over-Symposium review: Defining a pathway to climate neutrality for US dairy cattle production*

The US dairy industry has made substantial gains in reducing the greenhouse gas emission intensity of a gal-lon of milk. At the same time, consumer and investor interest for improved environmental benefits or reduced environmental impact of food production continues to grow. Following a trend of increasing greenhouse gas emission commitments for businesses across sectors of the economy, the US dairy industry has committed to a goal of net zero greenhouse gas emissions by 2050. The Paris Climate Accord’s goal is to reduce warming of the atmosphere to less than 1.5 to 2°C based on pre-industrial levels, which is different from emission goals of historic climate agreements that focus on emission reduction targets. Most of the emissions that account for the greenhouse gas footprint of a gallon of milk are from the short-lived climate pollutant CH 4 , which has a half-life of approximately 10 yr. The relatively new accounting system Global Warming Potential Star and the unit CO 2 warming equivalents gives the industry the appropriate metrics to quantify their current and projected warming impact on future emissions. Incorporating this metric into potential future emissions pathways can allow the industry to understand the magnitude of emissions reductions needed to no longer contribute additional warming. Deterministic modeling was performed across the dairy industry’s emission areas of enteric fermentation, manure management, feed production, and other upstream emissions necessary for dairy production. By reducing farm-level absolute emissions by 23% based on current levels, there is the opportunity for the US dairy industry to realize climate neutrality within the next few decades.


INTRODUCTION
Human activities that release greenhouse gas (GHG) emissions have increased the concentrations of GHG, such as carbon dioxide (CO 2 ), methane (CH 4 ), and nitrous oxide (N 2 O), in the troposphere (Myrhe et al., 2013;Forster et al., 2021).Since preindustrial times, the concentrations of CO 2 , CH 4 , and N 2 O have increased approximately 50, 150, and 22%, respectively (US EPA, 2021a).Dairy cattle production, defined as all activities from upstream inputs into feed production and animal management, contributes to the atmospheric increase of GHG emissions and the warming impacts associated with those increased GHG concentrations.These emissions include feed production N 2 O emissions from soils, enteric CH 4 , CH 4 and N 2 O emissions from manure, and CO 2 emissions from the combustion of fossil fuels used in farming equipment.
In the United States, it is estimated that dairy cattle production is responsible for approximately 99 to 172 million metric tonnes of CO 2 equivalents (CO 2 e), which represents approximately 1.9 to 2.5% of annual US GHG emissions (Thoma et al., 2013;Capper and Cady, 2020;Rotz et al., 2021;Uddin et al., 2022).The range in the estimated total GHG emissions from the US dairy cattle production are reflective of differences in modeling techniques, time periods assessed, system boundaries, and differences in the 100-yr global warming potentials (GWP100) used within the analysis.
The GWP100 of a GHG is a measure of how much energy the emissions of 1 ton of a gas will absorb over 100 yr, relative to the emissions of 1 ton of CO 2 , which is the reference gas (US EPA, 2022).Over time, substantial changes have been made to the GWP100 values of CH 4 , and as this gas is approximately 62% of the US dairy cattle production's total GHG emissions (Rotz et al., 2021), changes to the estimated warming impacts of CH 4 can significantly shift the total estimated contribution of the industry to US or global emissions inventories.
Recently, work has demonstrated that the GWP100 poorly links emissions to warming effects across a variety of emissions scenarios.Specifically, GWP100 over-Symposium review: Defining a pathway to climate neutrality for US dairy cattle production* estimates the warming contribution of CH 4 emissions in scenarios where CH 4 emissions are stable or declining, and underestimates warming in rising CH 4 emissions scenarios (Allen et al., 2018;Cain et al., 2019;Forster et al., 2021; Figure 1).Thus, the aggregation of all GHG emissions using GWP100 results in cumulative CO 2 e emissions, which do not necessarily represent the magnitude of future global surface temperature outcomes (Forster et al., 2021).The disconnect between CH 4 CO 2 e emissions derived from GWP100 and temperature change responses presents a potential accounting challenge as net zero commitments from industries and companies have increased in response to the Paris Climate Agreement (Pineda and Faria, 2019).
The Intergovernmental Panel on Climate Change defines net zero emissions (IPCC, 2018b; Table 1) as follows: "Net zero emissions are achieved when anthropogenic emissions of GHG to the atmosphere are balanced by anthropogenic removals over a specified period.Where multiple GHG are involved, the quantification of net zero emissions depends on the climate metric chosen to compare emissions of different gases (such as global warming potential, global temperature change potential, and others, as well as the chosen time horizon)." For many public commitments from governments, corporations, and industries, the specific time horizon is often the year 2050, relative to an earlier date (e.g., 2018).Specifically, the Paris Agreement has a goal to limit global warming to well below 2°C, preferable to 1.5°C, compared with preindustrial levels (UNFCCC, 2021a).The Paris Agreement is unique compared with past global climate agreements such as the Kyoto Protocol that centered on GHG emissions targets (UNFCCC, 2021b).Rather than GHG emissions targets, the Paris Agreement focuses on limiting warming.Consequently, it is important that climate metrics are fit-for-purpose in representing temperature change (i.e., warming) effects across future emission scenarios.
To stay within a 1.5 or 2°C of warming by 2050 relative to preindustrial global average temperatures, halting and achieving no additional warming effects will require a net zero CO 2 emissions balance with any residual CO 2 emissions balanced with CO 2 removals from the atmosphere (e.g., nature-based CO 2 removals such as afforestation or carbon capture and storage technologies; IPCC, 2018a).For non-CO 2 GHG gases, the IPCC (2018a) has estimated that halting and achieving no additional warming will not require net zero CO 2 e emissions using GWP100, though it will require significant reductions in CO 2 e emissions from current emissions levels.
The objective of this study was to conduct a deterministic analysis using direct emissions information from national GHG inventory data and estimates of indirect GHG emissions from life cycle assessments to estimate the emissions reductions required to achieve no additional net warming effects from the activities associated with US dairy cattle production at or before the year 2050, relative to the year 2010.

