yet on a route to rapid decarbonisation: fossil fuel emissions continue to increase year-on-year12 - with the caveat that the current disruption caused by Covid-19 means that 2020 is likely to depart significantly from general trends. Most projections suggest that it is very unlikely we will be able to stop CO2 emissions quickly enough that we can merely ‘stabilise’ the warming from methane at its current levels while also keeping to our 1.5-2°C targetsiv. Significantly reducing methane emissions from livestock would increase the amount of total CO2 that can be emitted while remaining under these temperature limits, and so may make them more attainable13. Integrated economic-climate models show how eliminating ruminant methane could therefore delay the rate at which a complete shift to clean energy is required, and so make this transition cheaper14. This also highlights some of the practical questions that might be raised in light of how this trade-off operates: “society may prefer a higher energy system transition cost to dietary change” (ibid.).
In the 1.5 degree pathways produced by global economic models as appraised in the IPCC Special Report on Warming of 1.5 degrees, global CO2 emissions reach net zero by about 2050, while agricultural methane emissions are reduced by about 24-47% below 2010 levels by 205015. Reducing CH4 emissions by less than this amount would mean that global CO2 emissions will have to decline even faster to meet this temperature objective, while reducing agricultural methane emissions by more could potentially delay – but not avoid – the date of net-zero CO2 emissions by a few more years.
Would a wider appreciation of these points have implications for how we view the mitigation action required from different countries or industries that are responsible for different mixes of gases? These are not issues that are introduced or solved through GHG emission metrics; questions of how, how fast and by whom different emission reductions are made are embedded in broader moral or political considerations. These will involve thinking about, for example, the historical and future responsibility of different countries and sectors in contributing to current and future climate change; differing developmental needs between countries; how much long-term warming we can accept from different activities; the costs, feasibility and trade-offs incurred in different options; and who wins and who loses as a result of the decisions made. A better understanding of what different metrics can tell us (or what they cannot) may, however, help inform policy choices.
If we overlook the dynamics described above, the main risk is that we might view different emission reductions as directly interchangeable, meaning that action to reduce methane emissions could potentially occur at the expense of action to reduce CO2.
As it affects the food system, misunderstanding these dynamics can sometimes lead to unhelpful framings of the food (and specifically ruminant) problem.
5.1 Cows compared with cars
Media representations and some non-governmental organisation (NGO) advocacy around food emissions invoke comparisons between agriculture and fossil fuels, often describing cattle emissions in terms of those of the transport sector. This may have helped draw attention to a topic by associating one activity widely appreciated to contribute to global warming (transport) with another less associated with the problem (agriculture and ruminant livestock). However, such comparisons have come in for criticism. Initially, this was because different accounting methods were applied to communicate emissions from either sector: typically a full life-cycle footprint was reported for livestock, but only exhaust/tailpipe emissions for transport16.
More fundamentally, however, and as the discussion above has shown, the methane emissions from the ruminant sector and the CO2 from the transport sector do not have directly equivalent impacts on the climate.
Transport is a major contributor to the problem of climate change via its current dependence on fossil fuels. As emphasised, the release of CO2 into the atmosphere has a permanent effect on global temperatures that will only stop getting worse once emissions stop, and can only be undone by actively removing past emissions. Some level of ruminant methane, on the other hand, may be compatible with even strict warming targets. Methane’s warming contribution is also not as irrevocable as that of CO2 - the impacts of ruminant methane on the climate can rapidly change (up or down), based on animal numbers or production methods.
To be clear however, methane is by no means the only climate consideration for ruminants: they also contribute N2O and CO2, which both have cumulative effects. This is discussed further below.
The ‘cows vs cars’ framing might also misleadingly imply that we have an either-or choice. But if we are serious about keeping warming to 1.5-2°C, we need to rapidly curtail emissions from both agriculture and transport.
5.2 Intensive vs extensive animal production
Similar considerations may apply over whether to favour intensive or extensive ruminant production systems. Intensification of a ruminant system may reduce methane emissions per unit of food but will likely lock in greater use of fossil fuels for feed production and housing. From a broader strategic perspective, additional energy demand may make overall decarbonisation more difficult.
