Rethinking methane from animal agriculture

As the global community actively works to keep temperatures from rising beyond 1.5 °C, predicting greenhouse gases (GHGs) by how they warm the planet—and not their carbon dioxide (CO2) equivalence—provides information critical to developing short- and long-term climate solutions. Livestock, and in particular cattle, have been broadly branded as major emitters of methane (CH4) and significant drivers of climate change. Livestock production has been growing to meet the global food demand, however, increasing demand for production does not necessarily result in the proportional increase of CH4 production. The present paper intends to evaluate the actual effects of the CH4 emission from U.S. dairy and beef production on temperature and initiate a rethinking of CH4 associated with animal agriculture to clarify long-standing misunderstandings and uncover the potential role of animal agriculture in fighting climate change. Two climate metrics, the standard 100-year Global Warming Potential (GWP100) and the recently proposed Global Warming Potential Star (GWP*), were applied to the CH4 emission from the U.S. cattle industry to assess and compare its climate contribution. Using GWP*, the projected climate impacts show that CH4 emissions from the U.S. cattle industry have not contributed additional warming since 1986. Calculations show that the California dairy industry will approach climate neutrality in the next ten years if CH4 emissions can be reduced by 1% per year, with the possibility to induce cooling if there are further reductions of emissions. GWP* should be used in combination with GWP to provide feasible strategies on fighting climate change induced by short-lived climate pollutants (SLCPs). By continuously improving production efficiency and management practices, animal agriculture can be a short-term solution to fight climate warming that the global community can leverage while developing long-term solutions for fossil fuel carbon emissions.


Background
The irreversible impacts of climate change have threatened the sustainability of the earth's eco-system (O'Gorman 2015; Sahade et al. 2015;Demertzis and Iliadis 2018). The decadal mean temperature has been increasing steadily, resulting in the past decade being the warmest on record (NASA 2020). According to the World Meteorological Organization (WMO), global temperatures during 2015-2019 were on average, 1.1 ± 0.1 °C higher than the pre-industrial level (WMO 2020).
The vital solution to stopping the warming trend is achieving net "zero-emission" of long-lived climate pollutants (LLCPs), primarily carbon dioxide (CO 2 ) and to a lesser degree nitrous oxide (N 2 O). However, there is growing recognition that minimizing the emissions of SLCPs will quickly, though temporarily, slow the warming of the atmosphere and buy time for the global community to develop solutions to keep temperatures from Page 2 of 13 Liu et al. CABI Agric Biosci (2021) 2:22 surpassing the 1.5 °C temperature goal set in the Paris Climate Accord (UNFCCC 2016). Primary SLCPs include methane (CH 4 ), black carbon, tropospheric ozone, and hydrofluorocarbons (Pierrehumbert 2014; Haines et al. 2017). These pollutants have a relatively shorter existence in the atmosphere, but have high warming potential (Table 1), contributing one-third of the current radiative forcing (RF) from GHGs (Ramanathan and Xu 2010;Shoemaker et al. 2013).
Methane is the second-most abundant GHG and an important contributor to climate warming. Globally, the annual emission of anthropogenic CH 4 was 572 (538-593) million metric ton (MMT) per year during 2008-2017, which is an increase of 3.6% from 2000-2010 level's (Saunois et al. 2016;. With a RF of 0.61 W m −2 (Etminan et al. 2016), CH 4 heats the atmosphere 86 and 28 times more efficiently than CO 2 over a 20-and 100year time horizon, respectively.