METHODS
Enteric CH 4 , managed-manure CH 4 , and managedmanure N 2 O emissions data from 1990 to 2019 were obtained from the US EPA GHG emissions inventory for all dairy cattle types, including milk cows, replacement heifers, and dairy calves.The US EPA estimates enteric CH 4 emissions from US dairy cattle via the Cattle Enteric Fermentation Model, which follows IPCC tier 2 methodology recommendations (IPCC, 2006).In brief, the process involves characterizing populations of cattle by type (e.g., mature cows vs. replacement heifers), characterizing representative diets by cattle type to calculate emissions factors, and finally, following tier 2 emissions methodology to estimate enteric CH 4 emissions by cattle type across the United States.The Cattle Enteric Fermentation Model process is also used to provide the necessary information for estimating volatile solids and N excreted by dairy cattle across types in the United States.From this excretion information, the US EPA estimates emissions factors and emissions of CH 4 and N 2 O (both direct and indirect N 2 O from N volatilization and leaching) based on climate types and manure management systems across the United States.Indirect GHG emissions associated with dairy cattle production in the United States were estimated from the work of Rotz et al. (2021) to account for emis-sions associated with cropland N 2 O, upstream inputs CO 2 e (e.g., fertilizer production, electricity generation), and on-farm fossil fuel combustion associated with feed production and dairy cattle feeding, milking, and management activities.Direct emissions inventory data were combined with indirect GHG emissions estimates to generate a historic emissions inventory for US dairy cattle production from 1990 to 2019.
Next, the emissions inventory was converted to both CO 2 e using the IPCC Assessment Report (AR5) GWP100 values for CO 2 , CH 4 , and N 2 O, which are 1, 34, and 298, respectively (Myhre et al., 2013), and CO 2 warming equivalents (CO 2 we) using an alternative to GWP100 known as Global Warming Potential Star (GWP*).The GWP* was developed by Allen et al. (2018) to overcome the poor linkage between emissions and warming effects of GWP100 for short-lived climate pollutants such as CH 4 in stable or falling emissions scenarios.Global Warming Potential Star considers the change in CH 4 emission rates over a specified time frame (typically 20 yr for CH 4 ) and the small stock component to calculate CO 2 we emissions.The following equation from Smith et al. (2021) was used to calculate CO 2 we emissions: CO 2 we = 4.53 × E100(t) − 4.25 × E100(t-20),

