There is consensus that, in comparison to 2 °C or even higher levels of global temperature increase, limiting global temperature increase to 1.5 °C will diminish the frequency and severity of extreme climate events in the next decades [1]. Methane (CH₄) atmospheric concentration has doubled since industrial times and is currently second to carbon dioxide (CO₂) in causing global warming [2]. In addition to reaching net zero emissions of CO₂, achieving a strong, rapid, and sustained decrease in CH₄ emissions is key to rapidly limiting global warming [3]. This is largely due to CH_4′s relatively high global warming potential (28 times greater than CO₂ in a 100-year period) and relatively short life (9.25 ± 0.6 years) and perturbation time (12.4 ± 1.4 years) in the atmosphere [4]. Other benefits of decreasing CH₄ concentration in the atmosphere include preventing premature death due to ground-level ozone pollution and increasing crop yields [2].

As part of the overall mitigation in the emissions of greenhouse gases (GHG) to limit global warming to 1.5 °C in this century, it is estimated that global anthropogenic CH₄ emissions must be reduced by 40 to 45% by 2030 from 2015 levels [2]. On the other hand, past and recent trends indicate continuous growth in the emissions of CH₄, with a recent acceleration and projected increases in the atmospheric concentration of CH₄ under the current scenario [2,4,5,6]. Agriculture is a major source of short-term global warming through its emissions of CH₄ [4]. Enteric CH₄ emitted by domestic ruminants is the main source of agricultural CH₄ and accounts for about 30% of total CH₄ emissions from human activities [2,7]. Emissions of CH₄ by livestock increased by 51.4% between 1961 and 2018 [7]. The necessary decrease in enteric CH₄ emissions between 2020 and 2030 across various socioeconomic scenarios and climate models, compatible with a maximal 1.5 °C increase in global temperature, was estimated to be 20% on average [2]. Decreasing enteric CH₄ emissions is, therefore, important as part of the effort to decrease the anthropogenic emissions of GHG. The objectives of this paper are to critically examine through a mathematical simulation the possibilities of decreasing enteric CH₄ emissions through sustainable intensification of ruminant agriculture and the use of feed additive inhibitors of methanogenesis as the most potent strategy for enteric CH₄ mitigation and to analyze the opportunities and barriers to widespread adoption of inhibitors of methanogenesis for pronounced mitigation of enteric CH₄ emissions.

Intensifying ruminant production increases the feed intake and productivity of the individual animal. Feed intake is the main driver of CH₄ production [8]. Increased feed intake resulting from improved feed availability and quality thus results in greater daily CH₄ emissions per animal. On the other hand, as animal productivity increases, a lesser proportion of dry matter intake (DMI) and of CH₄ emitted by an animal is associated with the animal’s maintenance requirements, which has been called the “dilution of maintenance” effect. The result is a decrease in CH₄ emitted per unit of milk [9] or meat [10] produced or CH₄ intensity. There are also other animal management and feeding practices that also improve animal productivity and decrease CH₄ intensity, such as reducing herd size to increase individual productivity, reducing mortality and morbidity, decreasing age at slaughter, and improving fertility [11].

Improvements in production efficiency between the 2000–2004 and 2014–2018 quinquennials led to declines in CH₄ intensity of meat and milk from dairy cattle, buffalo, sheep, and goat protein in most regions in the world, although this was more variable for beef. Despite the decreases in CH₄ intensity, total CH₄ emitted globally by ruminants increased in the same period of time [7]. Due to the forecasted increase in production of animal products, Chang et al. [7] projected a global increase in total emissions of livestock CH₄ (including pigs and poultry) of between 51 and 54% by 2050 relative to 2012 assuming constant CH₄ intensities. With decreasing CH₄ intensities due to improved production efficiency following past trends, total global emissions of CH₄ from livestock were estimated to increase less, by 15 to 21%, between 2012 and 2050 [7]. A similar analysis for wool production in Western Australia also revealed a relationship between increased animal productivity, mostly attributed to improvements in reproductive performance, and decreased CH₄ intensity, along with increased total emissions of CH₄ [12]. Therefore, whilst production intensification and resulting improvements in animal productivity and feed efficiency can ameliorate livestock CH₄ emissions relative to a scenario with constant CH₄ intensity, total CH₄ emissions from livestock will likely continue rising, as a result of the increases in animal production that are, in turn, driven by the increases in human population and per capita consumption of animal products, especially in developing countries [13,14].