While extensive grazing systems might emit more methane per unit of food than intensive production systems, some of them will be less dependent on fossil fuels and fertiliser inputs (see section below on ‘Ruminants and sustainability – the bigger picture’ for some system comparisons’). That said, many Life Cycle Assessments tend to report emissions aggregated using the GWP100 CO2e metric rather than reporting the gases separately, so there is not always the data available to confirm whether the balance of emissions really does substantially differ between alternative production systems17.
Once again, however, our current situation means that this is not an either-or situation: we need to reduce overall climate impacts rapidly, and this will ultimately put a limit on emissions from both intensive and extensive systems. No production methods would be able to meet the ever-increasing global demand for ruminant products without significant environmental (including climate) damage. In either type of system, the increasing methane emissions that would result will contribute to continued increases in temperature (see below), with additional environmental impacts depending on the specific system. For intensive production, these negative effects will likely come in the form of feed-food competition, potentially greater energy use, and dependence on chemical inputs; for extensive production, contributions to ongoing land-use change and its associated carbon and biodiversity impacts (discussed later) may be the more significant considerations. There will be other social, ethical and economic differences between systems as well (see here for a fuller discussion).
Unlike CO2, long-term sustained methane emissions do not necessarily always contribute to additional warming; so in principle a sustained rate of methane from an activity such as ruminant production may be compatible with our 1.5-2°C temperature goals. However, three points need to be highlighted.
Firstly, stabilising methane-induced warming (and thus keeping methane emissions at or close to current rates of release) at today’s levels is highly unlikely to allow us to keep to these climate targets since it will take up too much of the temperature ‘space’ (as described in further detail in section above on ‘Carbon dioxide, methane, and stabilising the global temperature’).
Secondly, even stabilising global ruminant methane emissions at today’s levels would require those of us in, for example, Europe, the Americas and Oceania who already eat much more than the global average to reduce our consumption, given the growth in the world’s population and the rising demand for meat and dairy in developing regions that currently have much lower consumption per capita.
The third point to note is that current trends indicate that stabilisation is in any case not on the cards. As figure 2 shows below, the projections are for an increase in ruminant production, and this translates into increases in methane (and other) emissions and, consequently, in extra warming (in reality, regardless whether reported using GWP100 or GWP*). So the ongoing trend of increased ruminant production is highly concerning, and failure to curtail the demand for ruminant products will make it harder, if not impossible, to meet the climate commitments set out in the Paris Agreement.
And it is still beneficial to decrease methane emissions, as emphasized above. Indeed, if GWP* is used, then climatic benefit of a sustained decrease of methane emissions is highlighted much more sharply than from GWP100.
When aggregated using the GWP100, the most widely accepted estimate suggests that globally, livestock production (including embedded feed production and land use change) contributes around 14.5% of annual greenhouse gas emissions, with methane accounting for about 44% of this - 39.1% of total livestock emissions from enteric fermentation methane alone19. The importance of methane is reflected in assessment of individual foods; ruminants are generally found to have the largest ‘carbon footprints’, with enteric methane a major contributor20.
As a result, methane has dominated the argument over the environmental impacts of ruminants. The introduction of the new metric, GWP*, into the public discourse has caused a great deal of discussion within the food sustainability community. One of the key insights it provides – that ongoing methane emissions can be compatible with temperature stabilisation – has been used by some stakeholders to argue that ruminants, particularly those in grazing systems, are (variously) not a problem, or inherently less problematic than monogastric pigs and poultry.
We have demonstrated above why, despite this, we still cannot ignore ruminant methane. It is even more important to emphasise that a narrow focus on methane alone can lead to the assumption that the ruminant ‘problem’ is synonymous with the methane ‘problem’ (or non-problem), when in fact ruminant systems can bring with them a range of other environmental (including climatic) and societal concerns.
Leaving aside methane for a moment, ruminants are responsible for other climate changing emissions as well: nitrous oxide and carbon dioxide - from the animals’ manure, feed and input production and from land-use change (including deforestation and the clearance of other vegetation for pasture or grain-feed production). Aggregated globally, total non-methane emissions from ruminants are still greater than emissions from monogastric livestock21. Even when scaling emissions per kg of protein we still see that ruminant production can have similar or greater emission intensities of CO2 and N2O, as shown in table 1.