Carbon neutral
The IPCC (2021) defines carbon neutral as the condition in which anthropogenic CO 2 emissions associated with a subject are balanced by anthropogenic CO 2 removals.The subject can be an entity such as a country, an organization, a district or a commodity, or an activity such as a service and an event.Carbon neutrality is often assessed over the life cycle including indirect (i.e., "scope 3") emissions, but can also be limited to the emissions and removals, over a specified period, for which the subject has direct control, as determined by the relevant scheme.Net zero CO 2 emissions Net zero CO 2 emissions are defined by the IPCC (2021) as the condition in which anthropogenic carbon dioxide (CO 2 ) emissions are balanced by anthropogenic CO 2 removals over a specified period.At a global scale, the terms carbon neutrality and net zero CO 2 emissions are equivalent.At subglobal scales, net zero CO 2 emissions is generally applied to emissions and removals under direct control or territorial responsibility of the reporting entity, whereas carbon neutrality generally includes emissions and removals within and beyond the direct control or territorial responsibility of the reporting entity (e.g., life cycle emissions).

Net zero GHG emissions
Net zero greenhouse gas (GHG) emissions are defined as by the IPCC (2021) as the condition in which metric-weighted anthropogenic GHG emissions associated with a subject are balanced by metric-weighted anthropogenic GHG removals.The subject can be an entity such as a country, an organization, a district or a commodity, or an activity such as a service and an event.GHG neutrality is often assessed over the life cycle including indirect (i.e., "scope 3") emissions, but can also be limited to the emissions and removals, over a specified period, for which the subject has direct control, as determined by the relevant scheme.The quantification of GHG emissions and removals depends on the GHG emission metric chosen to compare emissions and removals of different gases, as well as the time horizon chosen for that metric.

Net zero warming
Net zero warming is not formally defined by the IPCC; however, it has been described by Cain et al. (2019) as net zero (emissions plus removals) CO 2 warming equivalent emissions as calculated using GWP* for short-lived climate pollutants such as CH 4 .Net zero warming implies activities from an entity at the regional, subnational, or national scale would not lead to additional warming, and could be defined by reaching and maintaining net zero CO 2 warming equivalent emissions.Climate neutrality Climate neutrality is not formally defined by the IPCC; however, in common usage it can be viewed as equivalent to achieving no additional climate impact from activities from an entity at the regional, subnational, or national scale (Pineda and Faria, 2019).Climate neutrality can be viewed as equivalent to net zero warming and can be characterized by achieving and maintaining net emissions at zero CO 2 warming equivalents.
where E100 = the CO 2 e emissions calculated using GWP100, t = the year for which the CO 2 we are being calculated, and t-20 = the emissions in CO 2 e emissions calculated using GWP100 20 yr prior.
For longer-lived gases, such as N 2 O, GWP100 adequately represents warming responses to emissions rate increases or decreases, and the GWP100 value was used to calculate CO 2 we for those gases (Lynch et al., 2020).Therefore, all annual CO 2 we results include the warming effect contributed for a given year from CO 2 , CH 4 , and N 2 O.
As GWP* requires a time series of data to estimate emissions rate changes and the US EPA GHG inventory data begin in the year 1990, 2010 was used as the baseline year for future scenario analysis to determine emissions reductions required to achieve net zero CO 2 we, which is equivalent to net zero CO 2 (the point at which a system is no longer contributing to warming).
In addition to the constructed emissions inventory expressed both as CO 2 e and CO 2 we, dairy cattle inventories and milk production data from 1990 to 2019 were obtained from USDA NASS (2021).Future emissions trends from 2020 to 2050 were estimated in a case study scenario of estimated mitigation on a percent basis of absolute (e.g., in total) direct and indirect GHG emissions.Additionally, dairy cattle herd inventory was assumed to stay stable relative to 2019, and increasing milk production per cow was assumed at a rate similar to trends from 1990 to 2019.

