Livestock and greenhouse gas emissions

The production of animal source foods (ASFs) creates greenhouse gas (GHG) emissions. Arguing that climate change mitigation requires radical transition to plants overlooks that dietary change has a minor impact on fossil fuel-intensive lifestyle budgets, that enteric methane is part of a natural carbon cycle and has different global warming kinetics than CO2, that rewilding would generate its own emissions, that there are still ample opportunities to improve livestock efficiency, that livestock not only emits but also sequesters carbon, and that foods should be compared based on nutritional value. 
Emissions of greenhouse gasses (GHG) show an upward trend, mostly driven by fossil fuel combustion [OurWorldinData 2020]. The food system is a main source too, contributing up to 20-35% of total human-caused GHG emissions [Vermeulen et al. 2012; Crippa et al. 2021; NASA 2021]. Of these food emissions, 3/4 are situated in 'developing countries' and China; Asia yields half of the emissions [Crippa et al. 2021]. Agriculture and fisheries provide 40% of the food emissions, while land use/land-use change (mostly in developing countries) and supply chains provide each some 30% [Crippa et al. 2021]. Within the supply chains, global food miles account for some 20% of the total emissions within the food system [Li et al. 2022]. The largest part of the agricultural emissions are ascribed to livestock [>80% in the EU; Peyraud & MacLeod 2020]. Of the food emissions, 1/3 is due to methane, mainly from ruminants and rice [Crippa et al. 2021]. Contribution by artificial fertilizers, for both feed and food, is estimated at 1% of the global GHG emissions [IFS 2003], but this may be an underestimate based on the associated methane emissions [Zhou et al. 2019]. 
Since ASFs, and ruminant products in particular, most often have higher emissions per unit of mass or kcal than plants [Poore & Nemecek, 2018], they have become a primary target of dietary policies [Grasso et al. 2021]. Climate responsibilities are predominantly burdened on the shoulders of meat and dairy producers and their contributions to national and global emission budgets [Lazarus et al. 2021]. It has been suggested that lack of dietary change, including more plant-based eating, would preclude achieving the climate change targets of the Paris Agreement [Clark et al. 2020]. Some even claim that vegan eating is the 'single biggest way' to reduce one's environmental impact on the planet [Petter 2018], reducing footprints with >70% [Vegconomist 2019]. Such statements are exaggerations and a convincing moral obligation for climate veganism is not justified [Kortetmäki & Oksanen 2020]. Addressing GHG in the livestock sector, although important, should be done based on robust premises. Contrasting ASFs to plants is overly simplistic, as every dietary change comes with its own effects (some good, some bad). The ten points mentioned below need to be factored in holistically when evaluating the GHG emissions of livestock and ASFs.
1. Global data should not be used to evaluate local contexts
Globally, livestock represents 5-6% of the direct emissions [Mottet & Steinfeld 2018OurWorldinData 2020]. A more comprehensive life cycle assessment (LCA) in 2013 has put its contribution at 14.5%, mostly ascribed to feed production and its link with land-use change (45%) and enteric fermentation by ruminants (39%) [Gerber et al. 2013]. More recently, this estimate has been updated to 11%, using IPCC AR6 data [FAO 2023], but estimates remain relatively uncertain [BTI 2023]. Importantly, this estimate masks considerable heterogeneity worldwide [Gerber et al. 2013; Herrero et al. 2013]. It should be treated as such when discussing the climate impact of local systems. 
The same caution is needed at the level specific ASFs, with beef showing particularly large variability. Intensities are higher in sub-Saharan Africa and southern Asia (40-50 kg CO2-eq/kg carcass weight) than in Latin America (25 kg), North America and Oceania (10 kg), and Europe (5-10 kg) [Gerber et al. 2013; Thompson & Rowntree 2020]. The most comprehensive studies to provide baseline measures for US beef estimated carbon emissions per kg carcass weight at 21 kg CO2-eq [Rotz et al. 2019] and at 43 kg CO2-eq per kg of consumed beef [Putman et al. 2023]. Such heterogeneity is also found for dairy and - to a lesser degree - pork and poultry [Peyraud & MacLeod 2020]. This being said, smallholder farms of the Global South may lead to lower emission intensities than previously assumed, depending on productivity and herd management at farm level. About half of Kenyan smallholder farms displayed milk-related emission intensities that were comparable to the mean within Europe, even if the median was almost double [Ndung'u et al. 2022]. 
Regional differences are due to variations in feed digestibility, genetics, slaughter age and weight, climate conditions, and management [Gerber et al. 2013; Herrero et al. 2013], as well as a different focus on wealth, draft power, fuel, and religious significance [Smith et al., 2013; Thompson & Rowntree 2020], or nutrient security [Tedeschi et al., 2017]. In Europe, lower footprints are partially ascribed to the fact that 80% of beef comes from dairy animals [Gerber et al. 2013]. In the US, this is less the case even though finished dairy Holstein cattle have half the intensity of finished beef breeds [Rotz et al. 2020]. A dairy calf has a much smaller footprint because it is a by-product of milk production, while the lower footprint of a cull dairy cow compared to a finished beef breed is due to the fact that part of her life-long impact is allocated to milk [Rotz et al. 2019].
2. Further mitigation is possible
The argument against livestock neglects that production can still become considerably less carbon intensive in the future [Kortetmäki & Oksanen 2020]. Globally, a 30% reduction can be achieved if all producers would adopt the practices used by the 10% most efficient ones (or 18%, adopting the best 25%) [Mottet et al. 2018]. There is considerable margin for mitigation by focusing on feed strategies, veterinary care, smart use of manure, and herd management [Gerber et al. 2013; FAO/GDP 2018; Carrazco et al. 2020; Davison et al. 2020; Peyraud & MacLeod 2020; Ndung'u et al. 2022]. Margins are substantial for some regions of Sub-Saharan Africa and South Asia, but the potential is also present in industrialized countries. In Norway, for instance, recent improvements in the pork chain have been obtained based on better genetics and management [Bonesmo & Gjerlaug Enger 2021]. For cattle specifically, introducing seaweed into feed could reduce methane emissions by 82% [Roque et al. 2021]. Valorization of biogas also holds potential [Xue et al. 2019; Jiang et al. 2020], as well as an improved integration with crop agriculture [Liu et al. 2015; Lal 2020]. The Australian red meat sector considers the possibility of achieving a carbon neutral status by 2030, based on a combination of carbon sequestration [see below] and mitigation strategies to decrease emissions [Mayberry et al. 2019; Davison et al. 2020]. 