That said, there is likely to be very considerable variation by specific production system and country, as indicated in table 2. The data here shows gas-specific footprints for typical conventional and organic UK meat production (data from Smith et al., 201922). Across both production methods, ruminant systems in the UK had lower CO2 emissions per kg protein than monogastric systems, but greater emissions of N2O. Combining these CO2 and N2O emissions using the 100-year global warming potential (which, as illustrated and discussed above, provides a more direct equivalence than for methane over important policy timeframes) suggests that the ruminants can still have a greater climate impact per kg protein produced than monogastric livestock, even without considering the role of methane. Relating back to the discussion of intensive and extensive production above, organic livestock systems in the UK tend to be extensive (in addition to meeting the other requirement for organic certification), while ‘conventional’ in the UK context will span a range of more extensive and intensive systems (noting that definitions of intensive and extensive are not always clear cut). The data from Smith et al., 201922 suggest that organic production could provide lower emission footprints than conventional systems, and the lower CO2 emissions for organic ruminant systems is particularly notable. However, it is important to highlight the authors’ conclusion, particularly in light of the discussion below, that this emission reduction would be negated by the CO2 generated by additional land-use change required if we wanted to maintain the same meat output. In addition, we note that here we only provide a selected overview of some UK system comparisons – the impacts of different production methods are highly variable and dependent on the specific management of individual systems, with large ranges in relative performance between organic and conventional systems across different indicators23.
Table 1. Emissions (kg) of methane (CH4), carbon dioxide (CO2) and nitrous oxide (N2O) from production of 1kg of protein from average Global and Western European cattle, sheep, pig and chicken production (aggregated across all types of production system). Data from the FAO Global Livestock Environmental Assessment Model (GLEAM)
Table 2. Emissions (kg) of methane (CH4), carbon dioxide (CO2) and nitrous oxide (N2O) from production of 1kg of protein from organic and conventional cattle, sheep, pig and chicken in the UK. Based on data from Smith et al., 201922 applying carcass weight to meat and meat to protein conversions from GLEAM for consistency with table 1 above (conversion factors from table 9.1 in the GLEAM 2.0 model description document).
Even if ruminant production is not associated with ongoing land-use change for pasture or feed production (and at the moment these two processes are driving deforestation and hence CO2 emissions), there is arguably a potential opportunity cost in not using this land for other climate mitigation purposes, including for carbon capture and/or bioenergy, or for other reasons such as biodiversity conservation. Appraising the sequestration potential of land currently used by ruminants but which could be spared for other purposes is more difficult than it may first appear. It cannot automatically be assumed that land released from livestock will be used for something that has better consequences as regards climate and/or biodiversity, and the carbon fixed by any biological processes is also time-dependent and constrained by location-specific biophysical conditions (for further detail see the FCRN report Grazed and Confused). Nevertheless, potential non-agricultural uses of any spared land remain an important opportunity for climate change mitigation and other environmental aims, given political will. It is worth emphasising that even if we had a ‘separate basket’ climate policy that treated methane differently to CO2, land-use would likely prove the ultimate constraint on ruminant production, since most models suggest we will require substantial additional land for carbon sequestration and bioenergy if we are to reach net-zero CO2 emissions within a timeframe compatible with warming at 1.5-2 °C above pre-industrial temperatures.
The impacts of ruminant production also extend beyond climate change. The land-take of ruminants and the subsequent impact on biodiversity is the major concern here. While in certain specific contexts well managed grazing can help support conservation objectives and maintain biodiversity, the historical evidence and the aggregate picture clearly show that the expansion of ruminant production has occurred at the expense of native vegetation and has been a major cause of biodiversity loss24,25. In order to halt the decline in biodiversity it is imperative that we halt land expansion, especially in sensitive ecoregions.
Additionally, in many systems, fertilisers and pesticides are used to produce ruminant feed (both grass and grains) that can impact water quality and biodiversity, while other volatile compounds such as ammonia have air quality impacts.
On the other hand, there clearly is potential for some level of ruminant production within a climatically sustainable food production system. Well-managed ruminant systems might also be able to maintain soil carbon, or play a role in restoring degraded areas and, as noted above, maintaining biodiversity. Mixed rotations or agroforestry could integrate ruminants in systems that provide other important outputs, and some studies suggest that rearing ruminants on marginal lands and feeding them non-human digestible feedstuffs could contribute to a food supply with a potentially lower arable land footprint than an entirely animal-free food system, albeit with a higher overall land take26.
