A restatement of the natural science evidence base concerning grassland management, grazing livestock and soil carbon storage

Approximately a third of all annual greenhouse gas emissions globally are directly or indirectly associated with the food system, and over a half of these are linked to livestock production. In temperate oceanic regions, such as the UK, most meat and dairy is produced in extensive systems based on pasture. There is much interest in the extent to which such grassland may be able to sequester and store more carbon to partially or completely mitigate other greenhouse gas emissions in the system. However, answering this question is difficult due to context-specificity and a complex and sometimes inconsistent evidence base. This paper describes a project that set out to summarize the natural science evidence base relevant to grassland management, grazing livestock and soil carbon storage potential in as policy-neutral terms as possible. It is based on expert appraisal of a systematically assembled evidence base, followed by a wide stakeholders engagement. A series of evidence statements (in the appendix of this paper) are listed and categorized according to the nature of the underlying information, and an annotated bibliography is provided in the electronic supplementary material.


Introduction
Approximately 50% of the UK's land area is managed as pasture or grassland used for ruminant livestock production [1].The equivalent global figure is about 25% [2].The extent of such grassland, and how it is managed, is very significant for national and global greenhouse gas fluxes, and hence climate change.Pasture and grasslands contain substantial stocks of carbon, though can also act as carbon sources, particularly if overgrazed [2].There are also direct emissions of different types of greenhouse gases from the livestock that use these areas.Understanding the dynamics of carbon cycling and greenhouse gas emissions in pastures is critical in limiting climate change [3].
The extent to which climate change may be mitigated by producing and consuming fewer livestock products (meat and dairy) is highly contested.Some disagreements reflect the presence of vested interests or participants with strong ideological standpoints, but there is also considerable uncertainty around the natural science evidence base of relevance to policymakers.An important question is the extent to which carbon can be sequestered in the soil of grasslands grazed by livestock.This is difficult to answer for several reasons.First, carbon sequestration and stocks are influenced by the chemical and physical make-up of the soil matrix, by the local environment and by how the land is managed [4].The rate of carbon sequestration may also vary greatly over time.For example, rates can initially be very high on degraded soils that are depleted in soil carbon, but then decline as carbon builds up over time and ultimately reach an equilibrium.Second, in calculating the net benefits of pasture as carbon stores, account needs to be taken of the emissions associated with the livestock that feed on it, as well as further emissions in the meat and dairy food processing chains.This calculation is complicated by the fact that unlike many other sources of greenhouse gases, livestock production is associated with not only carbon dioxide emissions but also considerable amounts of methane and nitrous oxide [5,6].In formulating policies, it is important to take into account the different dynamics of these gases in the atmosphere.Third, the counterfactual of how much carbon could be stored by the land if it was managed in a different way, for example if it was forested, is relevant to policy; as is the question of whether any advantage of reducing meat or dairy production in one area might be reduced or reversed by increased production elsewhere which may be more or less carbon emission intensive.The wide range of estimates in the peer-reviewed scientific literature of the amount of carbon stored in pasture soil reflect these complexities.Outside the scientific mainstream there are some remarkable claims about the extent to which anthropogenic carbon emissions may be mitigated by sequestration in pastures and rangeland that need to be subjected to critical scrutiny.
Here, we attempt to summarize the science evidence base concerning carbon storage in pasture land used for livestock production, in a policy-neutral manner that is accessible to policymakers who have some background in this area but are not subject specialists.The format of the review is a 'Restatement' where the summary is given as numbered paragraphs in an appendix.Because of the size and complexity of the topic, we focus our attention on carbon storage in pasture and grassland used for livestock production in the UK, though we hope the review will also be useful in other countries.

Material and methods
The relevant literature on grassland management, grazing livestock and soil carbon storage was reviewed with particular focus on studies in the UK and a first draft evidence summary produced by a subset of the authors.The statements and their assessments were subsequently debated via correspondence until a consensus was achieved.The near-final draft was then sent to a wide set of stakeholders (see Acknowledgements) for comments.We use the following restricted terms to describe the evidence, indicated by abbreviated codes, which are similar to those used in previous Restatements [7].Codes at the end of paragraphs after full-stops indicate that they apply to the whole previous section; codes preceding full-stops or within a sentence apply to that sentence or clause alone.
