BURPS IN MERPS
Addressing agricultural methane emissions — including "burps" from cattle — in the U.S. Methane Emission Reduction Plan (MERP)
Agriculture accounts for the largest source of anthropogenic methane emissions in the U.S. Despite being the primary and growing source of methane emissions, these sources are not subject to any regulatory safeguards or mandatory mitigation strategies in the White House’s Methane Emissions Reductions Action Plan (MERP) or elsewhere. The MERP continues a trend of focusing regulatory efforts exclusively on oil and gas methane emissions. While these efforts are necessary, it will be impossible to meet climate targets without parallel efforts to address larger and increasing methane emissions from enteric fermentation, manure, and other agricultural sources. Thus, we urge that all efforts to address methane must also address agriculture’s large contribution and the need to reduce it, starting with improved measurement and monitoring and additional research.
Methane is nearly a quarter of anthropogenic U.S. greenhouse gas emissions when calculated according to policy-relevant timescales.
Methane emissions are typically reported as accounting for about 10% of total U.S. greenhouse gas emissions, with nitrous oxide and carbon dioxide accounting for the bulk of other emissions. However, comparing methane to other greenhouse gasses is not straightforward. To allow for comparisons based on the warming impact of emissions, scientists use global warming potentials (GWPs), which represent the amount of heat absorbed by a single unit of a greenhouse gas over a specified period of time. The choice of an appropriate time frame for this comparison can have a tremendous impact on the climate footprint associated with methane emissions. EPA’s commonly cited greenhouse gas inventory uses GWPs based on 100 year time-frames, but climate action is urgent and policy planning often occurs on the scale of years or decades.
Because methane remains in the atmosphere for about 12 years on average, EPA's use of a 100-year GWP for methane — representing its climate impact distributed across a century — results in lower estimates than when climate impacts are calculated over shorter, more policy-relevant timescales. This choice has a tremendous impact on the relative importance of methane compared to other greenhouse gas emissions. For example, converting to a 20 year GWP, as has been done in policy-contexts such as New York’s Climate Leadership and Community Protection Act , results in a much larger share of total greenhouse gas associated with methane emissions. While methane is 10% of U.S. GHG emissions with a 100-year GWP, it rises to 25% of total U.S. GHG emissions when it’s impact is analyzed using a 20-year GWP:
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Greenhouse gas emissions calculated according to 100-year and 20 year global warming potentials. Under GWP20, methane is equal to 25% of total U.S. greenhouse gas emissions.
Agriculture is the largest source of methane emissions in the U.S.
Regardless of which GWP is used, agriculture comprises the majority of methane emissions in the U.S.
Anthropogenic methane emissions by source. EPA 2019. Inventory of Greenhouse gas emissions and sinks.
Of 659.7 MMT CO2eq of methane emissions reported in EPA’s 2019 inventory:
- Enteric fermentation was the single largest emission source (27%, 178.6 MMT).
- Natural gas systems were the second largest emission source (24%, 157.6 MMT).
- Landfills were the third largest emission source (17%, 114.5 MMT), and a significant proportion of these emissions are from food waste. (The MERP states that food waste constitutes about 24% of landfill material.)
- Manure management was the fourth largest emissions source (9.5%, 62.4 MMT).
- Additional agricultural emissions were from rice cultivation (15.1 MMT), composting (2.3 MMT), field burning of agricultural residues (0.4 MMT), and anaerobic digestion at biogas facilities (0.2 MMT).
- Total agricultural emissions from enteric fermentation, manure management, and other agricultural activities combined were 258.6 MMT or 39% of U.S. anthropogenic methane emissions. (Compare to 196.7 MMT from natural gas + petroleum.)
- If we add on 24% of landfill emissions from food waste (rough conservative estimate based on percent of landfill waste that is food waste), the total rises to 286.1 MMT, or 43% of U.S. anthropogenic methane emissions.
While GHG emissions reduction policies have driven down climate pollution from the energy sector, GHG emissions from the agricultural sector have increased and are projected to continue to increase.
EPA 2019. Inventory of Greenhouse gas emissions and sinks.
Enteric fermentation emissions have risen from 164.7 to 178.6 MMT, and manure emissions have nearly doubled from 37.1 to 62.4 MMT between 1990 and 2019. Thus, agricultural emissions are of growing importance and will continue to pose a threat to meeting climate targets in the absence of aggressive mitigation measures.
EPA-Reported agricultural methane emissions are likely underestimated and in order to address this GHG pollution, the U.S. must invest in improvements in monitoring and modeling of agricultural methane emissions.
Recent studies suggest EPA’s models for estimating methane emissions from agriculture lead to systematic underestimation of methane emissions. For example, Hayek and Miller (2021) compared atmospheric measurements taken above and downwind of animal production regions to standard EPA and other models and found that direct measurements showed animal methane emissions 39%–90% higher than model estimates of animal methane emissions. (See figure below for comparison of EPA's bottom-up estimate across the US for 2007-2008 and a top-down estimate from Miller at al. 2013 ). These findings support targeted efforts to reduce methane emissions from concentrated animal feeding operations where animal emissions are highly concentrated.
Methane emission estimates from EPA compared to direct measurements.
A small number of facilities are responsible for the majority of enteric and manure emissions. Thus, mitigating emissions from the most concentrated facilities has the potential to make a large impact on total emissions.