RESULTS AND DISCUSSION
Trends in the 3 main categories of GHG emissions from dairy cattle production of enteric CH 4 , manure CH 4 , and manure N 2 O are shown in CO 2 e in Figure 2. Total direct GHG emissions from the US dairy industry have increased since 1990, with enteric CH 4 emissions increasing 11%, manure CH 4 emissions increasing 119%, and manure N 2 O emissions increasing 16% (US EPA, 2021b).Important context for these emissions trends is the changes in milk production and in dairy farm size and manure management systems.
From 1990 to 2019, the number of dairy cows in the United States decreased 6%, but milk production per cow increased 56%, translating into an increase in annual milk production in the United States by 32.1 billion kilograms of milk (USDA NASS, 2021).Over this period, dairy farms have relocated to the western part of the United States and this region currently accounts for almost 50% of the country's milk production (MacDonald et al., 2020).The Cattle Enteric Fermentation Model is used to estimate enteric CH 4 emissions accounts for the animal's dry matter feed intake, the digestibility of feeds, and the CH 4 yield per unit of gross energy the animal consumes (Mangino et al., 2003).As US dairy cattle have increased their productivity since 1990, they have increased feed consumption.Feed consumption is a key driver of CH 4 emissions; thus, enteric CH 4 emissions per cow in the US have increased.However, enteric CH 4 emissions per unit of milk have declined as increases in CH 4 emissions per cow have been offset by increased milk production.
Conversely, CH 4 emissions from manure management systems have increased per unit of milk.This has been driven by a shift in production systems of dairy farms with smaller herd sizes where manure is managed as a solid (e.g., daily spreading of manure), to dairy farms with larger herd sizes where manure is managed in liq- uid systems (e.g., anaerobic lagoons).As CH 4 production requires an oxygen-free environment, the switch to more long-term storage, liquid manure management systems has increased the CH 4 gas yield from dairy cattle manure in the United States.
Figure 3 shows how increasing CH 4 emissions from the dairy industry from 1990 through 2019 increases the assumed warming effects coming from the US dairy industry when expressed in CO 2 we (panel B) relative to CO 2 e (panel A).Cumulatively, the sum of the direct emissions from 2010 to 2019 from the US dairy industry were 1,047 million metric tonnes (MMT) of CO 2 e and 1,377 MMT of CO 2 we (Figure 3).Using GWP* versus GWP100 increases the assumed warming effect of the US dairy industry in this time frame by 32%.
There could be many iterations of a pathway to net zero warming by 2050 for US dairy production.The results outlined here should be considered a specific case study for the US dairy industry to make this concept more tangible and serve as a rightsizing of the magnitude of emissions reductions required to achieve no additional warming from dairy cattle production.However, there are many possible future GHG mitigation iterations to achieve the same outcome of net zero CO 2 we or a state of US dairy activities in which dairy cattle production no longer contributes to warming.
The key outcome changes of the case study are listed in Table 2.In this scenario, it is assumed that the dairy cattle herd will remain stable from the January 1, 2021 (herd size reported by USDA through 2050; USDA NASS, 2021).Figure 4 shows the annual CO 2 we emissions from US dairy cattle production from 2010 through 2050, whereas Figure 5 provides more context on the emission scenarios and highlights the cumulative emissions from 2010 to 2050 in both CO 2 e and CO 2 we emissions.Net zero warming is achieved when the annual dairy production activities do not add additional CO 2 we emissions to the total.In this scenario, this means US dairy cattle production would add to warming in the near term, but once annual CO 2 we emissions reach zero and are maintained at or below that level, the industry would not contribute additional warming thereafter.
In the outlined scenario, emissions need to decline per mass unit of milk produced, but also on an absolute basis, meaning the total emissions from the dairy industry must decline.As was aforementioned, this would be a departure from the trends of the past 30 yr according to US EPA data (US EPA, 2021b).Thus, although the emission reductions to achieve net zero warming will not be as large as what is required to achieve net zero CO 2 e emissions, they are still substantial departures from business as usual and will require development and adoption of new innovations.It is also assumed that the dairy industry will be able to reduce the net indirect CO 2 e emissions from feed production and other inputs per pound of milk produced.This could include moving to more non-CO 2 emitting energy sources for feed production and on-farm use, reducing N 2 O emissions from feed production, or increasing soil carbon stocks to offset CO 2 e emissions associated with feed production.
The reduction of enteric CH 4 emissions of 23% assumed here is consistent with reductions associated with the use of the feed additive 3-nitrooxypropanol (3-NOP; Thompson and Rowntree, 2020;Yu et al., 2021).Uddin et al. (2022) estimated the use of 3-NOP or dietary nitrate could reduce absolute CO 2 e emissions from the US dairy industry by 13.9 or 5.6 MMT of CO 2 e using the GWP100 value of 28 for CH 4 .Additionally, in the coming decades, there is the potential for adopting genetic selection strategies that lower CH 4 emissions from cattle or reduce emissions by indirectly selecting for feed efficiency traits such as residual feed intake (Lassen and Difford, 2020).This case study estimates a more aggressive reduction of absolute manure CH 4 emissions as compared with enteric CH 4 emissions of 31%.In part, we have made this assumption due to current trends in the industry of investing in new manure management techniques and policies that are driving change.
As an example, the state of California has set a goal to reduce dairy industry CH 4 emissions from manure management by 40% through the year 2030 based on 2013 levels.New manure management techniques, such as anaerobic biogas digesters, are one such strategy along with alternative manure management plans.To this point, the California dairy industry has already achieved a 22% reduction in emissions through the use of covered lagoons and conversion of captured biogas into vehicle fuels (California Air Resources Board, 2022).Alternative manure management practices beyond anaerobic digestion in California's plan include composting, acidification, and vermifiltration.As the reductions in CH 4 from dairy cattle manure in California illustrate, progress toward net zero warming will require the correct incentives and policies to help see this initiative to fruition.Although this policy only touches on the emissions associated with manure management, there may be further plans and policies to reduce CH 4 enteric fermentation emissions in the future.Given the success observed to this point in California, the timeline set should make it feasible for the US dairy industry to achieve its goals of reducing short-lived climate pollutants (SLCP) and achieving net zero warming.
In this case study, we estimate the US dairy industry will reduce its total CO 2 e emissions by 23% by 2050 (Table 2).This represents an aggregate reduction of CH 4 and indirect emissions of N 2 O and CO 2 from feed production, other inputs, and on-farm fuel use across the entirety of farms and production systems in the United States.Consequently, not all farms will reduce emissions by an equivalent amount.For example, for 2 farms with equal absolute emissions, one farm may not reduce its emissions from present day values, whereas another observes a 46% reduction.The percent per year reductions in absolute CH 4 and indirect CO 2 e emissions do influence the date at which US dairy cattle production could achieve climate neutrality.As highlighted in Figure 6, reducing the rate of annual mitigation of enteric CH 4 , manure CH 4 , and indirect CO 2 e by 50% compared with the base scenario can affect the date of achieving no additional warming, in particular, changing CH 4 emissions mitigation rates.Accurate and precise tracking of emissions at the farm level and national level will be required in the future to document progress toward the US dairy industry's emission reduction goals.
In addition to meeting the above climate goals in this proposed scenario, milk production expands while keeping herd levels relatively consistent.This trend is of upmost importance to continue to meet US consumer demands, along with growing export market demands (Alexandratos and Bruinsma, 2012).As the global population continues to grow, dairy is an important source of high-quality protein and micronutrients in the human diet (Pereira, 2014).Therefore, achieving net zero warming while still increasing total output will be valuable.