Another mitigation option consist of reductions in food wastage (losses plus waste), re-use of meat-processing by-products, and increased consumption of edible offal [Xue et al. 2019]. Cradle-to-grave emissions from food loss and waste (9.3 Gt CO2-eq/y in 2017) may account for half of the total GHG emissions from the global food system [Zhu et al. 2023]. More than one third of food wastage seems to occur at the consumption phase [FAO 2013]. In the West and industrialized Asia, the average footprint from wastage is 0.7-0.9 t CO2-eq/p/y, exceeding a global average of 0.5 t CO2-eq/p/y. In the US, avoidable food waste represents 2% of the total GHG emissions [Venkat 2012]. In UK households, >10% of meat purchases that are still safe to eat are thrown away [Taylor 2020]. Ultra-processed foods should be a particular target in such strategies, since they not only lead to up to one third of all food waste in high-income countries, they are also harmful in many other ways for both human and planetary health [Anastasiou et al. 2022].

In the EU27 plus UK, total agricultural emissions have substantially decreased already (by 20%) between 1990 and 2019 according to UNFCCC data, including methane (-21%) and N2O (-19%) emissions [Peters 2021]. For food waste, the decrease was equal to 44%, particularly driven by a drop in methane (-46%). Even if global methane emissions from ruminants has doubled from 50 to 100 Tg/y in 2012, compared to 1961, this increase mainly took place in the Latin America, Asia, the Near East, and Africa. In Europe and Russia, however, enteric methane emissions decreased with 31% and 54%, respectively, during the period of 1990–2012, whereas emissions in the US remained more or less stable [Chang et al. 2019].
3. Restricting ASFs only entails a small effect
Restricting ASF production will not lead to a simple subtraction from the GHG budget, but would generate its own effects related to the filling of the nutrient gap created by a shift to more crops. As a result, removal of livestock in the US would only lead to a net GHG reduction of 2.6% in national emissions. Similarly, removing all dairy would lead to a reduction of just 0.7%. At the same time, both transitions would create domestic deficiencies in critically limiting nutrients [White & Hall 2017; Liebe et al. 2020], which is not unexpected given that ASFs are valuable sources of essential nutrition [see elsewhere]. 