Debates about the nutritional role of animal products are complex and contested, but for many in low income countries, where diets are lacking in diversity and largely grain or tuber based, access to animal source foods can provide essential sources of micronutrients (particularly for children and women of childbearing age). And of course, there is significant, widespread ruminant production in the first place because ruminant products have long made a major nutritional, culturally significant and widely-enjoyed contribution to our diets.
Beyond food provision and environmental concerns, livestock rearing can play an important social role and is a major economic contribution to the lives and livelihoods of some of the world’s poorest communities. Animal husbandry is a way of life for large numbers of people across the world, and in many countries, landscapes and rural livelihoods are inextricably tied to livestock.
For some stakeholders these benefits serve to further justify the continuation of ruminant livestock production; for others they highlight the need to find alternative non-ruminant dependent ways of providing nutrition, and creating livelihoods and cultural value. Adding to the mix are complex and multi-layered concerns about animal ethics and welfare. These points further emphasise that a reductive focus on one issue – for example, the impact of methane – is only ever part of the story.
New ways of thinking about and reporting methane emissions may help us to better understand how different greenhouse gases contribute to global warming. A more nuanced perspective could even reveal a potential climatically sustainable space for ruminant production that may appear impossible under conventional ways of considering ‘equivalence’ between different emissions and net-zero emissions targets. But this climatic space is still limited, and not compatible with ever-increasing demand for ruminant products.
In short, the big picture conclusion from the last decade’s research on dietary sustainability does not change. At current numbers and as currently produced, livestock, including but not only ruminant livestock, are and continue to be important contributors to the problem of climate change and to other environmental harms. Even if the methane emissions associated with their on-going consumption do not result in additional warming, high consuming countries and individuals occupy a disproportionately large climatic space. Reduced consumption of livestock products by these high consumers, as well as development and adoption of measures to reduce emissions from livestock production, could allow for global livestock production at potentially sustainable levels.
i For this piece, we focus on ruminant livestock, as debates over the framing of methane have been especially prominent here, and we also seek to address a number of wider environmental sustainability concerns linked with ruminant production. The climate principles discussed below apply to any biogenic source of methane, however, and for food system sustainability could also be considered in the context of rice production and consumption, given that it is also a large contributor to anthropogenic methane emissions.
ii A distinction emerges here between biogenic methane (methane from biological sources such as agriculture or wetlands) and fossil methane (‘natural gas’ leakage). For biogenic methane, this CO2 resulting from methane breakdown does not represent an additional source of carbon in the atmosphere, while for fossil methane it does. For further information on the breakdown of methane, including this point, see section 3 of our explainer Agricultural Methane and its Role as a Greenhouse Gas
iii By treating methane in this way, GWP* can describe methane emissions within a ‘cumulative carbon’ framework. Each individual CO2 emission adds a relatively uniform amount of additional, long-term warming, which will persist even when emissions cease. This means there is a time-independent, linear relationship between cumulative CO2 emissions to date, and their contribution to global temperature increases8. GWP* can report methane emissions in a way that also conforms to this relationship7,9.
iv In addition, we need to keep some ‘temperature space’ for the removal of aerosol-induced cooling that is expected once we stop burning fossil fuels.
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Lynch, J., Garnett, T., Persson, M., Röös, E. & Reisinger, A. (2020). Methane and the sustainability of ruminant livestock (Foodsource: building blocks). Food Climate Research Network, University of Oxford.
- Dr John Lynch, Department of Physics, University of Oxford
- Dr Tara Garnett, Food Climate Research Network, University of Oxford
- Dr Martin Persson, Department of Space, Earth and Environment, Chalmers University of Technology
- Dr Elin Röös, Division of Agricultural Engineering, Swedish Agricultural University
- Dr Andy Reisinger, Deputy Director, New Zealand Agricultural Greenhouse Gas Research Centre
- Dr Richard Millar, Senior Analyst at the Committee on Climate Change
- Prof Pete Smith, Chair in Plant & Soil Science at the University of Aberdeen
Reviewing and advising do not constitute an endorsement. Final editorial decisions, including any remaining inaccuracies and errors, are the sole responsibility of the Food Climate Research Network.
- The Daniel and Nina Carasso Foundation
- The Oxford Martin School
- The Wellcome Trust, Our Planet Our Health (Livestock, Environment and People - LEAP), award number 205212/Z/16/Z.