IMPLICATIONS AND LIMITATIONS
Recent research has demonstrated that the widely used GWP100 for CH 4 poorly represents the effect of these emissions on global temperature change when emissions are stable or falling as it fails to account for the atmospheric removal of CH 4 (Figure 1; Smith et al., 2021).Thus, the aggregation of all GHG emissions using GWP100 results in cumulative CO 2 e emissions, which do not necessarily represent the magnitude of future global surface temperature outcomes (Forster et al., 2021).As the dairy industry strives to cut emissions rates of CH 4 in the future, accurate climate metrics such as GWP* are of critical importance to clarify the degree of CH 4 emission reductions required to achieve no additional warming.Critics of GWP* have focused on issues of fairness rather than how the metric accurately links emissions to warming effects.In essence, the critique is that the metric makes the path easier to climate neutrality for nations or industries with higher historical SLCP emissions, such as CH 4 , as there is a greater potential for reductions and development of key SLCP-emitting industries such as ruminant agriculture, which has happened decades prior, whereas the metric penalizes developing countries with growing industries that are major sources of SLCP emissions (Rogelj and Schleussner, 2019;Schleussner et al., 2019).Although it is important to be mindful of value judgments around fairness, Cain et al. (2021) noted issues of fairness arise in application of all climate metrics, and importantly, the authors note that transparent disclosure of emissions sources can address issues of unfair burden sharing, while maintaining accuracy in representing impacts on warming.The latest IPCC Assessment Report (AR6) makes clear that if metrics account for the differences in CO 2 and SLCP, goals of halting temperature increases can be met by achieving net zero CO 2 emissions combined with stable or gently declining emissions of SLCP such as CH 4 (Arias et al., 2021).
Increasingly, corporations and industries are making pledges to achieve net zero emissions for dairy production.When these emission pledges are expressed as net zero CO 2 e, the path to achieve these commitments may be difficult.Although CH 4 emissions from enteric fermentation can be lowered, they are unlikely to ever reach absolute zero.Thus, to reach net zero CO 2 e annually from US dairy cattle production, additional soil carbon stocks would be required from the whole industry to offset these ongoing emissions into the future.
From a temperature response perspective, a net zero CO 2 e emissions balance from the US dairy industry would likely exceed a goal of climate neutrality (no additional warming impacts) from dairy cattle production and lead to climate negative production (equivalent to removing CO 2 from the atmosphere).Achieving such a balance would be unnecessary for US dairy cattle production if the initial goal is to no longer add warming to the atmosphere within the next 20 to 30 yr, especially in a cost-effective manner.Importantly, reaching climate neutrality defined as achieving and maintaining no additional warming impacts is equivalent to the goal of many CO 2 -producing sectors when they aim to achieve net zero CO 2 emissions.Net zero CO 2 implies no additional contributions are added to the cumulative CO 2 emitted, either through emissions reductions alone or emissions reductions combined with atmospheric CO 2 removals.With a mixed gas emissions profile that includes CH 4 , achieving no additional contributions to the cumulative CO 2 we emitted is equivalent to the temperature change implications of halting contributions to cumulative CO 2 (Forster et al., 2021).Thus, cutting CH 4 emissions in the short term would have a greater effect on reducing climate warming compared with strategies that look at limiting CO 2 emissions alone (Balcombe et al., 2018).
The ability to reach climate neutrality, net zero warming, or a net zero CO 2 we emissions balance is more achievable for the US dairy industry than net zero CO 2 e calculated using GWP100.However, accomplishing such a goal will still require major reductions in emissions from business-as-usual US dairy cattle production.Climate neutrality or net zero warming would align with the Paris Climate Agreement's temperature change goal (Table 1).It is important to note that a constant level of CH 4 emissions from current day and 20 yr ago will still have a slight warming impact on the climate under GWP* accounting methods.However, this effect is substantially less than that of current accounting methods such as GWP100, and thus demonstrate the need to reduce CH 4 emissions to meet climate neutrality.
As mentioned previously, the case study presented here is a deterministic analysis of only one of many potential future iterations of emissions reductions from US dairy cattle production and should not be interpreted as the only way to climate neutral dairy cattle production.Rather, this analysis is meant as a first step toward rightsizing emissions reductions targets and inspiring research and development to focus on creating absolute GHG emissions reductions that do not sacrifice productivity.Additionally, the system boundaries of this analysis are US dairy cattle production; thus, achieving net zero CO 2 we emissions from the activities associated with dairy cattle production does not mean warming globally will cease as the vast majority (approximately >99.5%) of anthropogenic emissions globally are occurring outside these system boundaries.