A similar order of magnitude (3%) is found when approaching this issue at the level of Western individuals. On what is usually a dietary footprint of around 2 t CO2-eq/p/y [Hertwich & Peters 2009; Muñoz et al. 2010; Meier & Christen 2013; Arrieta & Gonzáles 2018; Crippa et al. 2021; Heller et al. 2021; Barnsley et al. 2021], a 60-% decrease in meat intake (from 200 to 80 g/p/d) would save 0.2 t CO2-eq [Meier & Christen 2013]. Vegetarianism and veganism lead to -0.5 and -0.8 t CO2-eq, resp., according to systematic reviews and summary reports [Hallström et al. 2015; Wynes & Nicholas 2017], although some individual studies claim somewhat higher savings [>1.0 t CO2-eq; e.g., Arrieta & Gonzáles 2018]. For the UK, specifically, the effect of a >40%-reduced-red-meat diet has been estimated at 0.5 t CO2-eq/p/y, some 3% of the total carbon footprint [Aston et al. 2012]. For the USA, halving all ASFs would save 0.6 t CO2-eq, or 0.9 t CO2-eq when beef is further reduced to 10% of the baseline [Heller et al. 2021]. Taken together and assuming, for instance, a total footprint of 12 t CO2-eq [data for France], these scenarios roughly translate into a 2-6% total carbon footprint reduction [Leroy et al. 2022]. The latter may arguably need to be adjusted to 1-3% because of rebound effects [cf. Grabs 2015]. A similar reduction (2-4%) has been found for a lifetime's total reduction in consumption-based emissions when adopting a meat-substituted diet in New Zealand [Barnsley et al. 2021]. Calculations of course depend on what is considered dietary equivalence, with many ASF-associated nutrients being more difficult to obtain from restrictive plants-only diets [see elsewhere].
The effect of a 'plant-based' shift is thus not only small on a yearly basis but especially so on a lifetime of emissions. Indeed, numerous vegans and vegetarians (up to 70-80%) rapidly revert to eating meat and other ASFs, a third even within three months of their change of diet [ScienceAlert 2014; Faunalytics 2014, 2015]. Only 12 to 24% of current vegans may be in the diet for >5 years [Kerschke-Risch 2015; FCN 2018; VOMAD 2019], 7% for >10 years, and 3% for >20 years [VOMAD 2019].
Some vegetarians may even have higher impacts than some omnivores [Rosi et al. 2017; Kortetmäki & Oksanen 2020], as certain plant products have elevated footprints. In the UK, this is valid for aubergines, peas, beans (3 kg CO2-eq/kg), asparagus (5), and Kenyan beans (6) [Frankowska et al. 2019a]. UK-produced tomatoes range from <1 (high-yield, seasonable), over 9 (average), to 50 kg /kg (cherry tomatoes, off season) [Berners-Lee 2010]. Mushrooms (2-3) [Robinson et al. 2019; CSS 2020], nuts (1-5) [Volpe et al. 2015; CSS 2020], grapes, pineapples, peaches, avocados (2-3), and mangoes (4) are non-negligible [Frankowska et al. 2019b]. Chocolate may be at 11 [CSS 2020] to >60 kg/kg [Poore & Nemecek 2018]. Vegetables and fruits also come with >1/3 of the food-miles emissions, which is substantial within the total food-systems emissions (20% of this total coming from food-miles)  [Li et al. 2022].
Meat substitutes (1-6 kg/kg) have footprints that are comparable to poultry or eggs (2-6) [Nijdam et al. 2012]. 'Chicken-free' quorn, for instance, has a footprint of 3 kg/kg [Finnigan 2010], while other mycoproteins are estimated at 6 kg/kg [Souza Filho 2019]. The contribution of in vitro meat has been estimated at around 7 (3-25) kg/kg, higher than poultry and pork [Mattick et al. 2015], and not prima facie climatically superior to beef (10-150 kg/kg) because of different warming potential effects (see point 10 below for info on cattle's GWP*) [Lynch & Pierrehumbert 2019]. But according to a more recent cradle-to-gate life cycle assessment factoring in the use of fine chemicals and higly refined growth media, the impact of in vitro meat (250-1500 kgCO2-eq/kg) could even be orders of magnitude higher (4-25x) than median beef production (60 kg CO2-eq/kg edible meat) [Risner et al. 2023].

Moreover, carbon tunnel vision should be avoided when attempting sustainable diet transitions. Gains in climate impact reduction due to a shift away from ASFs can potentially lead to trade-offs related to other environmental variables, such as increased water-scarcity footprints [see elsewhere].
4. Perspective: comparison with non-dietary lifestyle effects
From a climatic reasoning perspective, the overall carbon footprint should remain below one's 'harm budget'. This implies that the focus should not be on single actions but on aggregates [Kortetmäki & Oksanen 2020]. Overstating effects from dietary footprints distracts from the more impactful emissions related to extractive Western lifestyles and their reliance on fossil fuels. Overall, just 90 companies have been driving 63% of the GHG emissions between 1751 and 2010, with half of those emissions taking place after 1988 [Starr 2016]. Also, just 100 companies have been the source of more than 70% of the world’s greenhouse gas emissions since 1988 [Riley 2017].  