Methane's short atmospheric existence
Methane has a short atmospheric lifetime of 12.4 years (Myhre et al. 2013a). About 80-89% of the total atmospheric CH 4 is removed by oxidation with tropical hydroxyl radicals (OH), a process referred to as hydroxyl oxidation (Levy 1971;Badr et al. 1992;Kirschke et al. 2013;He et al. 2019). Other sinks include reactions with stratospheric chlorine and oxygen atoms, uptake by soil, and reactions with chlorine atoms in the marine boundary layer (Saunois et al. 2019;Kirschke et al. 2013). Different from CO 2 , which persists and accumulates in the climate system, CH 4 is constantly being removed from the atmosphere. This means that neutral warming will be achieved if the emissions equal the amount being oxidized and destroyed in the air. If emissions exceed the amount being removed, there will be warming. If emissions are less than the amount being removed, then there will be temporary cooling of the atmosphere. According to Cain et al. (2019), with an annual decline rate of 0.3%, a CH 4 emission source will not lead to warming in 20 years.
For example, a herd of 100 head of cattle will contribute new CH 4 to the atmosphere. But if the herd remains constant and reduces their emissions by 0.3% every year over the next 20 years-such as with improved genetics-their CH 4 emissions will approximate what is being removed from the atmosphere. As a result, the herd's warming from CH 4 will be neutral. Reductions beyond that, mean that less CH 4 is being emitted than removed from the atmosphere, and will induce cooling. Allen et al. (2017) illustrated the differences between the climate impacts of CH 4 versus CO 2 in Fig. 1. Entering the atmosphere with steady rising emissions, both CH 4 and CO 2 warm the climate (Fig. 1, left). While the CH 4 warms the climate linearly to its emissions, CO 2 warms it at an accelerated rate.
When emissions are constant, atmospheric CH 4 is in a dynamic equilibrium where sources and sinks approximately balanced each other. Therefore, it holds the elevated temperature but adds little additional warming. In contrast, the warming caused by constant CO 2 emissions further increases as the gas accumulates in the atmosphere (Fig. 1, middle).
In response to falling emissions, a decrease of current temperature occurs in response to decreasing CH 4 emission as the sinks outweigh the emission sources ( Fig. 1, right). The warming caused by atmospheric CH 4 will drop to near zero in a decade when its emission becomes zero. In contrast, the temperature continues to rise with decreasing CO 2 emissions and holds at elevated level as the emissions becomes zero.
Because of its comparatively short atmospheric lifetime, reducing CH 4 emissions will not contribute to lowering long-term peak temperature, which is still determined by the stock of atmospheric CO 2 . However, the near-term benefits of SLCP mitigation on human health, agriculture, ecosystems, and climate have been widely recognized (Allen 2015;Haines et al. 2017). (1) All GHGs can be converted into equivalent CO 2 emission by multiplying the corresponding conversion factor, GWP i (H) (Eq. 2).
where E CO 2− eq is CO 2 -equivalent emission; E GHG i is the emission rate of the GHG i; GWP i (H) is the conversion factor; H is the selected time horizon, year.
Throughout the IPCC assessment reports AR1 to AR5, continuous updates on GWPs have been made to account for various interactions and processes. Commonly used GWP values for CH 4 in the IPCC ARs (IPCC 1990(IPCC , 1995(IPCC , 2001(IPCC , 2007(IPCC , 2013 are listed in Table 2. GWP 100 , which is calculated based on a selected time horizon of 100 years, is the current universal GHG trading scheme. (2)   Table 2 GWPs for methane in IPCC assessment reports (ARs) AR1, the first IPCC assessment report (IPCC 1990); SAR, the second IPCC assessment report (IPCC 1995); TAR, the third IPCC assessment report (IPCC 2001); AR4, the fourth IPCC assessment report (IPCC 2007); AR5, the fifth IPCC assessment report (IPCC 2013) a GWP values in and out of the parentheses are for biogenic and fossil CH 4 , respectively