CONCLUSIONS
As society addresses climate change, it will be critical for all sectors of the US economy to do their part in stabilizing or reducing warming effects.The US dairy cattle industry is no different.However, it will be paramount to use metrics that are fit-for-purpose if the goal is not contributing to additional warming.As outlined in a case study scenario, dairy cattle production that no longer contributes to additional warming in 2050 could be achieved by lowering absolute CH 4 emissions, depending on the source, by 18 to 31% in the coming decades.However, these reductions only achieve net zero warming when also coupled with substantial reductions in emissions of CO 2 and N 2 O from indirect sources such as feed production, land use, energy use, and other inputs.Business-as-usual production will not allow the US dairy industry to achieve net zero warming; however, it is within reach as new and existing innovations that lower GHG emissions become more widely available and as the adoption of those innovations is incentivized.
Figure 1.Representation of the difference in atmospheric concentration responses for a scenario of constant annual emissions between longlived stock gases, such as CO 2 , and short-lived flow gases, such as CH 4 .Figure created by authors and adapted from Allen et al. (2018).
Figure 2. Trends in absolute direct greenhouse gas emissions from the US dairy industry from 1990 to 2019 according to US Environmental Protection Agency using the Intergovernmental Panel on Climate Change Assessment Report (AR5; Myhre et al., 2013) 100-yr global warming potential values of 34 and 298 for CH 4 and N 2 O, respectively.MMT = million metric tonnes; CO 2 e = CO 2 equivalents.