It is sometimes claimed, especially by animal right activists, that livestock creates more emissions than transport ("cows are worse than cars"), but this statement is erroneous [Mottet & Steinfeld 2018]. The transport sector’s 14% (IPCC data, direct emissions) cannot be directly compared to the commonly reported 14.5% figure for livestock (FAO data, LCA calculation), which moreover has been reassessed at 11% more recently [FAO 2023]. A comparison is only feasible at the level of direct emissions, which in the case of livestock are estimated at 5%. Also, living car free saves 1.0-5.3 t CO2-eq/p/y, [Wynes & Nicholas 2017], which exceeds the carbon gain for a shift to plant-based diets mentioned above (i.e. <0.8 t CO2-eq/p/y). Typical vehicles emit 3 [Belgium; DeLijn 2021] to 6-9 t CO2-eq/p/y [US; EPA 2020] depending on the country and average distances covered, with a kg CO2-eq/km load of 0.1 [EU; EEA 2018] to 0.7 [USA; EPA 2018]. For comparison, a cow emits about 1.5 t CO2-eq/y as methane equivalents [Davison et al. 2020], but this needs to be contextualized for global warming [see below]. 
Global tourism creates 8% of GHG emissions [Lenzen et al. 2018]. A roundtrip flight yields 0.7-2.8 t CO2-eq/p [Wynes & Nicholas 2017], offsetting years of veganism or decades of flexitarianism. This is still minor compared to the emissions of private jet users [up to 7,500 t CO2/y, Gössling & Humpe 2020] or superyachts [some 4,500 t CO2/y; Harding 2019], underlining a conflict between the virtue-signalling and carbon-intensive lifestyles of elites [Ahmed 2019Catenacci 2022]; Passifiume 2022]. For instance, a celebrity told her fan base that a vegan meal a day corresponds with driving from LA to New York [Starostinetskaya 2019], while her own air travels equalled 40x the total yearly emission of average Americans [cf. Gössling 2019]. For space tourism, increasingly popular among billionaires, CO2 emissions per flight represent 50-100x the emissions of a regular long-haul airplane flight [Marais 2021]. Taken together, some 1% of the world population emits half of the CO2 from commercial aviation [Gössling & Humpe 2020]. The latter represents >2% of global GHG emissions and is on the rise, while other sectors are reducing their emissions [ICCT 2018]. Moreover, these calculations largely underestimate the effects of non-CO2 emissions [Lee & Forster 2020], which account for more than half of the aviation net forcing [EASA 2020], so that aviation causes 5% of anthropogenic warming [T&E 2020]. OXFAM [2022] has estimated that the average personal consumption emissions from a sample of prominent billionaires amounts to no less than 8,190 t CO2/y, increasing to 3 million t CO2/y when taking into account their investments.
Increased digitalisation also come with heavy carbon costs [e.g., Energuide 2021]. Email use alone creates 0.1-0.6 t CO2-eq/p/y for professional and individual users [Berners-Lee & Clark 2010; Richards, 2018]. ICT may reach >14% of the global GHG budget by 2040 (smart phones surpassing the individual contribution of desktops, laptops, and displays) [Belkhir & Elmeligi 2018]. Communication technology (for consumer devices, communication networks, and data centers) could even become responsible for half of global electricity use and up to 23% of the GHG budget in 2030 [Andrae & Edler 2015]. 
There are various other important contributions to global GHG emissions that are inherent to contemporary lifestyles but are hardly addressed in the public debate. Fashion, for instance, generates 10% of the GHG budget [UNECE 2018; UNFCCC 2018], whereas the average person buys 60% more clothing than in 2000 [World Bank 2019]. Owning a dog or horse releases 1.0 or 3.0 t CO2-eq/y, resp. [Annaheim et al. 2019], while pet feed in the US generates 25-30% of the environmental impact from animal production [Okin 2017]. Worldwide, 1-3% of the GHGE can be ascribed to pet food [Alexander et al. 2020]. Yet, pets, fashion, travel, and smart phones receive little attention in comparison to the dietary quick-fix claims.

