Full adoption of the most effective strategies to mitigate methane emissions by ruminants can help meet the 1.5 °C target by 2030 but not 2050

To meet the 1.5 °C target, methane (CH₄) from ruminants must be reduced by 11 to 30% by 2030 and 24 to 47% by 2050 compared to 2010 levels. A meta-analysis identified strategies to decrease product-based (PB; CH₄ per unit meat or milk) and absolute (ABS) enteric CH₄ emissions while maintaining or increasing animal productivity (AP; weight gain or milk yield). Next, the potential of different adoption rates of one PB or one ABS strategy to contribute to the 1.5 °C target was estimated. The database included findings from 430 peer-reviewed studies, which reported 98 mitigation strategies that can be classified into three categories: animal and feed management, diet formulation, and rumen manipulation. A random-effects meta-analysis weighted by inverse variance was carried out. Three PB strategies—namely, increasing feeding level, decreasing grass maturity, and decreasing dietary forage-to-concentrate ratio—decreased CH₄ per unit meat or milk by on average 12% and increased AP by a median of 17%. Five ABS strategies—namely CH₄ inhibitors, tanniferous forages, electron sinks, oils and fats, and oilseeds—decreased daily methane by on average 21%. Globally, only 100% adoption of the most effective PB and ABS strategies can meet the 1.5 °C target by 2030 but not 2050, because mitigation effects are offset by projected increases in CH₄ due to increasing milk and meat demand. Notably, by 2030 and 2050, low- and middle-income countries may not meet their contribution to the 1.5 °C target for this same reason, whereas high-income countries could meet their contributions due to only a minor projected increase in enteric CH₄ emissions.

Global food systems contribute up to 30% of the worldwide greenhouse gas (GHG) emissions (1). The goal of the Paris Agreement, to limit global warming to 1.5 °C above preindustrial levels, is unlikely to be achieved if food systems continue operating on a business-as-usual (BAU) scenario (1). Among food-related GHG emissions, methane (CH₄) from livestock contributes 30% of the global anthropogenic CH₄ emissions (2), 17% of the global food system GHG emissions, and 5% of global GHG emissions (2, 3). Of the global livestock CH₄ emissions, 88% is contributed by enteric fermentation (4).Methane is a short-lived climate pollutant. Given its perturbation lifetime in the atmosphere of around 12.5 y, CH₄ contributes significantly to near-term global warming (5). Its global warming potential is 84 or 28 for 20- or 100-y time horizons, respectively (5). When evaluating the contribution of global food systems to CH₄ emissions over a 20-y period instead of the commonly used 100-y time period for national GHG inventories, the contribution of CH₄ to food system GHG emissions more than doubles, from 17 to 36% (3, 5).The realization of nationally determined contributions and 2050 climate neutrality goals depends upon the reduction of CH₄ emissions. Within sectoral reductions of CH₄ emissions, technical solutions to decrease CH₄ from agricultural production—especially strategies to mitigate CH₄ from enteric fermentation by ruminant livestock—are integral to meeting these climate targets, but quantitative data on mitigation potentials are scarce (6). Based on 2010 GHG emission levels and different mitigation scenarios to limit global warming to 1.5 °C, agricultural CH₄ emissions need to be decreased by 11 to 30% by 2030 and by 24 to 47% by 2050 (7).The global population is projected to increase by 23% between 2010 and 2030, with most of the increase occurring in low- and middle-income countries (LMIC) (8). Ruminants contribute about half of the animal protein produced by livestock (4). In LMIC, ruminant livestock play a crucial role in food security (9). Ruminants can convert human-inedible feeds, like those from pastures and grain commodity by-products produced on marginal lands or from subsistence agricultural production systems, into nutritionally dense human-edible foods. Ruminants also provide other benefits, such as traction and manure for fuel and fertilizer (10). In addition, human population growth is generally high in LMIC, while consumption of animal-sourced food is often below recommended dietary levels or reliant upon ruminant meat and milk for livelihoods and nutrition security (10, 11). Thus, from a feed-food competition perspective, ruminant production increases in LMIC should rely on human inedible feeds (i.e., forage and by-products). In contrast, in high-income countries (HIC) population growth is much lower and the consumption of animal protein is often above recommended dietary levels (9, 11).Sustainable strategies for enteric CH₄ mitigation that align with the 1.5 °C target should preferably avoid socioeconomic and environmental tradeoffs (12) and, ideally, increase production yield per unit of input. Reductions in both CH₄ emissions intensity (i.e., emissions per unit of milk and gain [CH₄I_M and CH₄I_G, respectively]) and absolute CH₄ emissions are therefore needed. Strategies that reduce CH₄I and increase production per unit of input could be used to expand food production from the existing ruminant population without increasing total CH₄ emissions (13–15), and thus contribute to the 1.5 °C target as well as to sustainable development goals. Several reviews indicate that animal and feed management, diet formulation, and rumen manipulation strategies could significantly decrease enteric CH₄ emissions (12, 16, 17). However, previous studies consisted of qualitative reviews (12), examined the quantitative effects of a single mitigation strategy (18–20), or compared CH₄ yield (CH₄Y; CH₄ per unit of feed intake) between multiple mitigation strategies (17). Methane yield is only one relevant measure, and other major CH₄ emission and animal performance metrics must be considered to determine the effectiveness and feasibility of mitigation strategies. Only one recent publication examined the quantitative effects of multiple mitigation strategies on CH₄ emission and animal performance metrics, but the analysis was limited to Latin America (21). Important CH₄ emission metrics include daily CH₄ emissions, CH₄Y, or CH₄-energy conversion factor [Y_m; CH₄ energy as a proportion of gross energy intake; a component of the tier 2 calculation for national GHG inventories recommended by the Intergovernmental Panel on Climate Change (22)], CH₄I_G, and CH₄I_M. Important animal performance metrics include feed intake, nutrient digestibility, and animal productivity (AP).The objective of this study was to conduct a comprehensive meta-analysis of enteric CH₄ mitigation strategies published in peer-reviewed journals by examining their quantitative effect on the aforementioned in vivo CH₄ emissions and animal performance metrics and to estimate their potential to contribute to the 1.5 °C target. As outlined above, there is an urgent need for strategies that can effectively mitigate enteric CH₄ emissions without negatively affecting AP by focusing exclusively on strategies that decouple CH₄ emissions from animal production (23). Mitigation effects were quantified on a global level as well as on a regional level. The African and European regions were selected to represent LMIC and HIC, respectively.