Figure 3 .
Figure 3. Direct greenhouse gas emissions from the US dairy industry from 2010 (reference year) to 2019 expressed as carbon dioxide equivalents (CO 2 e; panel A) and carbon dioxide warming equivalents (CO 2 we; panel B).MMT = million metric tonnes.
Figure 4. Annual US dairy cattle production cradle-to-farm gate carbon dioxide warming equivalent (CO 2 we) emissions expressed as million metric tonnes (MMT) from 2010 (reference year) to 2050 for the case study scenarios.This figure demonstrates achieving reductions in emissions as outlined in Figure 5 in 2050 emissions from US dairy cattle production of −89 MMT of CO 2 we, meaning that no additional warming would occur from dairy production activities in that year.

Figure 5 .
Figure 5. Cumulative carbon dioxide equivalents (CO 2 e) or carbon dioxide warming equivalents (CO 2 we) for US dairy cattle production from 2010 (reference year) to 2050 for the case study scenario.Assumed changes in emissions by time period are indicated on the graph.The point at which annual CO 2 we emissions do not add to further warming is indicated on the graph via the dotted line.

Table 1 .
Place et al.: PRODUCTION, MANAGEMENT, AND THE ENVIRONMENT SYMPOSIUM Definitions of carbon neutral, climate neutral, and net zero 1

Table 2 .
Thoma et al. (2013)CTION, MANAGEMENT, AND THE ENVIRONMENT SYMPOSIUM Case study scenario for US dairy cattle production to achieve net zero warming in 2050 relative to a base year of 2020 1The cradle-to-farm gate estimated here does not allocate any enteric and manure emissions from dairy cattle in the EPA GHG inventory to beef production; for comparison, a recent footprint analysis from Capper and Cady (2020) estimated a dairy cattle footprint of 1.7 kg CO 2 e/ kg of milk using 100-yr global warming potential (GWP100) values of 34 and 298 for CH 4 and N 2 O, respectively;Thoma et al. (2013)reported a cradle-to-farm gate US dairy average of 1.23 kg CO 2 e/kg of fat-and protein-corrected milk using the GWP100 values of 25 and 298 for CH 4 and N 2 O, respectively; and Rotz et al. (2021) reported a US dairy footprint of 1.01 kg CO 2 e/kg of FPCM using the GWP100 values of 28 and 265 for CH 4 and N 2 O, respectively.