5. Nutritional quality should not be overlooked
The widespread use of CO2-eq/kg in dietary scenarios is a reductionist metric that should not be used to compare foods with large differences in nutritional adequacy. Indeed, the higher carbon footprint of some nutrient-dense foods and beverages can be offset by their higher nutritional value [Smedman et al. 2010; Drewnowski et al., 2015; McAuliffe et al. 2018, 2020]. Even the common use of CO2-eq per kcal or per kg protein are poorly informative. Despite their apparent use of nutritional units, they overlook the complexity of dietary needs and optimal health [see elsewhere]. 
The main global food system challenges are not related to 'energy' or caloric efficiency, which is excessively and unfairly emphasized in  studies that express a food's impact in CO2-eq/kcal [e.g., Poore & Nemecek 2018], but to adequate essential nutrition. The latter is determined by protein quantity and quality as well as levels of key micronutrients [Nelson et al. 2018], of which the most crucial ones are best obtained from ASFs [see elsewhere]. Even when using ‘protein’ as a unit of comparison [e.g., Poore & Nemecek 2018], it is important to incorporate its biological value (determined by its digestibility and spectrum of essential amino acids), as this can considerably affect the outcome of the GHG comparison between foods [cf. Tessari et al. 2016; Marinangeli & House 2017; Sonesson et al. 2017; Moughan 2021; McAuliffe et al. 2023].
Developing dietary policies that aim at reducing GHG emissions but are nutritionally harmful or incomplete should be dismissed as unacceptable [Ridoutt et al. 2017]. For instance, ultra-processed foods drive overconsumption, so that this food group shows increasing greenhouse gas emissions over time in contrast to minimally processed and processed foods [da Silva et al., 2021]. Moreover, ultra-processed foods already account for up to one third of all diet-related emissions of adults in high-income countries [Anastasiou et al. 2022]. In Sweden, "sweet, snacks, and drinks (excluding dairy)" account for 18% of the dietary emissions [Moberg et al. 2020]. This is lower than the 67% coming from ASFs in that same diet, but the latter bring in much more valuable nutrition.

Lower GHG diets tend to decrease micronutrient supply and increase the content of sugar and discretionary foods, which have their own detrimental effects [Payne et al. 2016]. It needs mentioning that unhealthy diets lead to enormous health costs as well as important carbon footprints [Eckelman & Sherman, 2016], the pharmaceutical industry being more emission-intensive than the automotive industry [Belkhir & Elmeligi 2019]. Moreover, obesity not only brings carbon costs related to disease but also a 20%-increase due to greater metabolic demands, higher intake of food, and higher mobility costs due to greater body weight [Magkos et al. 2019; Kortetmäki & Oksanen 2020].
6. Co-product benefits should be accounted for

In addition to nutritional value, life cycle analyses of ASFs usually neglect to equitably share portions of the emissions profile with the non-edible products and services associated with livestock production (e.g., hides, wool, fats, organs, milk, bone, serum, manure, draught power, etc.) [Alao et al. 2017; Mullen et al., 2017; Lynch et al. 2018; Katz-Rosene 2020]. For the global livestock sector, the value of animal by-products (both inedible and edible) is substantial. This is also true in the West, for beef and pork co-products in particular [Marti et al. 2011].
Proper allocation is required to account for all of the functions of co-products in their various uses and markets, which is a complex task [Chen et al. 2015; Le Féon et al. 2020]. For instance, important differences are found when contrasting economic and mass allocation models, which is especially valid for beef [Gac et al. 2014]. The production of pet food needs to be carefully considered as such, with an economic allocation model being more meaningful than mass allocation (overestimation) or considering pet food as simple waste (underestimation) [Alexander et al. 2020]. Although such allocations in a multi-output system are difficult to quantify in a robust and fair manner, successful incorporation in life cycle analyses would further lower the carbon footprint of ASFs.