It has been estimated that agricultural emissions of CH₄ must diminish between 24 and 47% by 2050 relative to a 2010 baseline in order to contain the global temperature increase to 1.5 °C [15]. Given that the main source of agricultural CH₄ is livestock [16], it is reasonable to assume that enteric CH₄ will also need to be decreased by similar percentages between 2010 and 2050. In the same period, the consumption of bovine and ovine meat and dairy products is expected to expand by 58, 78, and 58%, respectively [13]. It follows that, in order to decrease enteric CH₄ emissions by 24% by 2050 relative to 2010 levels, global CH₄ intensity of beef, lamb, and milk production would have to decrease by 52, 57, and 52%, respectively, in relation to its 2010 levels. Likewise, decreasing enteric CH₄ emissions by 47% between 2010 and 2050 would require decreasing global CH₄ emissions intensity of beef, lamb, and milk production by 66, 70, and 66%, respectively (calculations not shown).

The same as with CH₄, intensifying animal production and improving animal productivity also decreases the emissions intensity of carbon dioxide equivalents (CO₂e; the sum of the main three GHG CO₂, CH₄, and nitrous oxide (N₂O), each weighted by its heat-trapping capacity over a 100-year period), i.e., CO₂e per kilogram of animal product, or carbon footprint. In some cases, decreasing the emissions of CO₂e per kilogram of animal product has allowed lowering of the total number of animals sufficiently in a country or region so as to decrease the total emissions of CO₂e of the livestock industries e.g., Capper et al. [9]. However, in various other cases, the decrease in the emissions of CO₂e per unit of animal product occurring as a consequence of intensification was insufficient to compensate for the increase in animal production, resulting, therefore, in increased total CO₂e emissions from milk and beef production [17]. Whilst producing meat and milk with a lower carbon footprint is an important goal, intensification of animal production alone is unlikely to stop the increase in total emissions of GHG from ruminant production, much less mitigate them. Specific additional measures to ameliorate the emissions of CH₄ and other GHG from the livestock industry are also needed.

It is challenging to reconcile the objectives of decreasing total emissions of enteric CH₄ from ruminant production and at the same time increase the global supply of animal products. Therefore, several strategies to mitigate enteric CH₄ emissions from ruminants are being investigated: increasing feed efficiency, genetic selection of animals with lower CH₄ production, modifying diet formulation and concentrate and forage processing, grazing management, the addition of oils to the diet, use of chemical inhibitors of methanogenesis, dietary inclusion of algae with antimethanogenic compounds, alternative electron acceptors, phytocompounds, defaunation (elimination of rumen protozoa), immunization against methanogens, early-life interventions, and archaeal phages, among others. For more information, readers are referred to various excellent published reviews [18,19,20,21,22,23,24,25].

The effectiveness of all [26,27,28] or some [29,30,31,32,33,34] enteric CH₄ mitigation strategies currently available has been quantified through various meta-analyses. In their meta-analysis, Arndt et al. [27] identified increasing the individual animal feed intake (by, on average, 58%, without altering the composition of the diet) as the most effective strategy to decrease the CH₄ intensity of milk production (by, on average, 16.7%), while simultaneously increasing animal productivity. Secondly, they identified the utilization of inhibitors of methanogenesis (including 3-nitrooxypropanol (3-NOP) and bromoform-containing red algae Asparagopsis spp.) as the most effective strategy to decrease total daily emissions of CH₄ per animal (by, on average, 35.2%) and emissions of CH₄ per kilogram of milk produced (by, on average, 31.8%), without negatively affecting animal productivity [27].

Using the average decreases in CH₄ production found in their meta-analysis, Arndt et al. [27] estimated that the adoption of increased feed intake or inhibitors of methanogenesis, or both antimethanogenic measures in combination, could allow containing of global temperature increase by 1.5 °C by 2030 but not by 2050, even if applied under an unrealistic scenario of global 100% adoption [27]. The conclusions of the analysis by Arndt et al. [27] illustrate the challenges and difficulties of increasing ruminant production while decreasing the emissions of enteric CH₄ and CO₂e.