[S] Strongly supported by a substantial evidence base where further information is unlikely to change the current consensus.
[L] Less strongly supported by the existing evidence base and where further information might alter the current consensus.
[E] Expert opinion based on information from related sources or general principles from different fields of science.

Results
The summary of the natural science evidence base relevant to grassland management, grazing livestock and soil carbon storage policymaking in the UK is given in the appendix, with an extensive annotated bibliography provided as electronic supplementary material.

Discussion
We comment here on several general themes that emerged from our attempt to summarize a disparate and sometimes contradictory literature.
First, part of the disagreement about the extent to which carbon can be sequestered in grassland is due to different methodologies in measuring carbon in the soil.Efforts to provide standards and platforms to allow more meaningful comparisons are valuable and important.There are also differences in methodologies to assess the totality of emissions from a grassland production system, and to make valid comparisons with other more intensive ways to produce meat and dairy.Very different results are obtained if emissions are compared per hectare of land versus per kg of product.It is also not straightforward to integrate factors such as point-emissions from the manure produced in intensive systems.Finally, there are no agreed ways to account for the indirect effects of different production systems such as displaced or replaced production, even though ignoring these can greatly distort the climate impact of different policies.Progress in developing better, standardized carbon accounting tools, as well as accounting for indirect effects, would greatly assist policymakers.
Second, developing policy around carbon sequestration in grasslands inevitably has an important temporal component.Carbon build up in soils can be very rapid when heavily degraded arable soils are laid to grass but the rate of accumulation eventually approaches zero.Some of the most dramatic estimates of the carbon sequestration potential of grasslands in the advocacy space take such initial accumulation figures and assume they can be applied to all pastures and in perpetuity.Measures of total emissions from grassland have to grapple with the different dynamics of carbon dioxide and methane in the atmosphere.Carbon dioxide accumulates in the atmosphere causing progressive warming while methane, produced by ruminant enteric fermentation or methanogens in waterlogged organic soils, relatively quickly reaches an equilibrium concentration so that constant rates of emissions cause a fairly stable warming.Reducing emissions of any greenhouse gas can contribute to climate change mitigation, royalsocietypublishing.org/journal/rspb Proc.R. Soc.B 291: 20232669 but the evidence base and the broader policy context must together determine which particular policies to consider and/or prioritize.For example, a broader policy focus might be the urgent need to prevent future warming in the next few decades to avoid potential Earth-system tipping points and give time for technological advances to help decarbonize other sectors.In that case, policymakers might place a high premium on measures to get carbon dioxide out of the atmosphere now (carbon capture in heavily degraded soils) and on reducing methane emissions (which acts to cool the environment).Landowners and farmers who make these land use changes or reduce livestock emissions are producing 'public goods' (benefits to society), an argument for public funding.A further complication is the possibility that limiting meat or dairy production in one place is compensated for by increased production elsewhere which may be less (or more) carbon efficient.
Third, though the evidence base is inevitably not as comprehensive as desirable, it does suggest some obvious no-regret policies.A variety of management practices on pasture, discussed in this Restatement, have been shown to have both environmental and economic benefits to the landowner.Many farmers and landowners are implementing them but more would be encouraged to if provided with better advice and guidance.A different type of policy where the evidence base is clear, concerns the importance of maintaining peatlands as carbon stores and ensuring that if they are used for grazing, livestock densities are kept below a level at which peat damage causes greenhouse gas emissions.