7. Livestock farming also sequesters carbon 
Based on accountancy methods for anthropogenic emissions, agriculture is often discussed seen as one of the most harmful sectors, whereas in reality a holistic interpretation is needed that also factors in how much carbon is sequestered [Frankelius 2020]. Grazing lands co-evolved with herbivores and act as massive reservoirs of soil organic carbon (10-30% of the global total) [Schuman et al. 2002; Kleppel 2020]. Their vast expansion over 40 million years, covering almost half of the land mass [Zimov 2005], may even have induced global cooling and Pleistocene glaciation [Retallack 2013]. At the Holocene boundary, however, megafauna extinction (likely due to overhunting) led to a shift from the 'mammoth steppe' to tundra [Zimov et al. 1995], while impacting on the methane budget [Zimov and Zimov, 2014; Smith et al. 2016]. 
Climate change mitigation strategies should factor in the interplay between soil biology, vegetation, grazers, and predators. Because proper grassland management improves soil carbon stocks [Conant et al. 2017], offsetting of emissions can be high in regions where grazing lands are a dominant biome [Viglizzo et al. 2019]. In the Welsh uplands, a drop from 12 to 9 kg CO2-eq/kg beef is obtained when accounting for sequestration estimates [Hybu Cig Cymru 2020]. In South-West Spain, sequestration offsets by soil and vegetation (trees) ranged from 54% to 95%, sometimes even resulting in negative carbon footprints [Reyes-Palomo et al. 2022]. Carbon stocks may deplete, however, in the case of harmful grazing management and grassland degradation [Ganjegunte et al. 2005; Chang et al. 2021].
The sequestration process is based on photosynthesis (via vegetation) and carbon storage in soils [Kleppel 2020]. The latter is driven by complex interactions of above-ground herbivory, saliva, dung and urine, and below-ground biology (involving fine root exudates and microbial biomass) [Merill et al. 1994; Bardgett et al. 1998; Wilson et al. 2018]. As a result, gains are seen in topsoil, fertility, and forage biomass [Gullap et al. 2011; Franzluebbers et al. 2012; Hillenbrand et al. 2019]. Soil thereby acts as an extended composite phenotype of a resident microbiome, responsive to organic inputs [Neal et al. 2020]. Success depends on the effectiveness of microbially-mediated  and soil-dependent micro-structure remodelling.
Results are context‐specific, implying that managerial adjustment is needed in different regions [McSherry & Ritchie 2013]. Where conditions allow it, some pastoral management systems drastically offset emissions [Allard et al. 2007; Beauchemin et al. 2011; Teague et al. 2016; Assouma et al. 2019]. On US rangelands, grazing management increases soil carbon with 0.1-0.6 t C/ha/y [Schuman et al. 2002; Conant et al. 2003; Liebig et al. 2010; Pelletier et al. 2010; Lupo et al. 2013], but rotation systems have even reported rates of 2.3 [multi-species; Rowntree et al. 2020] and 3.6 t C/ha/y [beef cattle; Wang et al. 2015; Stanley et al. 2018]. Starting from degraded cropland, this can be up to 8.0 t C/ha/y [Machmuller et al. 2015].
Despite such impressive results, the effects of 'regenerative' agriculture are ignored in conventional assessments. This is partially due to a lack of data and partially due to a dismissal of the potential, based on (1) the fact that more land is required [Stanley et al. 2018; Rowntree et al. 2020], (2) the assumption that soils are generally in a long-term equilibrium of near-zero carbon sequestration [Viglizzo et al. 2019], and (3) the higher methane emissions for grass-only cattle compared to those finished on grain because of differences in feed digestibility [Lupo et al. 2013; Heflin et al. 2019]. Although these limitations need consideration, they are often overstated for a number of reasons outlined hereafter.
A lot of degraded grassland and arable land is available still, which is poorly suited for cropping [Yang et al. 2019; see also elsewhere]. Moreover, these soils are degraded to such an extent, that their long-term sequestration potential may provide the best opportunity to reduce the footprint of ruminants [Rowntree et al. 2020]. While conversion of grasslands into arable land leads to rapid carbon losses, mainly in North America, Europe, and South Asia [Chang et al. 2021], the inverse action indeed leads to more carbon in the surface and deeper layers, while enhancing soil fertility [Powlson et al., 2011; Conant et al. 2017; Mathew et al. 2017; Minasny et al. 2017]. The saturation argument thereby overlooks the biology-driven sequestration at the deep horizons of the soil [Wei et al. 2012], and the fact that additional new topsoil can be formed. Unfortunately, the scientific literature seems to lag behind what is being achieved by regenerative farmers on the ground [e.g., 30 cm of new topsoil built directly on top of a gravel and sand layer, in 10 years; Williams 2018]. Finally, the increased land demands can at least partially compensated for by higher stocking densities (livestock units/ha) in the case of rotational grazing. The degree of change in stocking density is uncertain but may reach 30% [Hillenbrand et al. 2019] to 50% [Cunningham 2021].
As far as the higher methane emissions of grass-only cattle are concerned, nuance is once more needed [Kleppel 2020; Rowntree et al. 2020]. Feed inputs are usually lower in extensive systems compared to conventional ones [Reyes-Palomo et al. 2022]. Grain feeding comes with its own trade-offs, increasing fossil-fuel derived emissions that have a more worrying impact on global warming [Picasso et al. 2014; see point 10 below]. Moreover, grain-finished cattle also spend a substantial (and sometimes most) of their lives on grass, so that their management can in principle be combined with a 'regenerative' component.