Proportion of animals produced by largest facilities.
Greenhouse gas emissions from enteric fermentation and manure are largely dependent on the number of animals raised in facilities, which are heavily concentrated in a small proportion of the largest operations: over 50% of dairy cows in the United States are in the 4% of operations that stock 1,000 or more dairy cows. More than 90% of hogs in the United States are in the 12% of facilities that stock 2,000 or more hogs, and more than three-quarters of all cattle on feed in the United States are in the 5% of facilities that stock 1,000 or more cattle ( USDA NASS Census of Agriculture ). Reducing emissions from these largest facilities has the potential to cut back total emissions significantly.
The MERP has no mandatory strategies for reducing agricultural methane emissions and insufficient voluntary programs.
Despite the fact that agriculture is the largest source of methane in the U.S., the MERP proposes only “expanding incentive-based and voluntary partnership efforts.” The plan appropriately supports additional financial and technical support for transitioning to manure system that generate less methane, such as dry manure management and alternative systems such as composting or pasture. However, the plan largely ignores the manure methane emissions from the pasture phase of cattle production. There is no mention of improving grazing practices to reduce these emissions.
Even more significant, the plan entirely ignores enteric emissions. It is critical that the USDA and other agencies accelerate R&D of practices to reduce enteric emissions by altering animal diets and management. In addition, there must be immediate effort to deploy existing opportunities such as shifts in dietary demand to address enteric emissions.
These strategies should be a top priority for adoption as they reduce the quantity of methane generated that then must be captured and treated in contrast to strategies that only seek to capture emissions.
Voluntary programs that rely on methane capture and use are unlikely to be successful and have many harmful effects.
Many of the voluntary programs in the MERP and elsewhere rely primarily on biogas – systems such as anaerobic digesters to capture some of the manure methane and destroy it (e.g. by flaring) or convert it to energy. Industrial-scale anaerobic digesters are costly and ineffective with limited overall climate mitigation potential. Biogas schemes rely on the production of methane to be profitable. Therefore, rather than reducing methane emissions they incentivize increasing methane production and promote waste generation. Furthermore, industrial-scale anaerobic digesters continue to pose hazards through groundwater contamination, subsequent nitrous oxide emissions, and runoff of nitrogen into waterways. Methane leakage from these facilities can be substantial, causing them in some cases to be net sources of methane rather than offsetting methane emissions ( Bakkaloglu et al. 2022 , Dumont et al. 2013 , Flesch et al. 2011 , Miranda et al. 2015 , Montes et al. 2013 ). In addition, these large digesters release additional pollutants such as NOx, sulfur oxide, and particulate matter ( Battini et al. 2014 ; Camillo et al. 2011 , Carreras-Sospedra et al. 2015 ). Subsequent storage and spreading of digestate can also result in air, water and climate pollution ( Bakkaloglu et al. 2022, Holly et al. 2017 , Miranda et al. 2015 ).
Reducing the production and concentration of manure itself — for example, through reducing concentrated animal production, shifts in dietary demand, or altering animal feed diets to improve production efficiency — are necessary to curb manure production and reduce the magnitude of the problem. These strategies should be a top priority for adoption as they reduce the quantity of manure required to treat, in contrast to mitigating strategies like biogas that only address emissions and pollution from already-produced manure.
EPA can regulate methane emissions.
In order to effectively regulate CAFO emissions, EPA must establish methods by which CAFOs can measure and monitor their emissions. EPA should complete its development of emissions estimating methodologies (EEMs) for CAFOs, as it committed to do by 2009 under a 2005 consent agreement but has not yet done . This is essential to EPA’s ability to effectively regulate and reduce toxic air and GHG emissions from CAFOs. EPA should ensure that it meets the current deadline posted on its website for issuing the proposed EEMs, and that it then proceeds expeditiously to finalize them after notice and comment.
EPA can use Section 111 of the Clean Air Act to set emissions standards for methane. EPA should assess achievable limits by looking at a number of actions new CAFOs can take to reduce methane emissions, including the use of methane-reducing feed additives, grazing animals, or cover and flare manure management systems (a cost-effective method of reducing GHG emissions while reducing odor and protecting water quality). If a methane emissions standard for new CAFOs proves to be impractical (for instance if emissions monitoring difficulties preclude adequate enforcement), EPA should promulgate a “design equipment, work practice, or operational standard, or combination thereof” to limit emissions. For example, EPA could require dry manure handling practices, which generate less methane. EPA could also distinguish sizes based on animal units, as EPA does under the Clean Water Act, or amounts of pollution emitted as it does under §112 of the Clean Air Act.
EPA should require the largest CAFOs to comply with Title V permitting requirements. As the EPA Office of Inspector General has noted, animal feeding operations that emit air pollution in sufficient quantities can trigger Clean Air Act permit requirements. While the EEMs are designed to allow CAFOs to measure and report their emissions and thereby determine the applicability of permitting requirements, even in the absence of precise estimating methodologies, the largest CAFOs unquestionably emit certain substances in quantities far above the regulatory thresholds. Therefore, as part of its effort to enforce environmental laws and ensure clean and healthy air for all communities, EPA should implement permitting requirements for the largest CAFOs that clearly exceed threshold emissions levels and thus do not need to wait for completion of the EEMs to take measures to reduce their emissions and comply with the Clean Air Act.