All high-income countries, to differing degrees, subsidize their agricultural and land sectors, in large part because their labour costs are higher than in lower-income countries.Subsidies may be explicit as in the area-related cash-transfers in the European Union's Common Agricultural Policy, or more indirect as in the United States' Farm Bills' provisions which, for example, provides low-cost insurance against yield losses.Some will argue that it is economically inefficient to support a particular sector in this way, but the political reality is that these transfers will continue.Accepting this, there is the opportunity for the state to get more out of its investment.The challenge is to design the means to incentivize the provision of public goods in a way that is transparent, easily implementable, avoids major transaction costs and takes into account indirect effects.To do this successfully, policymakers need to be able to access summaries of the evidence base that are policy neutral in the sense of not being designed to support a particular advocacy position.A Restatement seeks to summarize the complex literature in an area in a way that is useful to policymakers who have knowledge of the issue but are not deep subject specialists.

Data accessibility.
The data are provided in the electronic supplementary material [8].
Declaration of AI use.We have not used AI-assisted technologies in creating this article.
Appendix A (a) Aims and scope  royalsocietypublishing.org/journal/rspb Proc.R. Soc.B 291: 20232669 warranted regardless of the lack of a direct beneficial effect on net emissions in order to increase yields, thus maintaining food production while reducing the overall land required for agriculture.
[E] (f ) The composition and diversity of soil microorganisms also influence the rates of SOC turnover and stabilization.Organic material from dead microbes is the source of around 50% of total SOC in grassland topsoil.Fungi play a greater role in SOC accumulation than bacteria, due to their close association with plant roots, greater carbon use efficiency and production of more stable types of organic matter.Fungal abundance is reduced in acidic soils (see ¶15(b)).
[S] (16) Soils can also be responsible for CH 4    Other systems prioritize improvements in soil health and balancing the energy to fibre ratio in livestock diet for optimum ruminant functioning.In these systems, longer recovery periods without grazing are used to maximize forage accumulation, root growth and forage is frequently allowed to reach maturity before grazing.The precise soilplant-livestock emissions balance across such variations in management regimes are highly context specific and have not been well quantified.
[E] (d) The impact of rotational grazing depends on environmental conditions (temperature, rainfall, soil type) being suitable for vegetation regrowth between grazing episodes and this may limit its benefits.[B] (I) By contrast, adaptive and holistic grazing approaches aim to respond to changing environmental conditions.For example, areas of pasture can be removed from or added to the grazing rotation to match rapid versus slow rates of forage growth, respectively, at different points in the grazing season.However, 'stockpiling' forage biomass in this way can result in a tradeoff with forage digestibility and therefore livestock performance and hence emissions per unit of meat or dairy output.[E] (e) Although well-managed rotational grazing approaches can increase soil carbon and agricultural productivity, the extent and magnitude of possible benefits is contested.
[E] (I) Bold claims have been made by some advocates that approaches such as Holistic Planned Grazing could reverse desertification and fully mitigate anthropogenic climate change.However, the proposed mechanisms underpinning these assertions lack an evidence base [S], and the scale of the claims are implausible [E].(II) It has been suggested that adaptive, holistic or mob grazing approaches could help form 'new' soil and there is some evidence for increases in topsoil depth.True soil creation occurs very slowly and requires bedrock weathering and mineralization, and the observed increases in topsoil depth are likely to result from: (a) increase in the fine root mat typical of grasses at the soil surface and deeper roots increasing organic inputs to the subsoil, (b) partially decomposed plant litter accumulating on the soil surface and (c) improved royalsocietypublishing.org/journal/rspb Proc.R. Soc.B 291: 20232669 soil particle aggregation and soil porosity reducing soil density for an equivalent mass.It is implausible that perpetually high rates of soil carbon sequestration will occur due to the saturation point of MAOM eventually being reached.
[E] (27) Allowing livestock to graze temporary pasture in leyarable rotations leads to a greater increase (2-20%) in SOC than if grass is cut and removed [L].Livestock excreta also increase nutrient availability for subsequent crops and may benefit soil biodiversity, although the direct and indirect emissions associated with livestock must be considered [S].Livestock can also be used to graze over-winter cover crops in arable rotations, although there is currently insufficient evidence to determine the impact of this on soil carbon stocks [E].(28) In assessing the net benefits of soil carbon sequestration due to grazing, it is important to include the direct and indirect greenhouse gas emissions from the livestock involved.
