Editing IPCC:AR6/WGIII/Chapter-7 (section)

==== 7.4.3.4 Enteric Fermentation ====

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'''Activities, co-benefits, risks and implementation opportunities and barriers.''' Mitigating CH 4 emissions from enteric fermentation can be direct (i.e., targeting ruminal methanogenesis and emissions per animal or unit of feed consumed) or indirect, by increasing production efficiency (i.e., reducing emission intensity per unit of product). Measures can be classified as those relating to (i) feeding, (ii) supplements, additives and vaccines, and (iii) livestock breeding and wider husbandry ( [[#Jia--2019|Jia et al. 2019]] ). Co-benefits include enhanced climate change adaptation and increased food security associated with improved livestock breeding (Smith et al. 2014). Risks include mitigation persistence, ecological impacts associated with improving feed quality and supply, or potential toxicity and animal welfare issues concerning feed additives. Implementation barriers include feeding/administration constraints, the stage of development of measures, legal restrictions on emerging technologies and negative impacts, such as the previously described risks (Smith et al. 2014; [[#Jia--2019|Jia et al. 2019]] ; P. [[#Smith--2019|Smith et al. 2019]] a).

'''Conclusions from AR5 and IPCC Special Reports (SR1.5, SROCC and SRCCL); mitigation potential, costs, and pathways.''' The AR5 indicated medium (5–15%) technical mitigation potential from both feeding and breeding related measures (Smith et al. 2014). More recently, the SRCCL estimated with ''medium confidence'' , a global potential of 0.12–1.18 GtCO 2 -eq yr –1 between 2020 and 2050, with the range reflecting technical, economic and sustainability constraints (SRCCL, Chapter 2: [[#Hristov--2013|Hristov et al. 2013]] ; [[#Dickie--2014a|Dickie et al. 2014a]] ; [[#Herrero--2016|Herrero et al. 2016]] ; [[#Griscom--2017|Griscom et al. 2017]] ). The underlying literature used a mixture of IPCC GWP100 values for CH 4 , preventing conversion of CO 2 -eq to CH 4 . Improved livestock feeding and breeding were included in IAM emission pathway scenarios within the SRCCL and SR1.5, although it was suggested that the full mitigation potential of enteric CH 4 measures is not captured in current models ( [[#Rogelj--2018b|Rogelj et al. 2018b]] ; [[#IPCC--2018|IPCC 2018]] ).

'''Developments since AR5 and IPCC Special Reports (SR1.5, SROCC and SRCCL).''' Recent reviews generally identify the same measures as those outlined in the SRCCL, with the addition of early life manipulation of the ruminal biome ( [[#Grossi--2019|Grossi et al. 2019]] ; [[#Eckard--2020|Eckard and Clark 2020]] ; [[#Thompson--2020|Thompson and Rowntree 2020]] ; [[#Beauchemin--2020|Beauchemin et al. 2020]] ; [[#Ku-Vera--2020|Ku-Vera et al. 2020]] ; [[#Honan--2021|Honan et al. 2021]] ). There is ''robust evidence'' and ''high agreement'' that chemically synthesised inhibitors are promising emerging near-term measures ( [[#Patra--2016|Patra 2016]] ; [[#Jayanegara--2018|Jayanegara et al. 2018]] ; [[#Van%20Wesemael--2019|Van Wesemael et al. 2019]] ; [[#Beauchemin--2020|Beauchemin et al. 2020]] ) with high (e.g., 16–70% depending on study) mitigation potential reported (e.g., [[#Hristov--2015|Hristov et al. 2015]] ; [[#McGinn--2019|McGinn et al. 2019]] ; [[#Melgar--2020|Melgar et al. 2020]] ) and commercial availability expected within two years in some countries ( [[#Reisinger--2021|Reisinger et al. 2021]] ). However, their mitigation persistence ( [[#McGinn--2019|McGinn et al. 2019]] ), cost ( [[#Carroll--2019|Carroll and Daigneault 2019]] ; [[#Alvarez-Hess--2019|Alvarez-Hess et al. 2019]] ) and public acceptance ( [[#Jayasundara--2016|Jayasundara et al. 2016]] ) or regulatory approval is currently unclear while administration in pasture-based systems is likely to be challenging ( [[#Patra--2017|Patra et al. 2017]] ; [[#Leahy--2019|Leahy et al. 2019]] ). Research into other inhibitors/feeds containing inhibitory compounds, such as macroalga or seaweed ( [[#Chagas--2019|Chagas et al. 2019]] ; [[#Kinley--2020|Kinley et al. 2020]] ; [[#Roque--2019|Roque et al. 2019]] ), shows promise, although concerns have been raised regarding palatability, toxicity, environmental impacts and the development of industrial-scale supply chains ( [[#Abbott--2020|Abbott et al. 2020]] ; [[#Vijn--2020|Vijn et al. 2020]] ). In the absence of CH 4 vaccines, which are still under development ( [[#Reisinger--2021|Reisinger et al. 2021]] ) pasture-based and non-intensive systems remain reliant on increasing production efficiency ( [[#Beauchemin--2020|Beauchemin et al. 2020]] ). Breeding of low emitting animals may play an important role and is a subject under ongoing research ( [[#Pickering--2015|Pickering et al. 2015]] ; [[#Jonker--2018|Jonker et al. 2018]] ; [[#López-Paredes--2020|López-Paredes et al. 2020]] ).