8. Rewilding comes with its own climate impact
Discussions on dietary GHG emissions extend to the notion of missed 'opportunity costs', i.e., carbon saving via rewilding and afforestation [Searchinger et al. 2018; Hayek et al. 2021]. Besides the fact that land use change is approached simplistically [see elsewhere], such scenarios underestimate the role of grasslands as carbon sink [see above] and the fact that rewilding comes with its own emissions (via the digestive processes of wildlife or, more directly, via decomposition of plant matter). 
In rewilding scenarios, livestock would be replaced by other methanogenic animals, at least partially, which are moreover less efficient feed converters [Manzano & White 2019]. Also, wild animals can diminish forest carbon sequestration because of browsing, as has been argued for moose in Norway, even if part of it can be offset because of albedo effects [Salisburry et al. 2023]. As a result, rewilding effects on GHG emissions may not be all that high as expected. This point is underlined by the fact that current levels of enterogenic methane production are not very different from the historical/natural ones produced by wild animals, such as termites, deer, elk, or bison (pre-European settlement in the US), and especially when compared over a larger time span to Paleolithic megafauna [Doughty & Field, 2010; Hristov 2012; Zimov and Zimov, 2014; Smith et al. 2016]. Even more recently, warming potential caused by livestock increases since the mid-19th century has been partially balanced by a cooling from a reduced number of wild grazers [Chang et al. 2021].
9. Large-scale afforestation of grasslands is not a panacea
Deforestation is partially driven by livestock expansion and contributes to climate change. Carbon emissions from deforestation to pasture are mostly found in South America (70%) and East and Southeast Asia (21%) [Chang et al. 2021]. As this is environmentally harmful, a case for reforestation should be considered where relevant. Some authors, however, want to go further and argue for massive global afforestation programs. Although this indeed holds high theoretical potential in carbon sequestration [Searchinger et al. 2018; Bastin et al. 2019; Hayek 2020], in practice this is unrealistic. 
Forests are not the most reliable ecosystems, prone to large forest fires in Mediterranean and semi-arid climates [Dass et al. 2018], and vulnerable to climate change [Choat et al. 2012]. Their sequestration advantage may not be effective until decades or centuries and even result in a transitory net loss [Poeplau et al. 2011; Friggens et al. 2020], whereas classic forest management may not be effective at all [Naudst et al. 2016]. Satellite analysis in California, for instance, has shown that forest offsets may not be doing much for the climate [Coffield & Randerson 2022Coffield et al. 2022]. Worse, the changes in albedo that are caused by afforestation can outweigh the carbon storage benefits, so that a net warming effect is obtained [Kirschbaum et al. 2011].
Grazing livestock sequesters carbon at a similar rate of afforestation (0.5 t C/ha/y during the first two decades) [Peyraud & MacLeod 2020], but rates can be higher [see above]. Also, assessment of the deeper layers is crucial when comparing forests and grasslands. Although carbon storage increases sharply in the top 30 cm of secondary forest soil, mainly due to surface litter, effects in grasslands also take place at deeper levels (> 1 m), due to higher carbon input from roots [Wei et al. 2012]. 
Although region-dependent, restoring grasslands can be more effective where soil depth and climate allow it. Whereas forests in temperate zones sequester 60-80 t C/ha above ground and 100 t C/ha below, grasslands store up to 100-300 t C/ha [Alonso et al. 2012]. As below-ground sinks, they are also more reliable in regions vulnerable to wildfires [Dass et al. 2018]. In the UK, a decrease in soil carbon with concomitant GHG release would be obtained for any change from permanent grassland to other use, incl. conversion to arable land (-9.3 t CO2-eq/h/y) or forestry (-2.3) [Smith et al. 2010]. 
Woodland cover can be strategically increased by integrating it within existing farms systems, rather than by displacement [Hybu Cig Cymru 2020]. Agroforestry, and other discrete tree planting options, would be more resilient to fire and climate change, hold more biodiversity, and effectively sequester carbon without necessarily suppressing livestock [De Stefano & Jacobson 2017; Prevedello et al. 2017; Assouma et al. 2019]. Livestock grazing can also be an effective tool against forest fires [Lasanta et al. 2018]. Traditionally inspired agricultural systems that combine animals and plants synergistically may further enhance yields and ecological benefit [Plexida et al. 2018; Khumairoh et al. 2018].