[B] (a) Where soils are initially degraded, improved livestock management, such as reducing grazing intensity when overgrazing has occurred ( ¶26b), adopting rotational rather than continuous grazing ( ¶26) or integrating livestock into arable systems, can lead to an increase in soil carbon stocks particularly over the short term.Understanding the full carbon-budget implications requires considering system productivity and any indirect effects of higher or lower outputs.

[E] (b) If introducing livestock into ley-arable rotations results
in an overall increase in their numbers, then the overall carbon-budget effect is likely to be negative.But if livestock numbers do not change, positive outcomes may occur, for example because: (i) the temporary pasture that is used for grazing or forage production replaces other feed sources and their associated emissions, (ii) land is released for targeted sequestration projects, (iii) inorganic fertilizer and herbicide use is reduced on cropland due to livestock manure and grazing, respectively.Such seasonal reallocation of existing livestock from pasture areas to temporarily graze cropland is gaining traction with UK practitioners, but is limited by the spatial separation of the main cropland and livestock areas in the UK.
[E] (c) In calculating the net climate benefits of altering grazing patterns to promote carbon sequestration, using the GWP 100 CO 2 e metric may weight too highly the negative effects of CH 4 emission (if livestock numbers do not change there will be no net increased warming through this route).Using the alternative GWP* metric allows the direct effects of changes in CO 2 emissions and sequestration, and of changes in CH 4 emissions, on climate warming over different time scales to be better integrated (see also ¶(47)).
[S] (29) Relatively low intensity 'conservation grazing' by domestic livestock at appropriate times can be an important management tool in maintaining a variety of semi-natural habitats of high biodiversity value (figure 1b), in some cases substituting for wild herbivores that are locally absent or extinct [B] .However, the net impact on greenhouse gas emissions varies between studies, due to a simultaneous increase in soil CO 2 emissions from increased microbial activity as soil acidity is neutralized.Emissions associated with the production, transport and application of lime also need to be accounted for.
(g) Indirect effects (39) The creation or destruction of pastures, and how they are managed, affects greenhouse gas fluxes and the amount of carbon stored above and below ground.In estimating the overall benefits or downsides of these changes, it is essential to consider any indirect changes to emissions and storage elsewhere.[B] (40) Conversion of cropland to pasture will result in reduced crop production and increased or redistributed ruminant livestock production.However, where arable soils are significantly degraded, temporary grass leys can recover soil organic matter and fertility thus enabling a return to more productive arable cropping on rotation (see ¶20 and ¶27).
Changing management practices to increase soil carbon stores may affect food production, positively or negatively.Reduced production in one place will tend to stimulate more production elsewhere, in the absence of policies to influence consumption practices.
[E] (a) It is seldom possible to identify precisely where and how compensatory responses take place and instead they must be estimated by food system models that take into account how prices and demand are affected by changes in supply, and which incorporate realistic assumptions about within-nation and international trade.
[E] (b) Deforestation, especially in the tropics, is a major source of greenhouse gas emissions, biodiversity loss and soil degradation, so where compensatory production to reduced UK outputs involves clearing tropical forests, the net effects on climate and other environmental outcomes are likely to be very unfavourable.
[S] (41) There is substantial variation in the emissions produced from different beef production systems.Grass-fed systems typically incur higher CH (47) There are always multiple objectives when managing landscapes and grasslands, and these need to be considered in decisions around increasing carbon sequestration.For example, well-managed grasslands can improve water quality and reduce flood hazard.Both globally and in the UK, grasslands used as pasture produce food and support livestock farmers and allied rural businesses, and pastoralists with strong heritage and cultural value.Some grasslands also have important biodiversity value, and species-rich grasslands are threatened in the UK, although these frequently require specific management interventions that reduce their value for livestock production. [S] (a) Climate change, changing weather patterns and extreme weather events are likely to impact the ecosystem service of temperate grasslands.For example, changes in temperature and moisture will influence soil carbon dynamics ( ¶15(b-c)) and the resilience of forage productivity, while extreme weather events can drive shifts in soil microbial and plant communities.

Figure 1 .