Approaches differ regionally, with more focus on direct, technical options in Developed Countries, and improved efficiency in developing countries ( [[#Caro%20Torres--2016|Caro Torres et al. 2016]] ; [[#Mottet--2017b|Mottet et al. 2017b]] ; [[#MacLeod--2018|MacLeod et al. 2018]] ; [[#Frank--2018|Frank et al. 2018]] ). A recent assessment finds greatest economic (up to USD100 tCO 2 -eq –1 ) potential (using the IPCC AR4 GWP100 value for CH 4 ) for 2020–2050 in Asia and the Pacific (32.9 MtCO 2 -eq yr –1 ) followed by Developed Countries (25.5 MtCO 2 -eq yr –1 ) ( [[#Roe--2021|Roe et al. 2021]] ). Despite numerous country and sub-sector specific studies, most of which include cost analysis ( [[#Hasegawa--2012|Hasegawa and Matsuoka 2012]] ; [[#Hoa--2014|Hoa et al. 2014]] ; [[#Jilani--2015|Jilani et al. 2015]] ; [[#Eory--2015|Eory et al. 2015]] ; [[#Pradhan--2017|Pradhan et al. 2017]] ; [[#Pellerin--2017|Pellerin et al. 2017]] ; [[#Ericksen--2018|Ericksen and Crane 2018]] ; [[#Habib--2018|Habib and Khan 2018]] ; [[#Kashangaki--2018|Kashangaki and Ericksen 2018]] ; [[#Salmon--2018|Salmon et al. 2018]] ; [[#Brandt--2019b|Brandt et al. 2019b]] ; [[#Kiggundu--2019|Kiggundu et al. 2019]] ; [[#Kavanagh--2019|Kavanagh et al. 2019]] ; [[#Mosnier--2019|Mosnier et al. 2019]] ; [[#Pradhan--2019|Pradhan et al. 2019]] ; [[#Sapkota--2019|Sapkota et al. 2019]] ; [[#Carroll--2019|Carroll and Daigneault 2019]] ; [[#Leahy--2019|Leahy et al. 2019]] ; [[#Dioha--2020|Dioha and Kumar 2020]] ), sectoral assessment of regional technical and notably economic ( [[#Beach--2015|Beach et al. 2015]] ; [[#USEPA--2019|USEPA 2019]] ) potential is restricted by lack comprehensive and comparable data. Therefore, verification of regional estimates indicated by global assessments is challenging. Feed quality improvement, which may have considerable potential in developing countries ( [[#Caro--2016|Caro et al. 2016]] ; [[#Mottet--2017a|Mottet et al. 2017a]] ), may have negative wider impacts. For example, potential land-use change and greater emissions associated with production of concentrates ( [[#Brandt--2019b|Brandt et al. 2019b]] ).

'''Critical review and conclusion.''' Based on studies to date, using a range of IPCC GWP100 values for CH 4 , there is ''medium confidence'' that activities to reduce enteric CH 4 emissions have a global technical potential of 0.8 (0.2–1.2) GtCO 2 -eq yr –1 , of which 0.2 (0.1–0.3) GtCO 2 -eq yr –1 is available up to USD100 tCO 2 -eq –1 (Figure 7.11). The CO 2 -eq value may also slightly differ if the GWP100 IPCC AR6 CH 4 value was uniformly applied within calculations. Lack of comparable country and sub-sector studies to assess the context applicability of measures, associated costs and realistic adoption likelihood, prevents verification of estimates.

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