In the Spanish dehesa agroforestry system, carbon sequestration in both trees and soils offset 54% and 95% of the GHG emissions on conventionally and organically managed cattle farms, respectively; in some cases, even leading to negative carbon footprints [Reyes-Palomo et al. 2022]. Depending on the source, woody vegetation on New Zealand's sheep and beef farms offsets 30% (18-50%) [Ministry of the Environment 2021Mazzetto et al. 2023] to 90% (63-118%) [Case & Ryan 2020] of the on-farm agricultural emissions.  Most vegetation on farms does, however, not meet the definition of a forest, and does not qualify for inclusion in the ETS. Yet, it would only be fair if farmers get credit for their sequestration.
10. Methane should be treated differently than CO2
The above-cited arguments add essential context but still overstate livestock's impact on global warming. This is particularly true for ruminants, said to have worse GHG emissions than other livestock categories due to the production of methane [Eshel et al. 2014]. The usual data for beef or lamb (10-150 kg CO2-eq/kg of food) are indeed higher than for pork (3-10) or poultry (2-6) [de Vries & de Boer 2010; Nijdam et al. 2012; Röös et al. 2014; Apostolidis & McLeay 2016; Jiang et al. 2020; Bonesmo & Gjerlaug Enger 2021], and saltwater fish (2-3) [CSS 2020]. However, there are various reasons to caution against a simplistic replacement of ruminants by monogastrics based on the above-mentioned differences in emissions [Cheng et al. 2022]. In addition, it is important to factor in farm management [Nijdam et al. 2012] and adjustment for the nutritional value of each meat type [McAuliffe et al. 2018]. Moreover, such comparisons are also unfair with respect to the global warming potential (GWP) of different meats because of key differences in atmospheric kinetics between methane and CO2

Although methane is more potent, both gasses should be treated differently when assessing their global warming potential (GWP*). This is not trivial: the choice of the GWP versus GWP* framework has important effects on the outcomes. A study from New Zealand estimated the carbon footprint of sheepmeat and beef below 15 and 22 kg CO2-eq per kg of meat, respectively, when taking into account sequestration and using GWP100, but found that sheepmeat is climate neutral and beef is heading in that direction when using GWP* (for 1998-2018) instead [Mazzetto et al. 2023].
In contrast to CO2, which is a long-lived stock pollutant that accumulates in the atmosphere and generates warming, methane is a short-lived flow pollutant [Allen et al. 2018; Cain et al. 2019; Lynch et al. 2020]. However, methane is also 30x more potent than CO2 (over the span of a century) and its atmospheric concentration has more than doubled in the last 200 years, being responsible for 20% of global warming since the Industrial Revolution [NASA 2021]. Yet, whereas CO2 originates mostly from the mobilization of fossil carbon that took millions of years to form, methane from the enteric digestion by ruminants is part of a biological cycle which does not bring in new carbon (provided there is no increase in emissions, as could be caused by increasing herd sizes). 

Taken together, slightly reducing the herd’s methane emissions (0.3% per year) would imply zero CO2-warming-equivalent emissions and, therefore, not cause further global warming at all in aggregate. Going beyond that may even induce cooling. In the UK, for instance, total agricultural emissions under GWP* would be just 20% of the GWP-based value, due to a considerable drop of methane over the last decades [Costain 2019]. Calculated for 2016, emissions would thus drop from 46.5 MtCO2eq to 9.5 MtCO2eq* because methane levels have fallen since the base year of 1996 (leading to a negative value of -10.6 MtCO2eq*), while CO2 and N2O across that period have remained roughly the same.
Explanatory videos 💬  del Prado 2019; CLEAR 2020, BAMST playlist
As argued above, this is not wishful thinking. There is still ample potential for mitigation of biogenic methane from livestock in global food systems. In the EU27 plus the UK, for instance, total methane emissions have decreased with 40% between 1990 and 2019, with a 21-% decrease for agriculture [UNFCCC data; Peters 2021], corresponding with a 1.4-2.8 Tg/y decrease between 2000-2006 and 2017 [Jackson et al. 2020]. In the US, where livestock contributed to 10% of methane emissions in 2015 [NASA 2021], agricultural methane emissions have stabilized although total methane increased due to fossil fuel industries [Jackson et al. 2020]. Globally, methane emissions nevertheless remain a challenge, as Europe is the only continent whose methane emissions have been decreasing during that period [NASA 2021]. To which degree this challenge relates to global livestock production, and how much mitigation within animal agriculture can still be envisaged, remains the topic of debate.
The global cattle population has not been increasing during the last decade [Shahbandeh 2020]. The steepest recent increase in cattle was between 2000 and 2006 [FAO], when methane levels were flat. Part of the post-2007 increase in global methane may be due to a relative surge in herd sizes in regions with low efficiency, but also to a multitude of potential reasons, incl. geological and fossil fuel emissions, wetlands, rice farming, and landfills [Gramling 2016; NASA 2016Nisbet et al. 2016; Alvarez et al. 2018; Rasmussen 2018; Etiope & Schwietzke 2019; Jackson et al. 2020; Malik 2021]. Alternatively, a decrease in hydroxyl radical levels has been suggested, being the main sink for atmospheric methane [Turner et al. 2017]. Highly variable aquatic ecosystem sources, including flooded rice paddies, aquaculture ponds, wetlands, lakes, and salt marshes, have been underaccounted for and may represent half of the global methane emissions [Rosentreter et al. 2021].

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