Figure1.(a) UK agricultural area that is grassland or used for grazing, total 12.4 Mha (million hectares) (1).Typical stocking rates in livestock units per hectare (LU.ha−1) are also given, based on industry recommendations, where cattle over 2 years are 1 LU, and a lowland ewe with lamb is 0.12 LU.(b) UK land cover of habitat types that can be used for grazing(2).

c o n t i n u o u s g r a z i n g l i m i n g f e r t i l i s e r d i v e r s e p a s t u r e s i m p r o v e d g r a s s s p e c i e s l e g u m e s r o t a t i o n a l g r a z i n g b r o a d l e a f w o o d l aFigure 2 .
Figure 2. Carbon sequestration or emission rates from adopting grassland management practices, assumed to be adopted over 20% of UK improved pasture area from figure 1b (i.e.1.07 M ha).Positive values are carbon sequestration, negative values represent emissions.Average sequestration rates presented from years 0-10, 10-25 and 25-50 following intervention adoption, demonstrating the declining rates of sequestration/emissions over time as soil carbon stocks approach a new equilibrium.The numbers above each group of bars indicate the number of primary studies underpinning that data point.The red dashed line at 22.3 Mt CO 2 e.yr represents half (50%) of UK agriculture emissions in 2020 (total 44.6 Mt CO 2 e (3)), included as a point of reference.
1.5 and 1.9 M breeding females in the beef and dairy herds, respectively.The UK has 33 M sheep, of which 16 M are breeding females.Agriculture accounted for 11% of UK emissions in 2020 (reflecting food produced rather than consumed), of which 45% was due to enteric fermentation by sheep and cattle, and 12% due to livestock manure.[B](b)Formeatproduced in the UK, typical greenhouse gas emissions are substantially lower than global averages.UK-specific emissions have been estimated at: beef, 11-25; lamb, 17; pork, 6.4 and chicken, 4.6.All figures given as kg CO 2 e per kg of carcass weight including bones from 'cradle' to farmgate.A litre of milk is responsible for 1.1 kg CO 2 e. [S] (I) Emissions are sometimes expressed per kg of edible protein or per total (or digestible) amino acids or micronutrient content [B].Such metrics can be useful in comparing among substitutable food types, but selecting measures that favour particular food types has also been used for advocacy purposes [E].For example, the use of female-sexed semen in UK dairy cows has reduced the number of pure dairy calves required to breed replacement cows, enabling higher meat yield beef sires to be used on much of the herd, resulting in more meat produced for the same number of animals.[B](II)Genetic breeding for increased efficiency may have negative welfare impacts or result in animals less suited to outdoor production systems; for example, by reducing robustness and resilience to variable environmental conditions.[B](III)CH 4 production in ruminant animals is a heritable trait (heritability of 0.14-0.26),mediatedthroughhost genomic influences on the rumen microbiome, suggesting the potential for breeding programmes to substantially reduce CH 4 production over a small number of generations.[L]forenergy and released as CO 2 , and some used for growth (increasing microbial biomass).[S](b) Mineral associated organic matter (MAOM) is strongly bound to small soil mineral particles (less than 53 µm), particularly clay minerals, and predominantly originates from plant root exudates and microbial residues.Because microbes are tiny and live on soil particles and in the small pores between them, when they die their residues can get stuck to the soil minerals.MAOM is important for soil structure because it helps to hold soil particles together.
(7)A number of different measures and interventions have been proposed or are being researched to reduce the direct greenhouse gas emissions from livestock.Improved animal husbandry and genetics can increase production efficiency leading to lower emissions per kg of meat or dairy produced, while livestock nutrition and feed supplements may reduce methane production.[S](a) CH 4 is produced from fermenting cellulose-rich material, so increasing the digestibility and starch or fructan content of forage by manipulating plant species or variety composition, or plant age at time of grazing, can reduce emissions.Cereal-based and 'concentrate' feeds also lead to lower methane emissions.[S] Although, full life-cycle assessments are required to assess the overall benefits of different strategies [E].(b) Manipulation of rumen chemistry and microbiota can also reduce emissions.(I) Feed additives, such as nitrates and ionophores, inhibit methanogenic bacteria or provide alternative metabolic pathways to those ending with CH 4 .Feed additives can be difficult to deliver when livestock are not fed in confined systems.[L] (II) Feeding cattle diets rich in lipids, condensed tannins or seaweeds, particularly red algae, also reduces CH 4 emissions.Condensed tannins are naturally available in some forage species such as sainfoin (Onobrychis sp.) and birdsfoot trefoil (Lotus sp.), as well as in willow trees (Salix sp.) which can be browsed by livestock in agroforestry systems.There is uncertainty about the overall emissions reduction potential in farm contexts, as well as possible animal welfare and health, and environmental impacts.For example, the bromoform and iodine contents of seaweed currently limit how much can be safely fed to animals, thus restricting their emissions reduction potential.[L] (c) Genetic breeding programmes can produce animals that grow faster, have greater muscle volume or royalsocietypublishing.org/journal/rspb Proc.R. Soc.B 291: 20232669 milk yield, improved fertility and lower maintenance requirements.[B] (I) (c) Soil carbon dynamics (8) Carbon exists in the soil in two broad forms-organic and inorganic.Soil organic carbon (SOC) is derived from the remains of living organisms, while soil inorganic carbon (SIC) comes from carbonate rocks and minerals, such as chalk and limestone.Both components influence the global carbon cycle, but the dynamics of SOC are more rapid and more relevant to the degree to which soils can be managed to store and sequester carbon.[B] (9) The existing level of SOC in the soil is referred to as the stock, and is typically measured in tonnes of carbon per hectare (t C ha −1 ).Any increase in soil carbon stocks is referred to as sequestration.Soil carbon sequestration achieves a net removal of CO 2 from the atmosphere and is typically measured in tonnes of CO 2 -equivalent per hectare per year (t CO 2 e ha −1 yr −1 ).Maintaining existing soil carbon stocks is important to avoid releasing additional CO 2 into the atmosphere, but only soil carbon sequestration removes CO 2 from the atmosphere and hence contributes to climate change mitigation.[B] (10) CO 2 is absorbed from the atmosphere by photosynthesis in green plants.Stocks of SOC in the soil are increased by the addition of dead plant material from aboveground plant litter, growth of below-ground roots, root exudates from living plants and through the addition of any animal excreta containing partly digested plant material.Microorganisms in the soil, which make up a small (2-4%) but important One widely adopted approach measures two broad pools of soil organic matter (SOM; approximately 50% carbon by weight and so proportional to SOC) that decompose at different rates.[B] (a) Particulate organic matter (POM) is relatively undecomposed material present in large particles (53-200 µm).POM can be decomposed quickly by soil microbes with some used activity is reduced and soils can continue to accumulate large amounts of carbon as partially decomposed plant material, albeit very slowly.[B] (14) Soil carbon sequestration is reversible if organic inputs to the soil (i.e.plant residues and livestock excreta) are not maintained or if the rate of microbial degradation is increased, even after the SOC has reached sink saturation.[S] royalsocietypublishing.org/journal/rspb Proc.R. Soc.B 291: 20232669 .org/journal/rspb Proc.R. Soc.B 291: 20232669 royalsocietypublishing.org/journal/rspb Proc.R. Soc.B 291: 20232669 contain more POM[S].If tree planting involves major soil disturbance, it can take 5-10 years for net carbon sequestration to occur (including tree biomass) and over 20 years for the soil carbon stocks to be restored[E].(b)Onsoils with high organic content such as shallow peats and organo-mineral soils, more soil carbon is lost during tree planting, so net sequestration and(e) Grazing management and soil carbon (24) The presence of livestock, and how livestock are managed, have multiple effects on soil carbon dynamics and storage, as well as on plant net primary productivity (NPP), biomass removal and livestock emissions.Plant productivity in turn underpins the CO 2 flux of soils.royalsocietypublishing(b)A subset of rotational practices grazes high densities of livestock for short periods of time with long rest and recovery intervals; this can be called mob, tall grass, holistic planned or adaptive multi-paddock grazing.Grazing in this way can result in some plant foliage being trampled down rather than . Because of their low stocking rate, the effects on soil carbon dynamics and net emissions are small [E].
4emissions and require royalsocietypublishing.org/journal/rspb Proc.R. Soc.B 291: 20232669 more total land than grain-fed systems, although less cropland.N 2 O and CO 2 emissions are normally higher in grain-fed systems.There are not major differences in emissions per kg of animal liveweight or meat between the two production systems, though a great range of values within each.[S] The overall technical potential for soil carbon sequestration from appropriate pasture management in the UK has been estimated at 2.95 Mt CO 2 e per year (approximately 0.7% of UK current emissions), informed over 20 years.[L] (43) The permanence of carbon storage needs to be considered when developing land use policy to increase carbon sequestration.Much carbon in soils, especially the POM fraction, can be quickly lost if appropriate management is not continued or if the land is ploughed or severely disturbed [S].(a) It will be important to explore the resilience of soil carbon stores to climate change.For example, changes in temperature and moisture will influence soil carbon dynamics (see ¶(25)), while extreme weather events can affect soil microbial and plant communities and lead to episodes of soil loss and erosion.Climate change may also have an indirect effect through its influence on forage production and livestock husbandry.[E] (44) Ruminant livestock production systems in the UK are predominantly pasture based with little recent history of land use change and only moderate use of imported feed crops, meaning direct emissions from land use change are much less than the global average.UK and Eire CH 4 emissions, per kg unit output, are also well below global average, due to existing high productivity and efficiency, and a suitable climate.[E] (a) A reduction in UK livestock production could have net negative climate implications if the displaced production was made up from less carbon efficient production systems elsewhere (and for the same reason increased production might be positive).Whether this occurs depends on the details of trade networks and different product substitutability.[E] (b) For similar reasons a switch from more animal-to more plant-based diets in the UK, which would reduce emissions, need not imply a corresponding fall in livestock production if UK livestock products are competitive on global markets, especially were carbon trading to be widely introduced in the food sector.[E] (45) Greenhouse gas emissions from the livestock sector can be reduced by supply-side measures ( ¶7), but for nations and the world to halt global warming demand-side interventions such as constraining global per capita meat consumption and reducing food loss and waste are unavoidable.[B] (a) People can be encouraged to reduce their consumption of high-emissions food types by education, persuasion, fiscal measures and regulation.The effect of the change depends on the quantity and emission intensity of the foods that they switch to.On average, appropriate plant-based substitutes such as pulses have substantially lower emissions than animalbased foods and so policies to reduce demand for meat and dairy would result in lower emissions.[S] (b) Some groups in low-income countries rely on animalbased products for their nutrition and should not be subject to meat consumption reduction policies in the absence of alternatives.The availability of minerals and vitamins differs between animal-and plantsourced food which needs to be considered in formulating population nutrition policy.[E] (46) As stated above ( ¶ 4(b)28(c)) emissions policies around livestock production need to take into account the different residence times of CH 4 , N 2 O and CO 2 in the atmosphere.[E] (a) To stop further temperature increases, the world must achieve net zero CO 2 emissions, but only prevent greater than present emissions of agricultural CH 4 .If livestock production were to remain constant with the same emissions intensity then its CH 4 emissions would not contribute substantially to further global temperature increases beyond the amount of warming already caused by livestock CH 4 emissions, though any CO 2 and N 2 O emissions would.[E] (b) A sustained reduction in livestock production today would reduce the equilibrium concentration of CH 4 in the atmosphere which would reduce the level of temperature increase from livestock.By contrast, the concomitant reduction in CO 2 emissions would only limit further warming.[E] (c) Different metrics (GWP 100 , GWP*) report the warming effects of CO 2 , CH 4 and other gases in different ways, and the most suitable metric is influenced by the precise policy goal it is intended to support.The widely used GWP 100 can overestimate the long-term advantages of reducing CH 4 emissions and underestimate the short-term benefits, an issue that GWP* or climate modelling approaches can address.[E] royalsocietypublishing.org/journal/rspb Proc.R. Soc.B 291: 20232669