Global Warming Potential Star (GWP*)
An alternative method denoted as GWP* has been recently proposed to assess the climate effects of SLCPs as a supplementary to GWP (Allen et al. 2017(Allen et al. , 2018Cain 2018). GWP* was first proposed in the form of Eq. 3, which equates the temperature impact of a sustained one-ton-per-year increase in SLCP emission to that of a one-off pulse emission of GWP H × H tons of CO 2 (Allen et al. 2018). The recently updated GWP* (Eq. 4) is comprised of a "flow" term ( r × E SLCP t × H ), which characterizes the fast climate response from the atmosphere-ocean surface interface to the changed RF caused by SLCP emission, and a "stock" term ( s × E SLCP ), which represents the slower climate response from the deep ocean (Cain et al. 2019).
where E CO 2 −we is CO 2 -warming equivalent emission; E SLCP is the change in SLCP emission rate over the time interval t (year); E SLCP is SLCP emission rate; r is the weighting assigned to the climate impacts of the change in SLCP emission rate; s is the weighting assigned to the climate impacts of the current emission (r + s = 1). Equation 3 can be considered a special case of Eq. 4 (r = 1 and s = 0) and can be applied to SLCPs that have only been released in recent years.
The weights r and s are scenario-dependent, and the exact values are estimated by multiple linear regression onto the response to CH 4 emissions during 1900-2100 in different scenarios (Cain et al. 2019). Lynch et al. (2020) found that a combination of r = 0.75 and s = 0.25 provide a good estimation of both historical and predicted warming impacts of CH 4 with different scenarios. Allen et al. (2018) showed that scaling the change in SLCP emission E SLCP over a t = 20 year provided a good fit for modelled warming. Considering the near to medium effects with all recommended parameters, Eq. 4 can be simplified to Eq. 5: where E SLCP(t) and E SLCP(t−20) indicate a current and a 20 years ago SLCP emission, respectively.
It is shown in Eq. 5 that GWP* weighs the climate effects caused by the current CH 4 emission ( E SLCP(t) ) four times as high as that estimated by GWP. In the meanwhile, it considers most of the CH 4 emitted 20 years ago as having been removed ( E SLCP(t−20) ).
Rather than being a brand-new metric, GWP* is a new way of applying GWP to SLCPs like CH 4 . GWP* does not convert the GHG emissions to an equivalent amount of CO 2 ( E CO 2 -eq ), which is always a positive number. Instead, it equates the climate impacts from a one-step permanent change of SLCP emission to that caused by a one-off "pulse" change of CO 2 ( E CO 2 −we , CO 2 -warming equivalent). Therefore, E CO 2 −we can be either positive or negative to indicate the "warming" and "cooling" of the temperature compared with 20 years ago, related to an increase and decrease of CO 2 , respectively. Lynch et al. (2020) compared GWP 100 and GWP* in different emission scenarios by using the FaIR v1.3 climate-carboncycle model. They demonstrated that GWP* provided a reliable link between CH 4 emission and its warming impacts while GWP overestimated the climate impacts when the emissions were constant or decreasing.

Methane from animal agriculture
Methane from livestock production is primarily from enteric fermentation and manure management. Methane from enteric fermentation is a byproduct of digestion of feed materials, chiefly roughage. The majority of CH 4 from ruminants is produced in the rumen and is exhaled or belched by the animal. During enteric fermentation in the rumen, methanogenic microorganisms generate CH 4 from hydrogen (H 2 ) and CO 2 produced by protozoa, bacteria and anaerobic fungi Morgavi et al. 2010;Tapio et al. 2017). The amount of CH 4 emissions depends on animals (i.e. the type of digestive tract, production stage, age, and weight), feed (i.e. quality, quantity, and composition), and ambient temperature (Shibata and Terada 2010;IPCC 2019). The quantity and quality of feed affect the energy, nitrogen, and minerals available to the microorganisms in the rumen (Shibata and Terada 2010). The protein content in the feed negatively influences CH 4 production, and the fiber content positively affects it (Shibata and Terada 2010). Only a small portion of CH 4 is produced in the large intestines of ruminants and expelled via flatulence (EPA 1995).
Methane from livestock manure is a product of anaerobic decomposition of the organic residues in the excreta of animals through a two microorganism mediated processes: "liquification", where organic substances are converted into organic acids with acetic and propionic acids being the primary products, and "methanogenesis", where organic acids are broken down into CH 4 within a pH range of 6.5-8.0 (Lapp et al. 1975). The anaerobic condition largely determines the production of CH 4 during manure storage and handling. Methane emission from manure management is largely dependent on ambient temperature Page 5 of 13 Liu et al. CABI Agric Biosci (2021) 2:22 and the composition and management practices of manure, including treatment, storage, and application methods (Petersen et al. 2013). Methane emissions vary significantly among different animal production systems. Liu et al. (2014) reviewed CH 4 emissions data per animal unit (AU) in the literature and reported average emission rates from poultry layer houses (12-13 g d −1 AU −1 ), swine (24-16 g d −1 AU −1 ), beef steer (56-118 g d −1 AU −1 ), beef heifer (161-194 g d −1 AU −1 ), and dairy cows (281-323 g d −1 AU −1 ). According to the U.S. EPA GHG inventory (EPA 2020), beef and dairy cattle contribute 72% and 24.7% of the total CH 4 from enteric fermentation (7.0 MMT), respectively; and beef and swine contribute 55.9% and 32.4% of the total CH 4 from manure management (2.5 MMT), respectively.
By using the life cycle approach, Gerber et al. (2013) reported that global animal agriculture contributes to approximately 7.1 × 10 3 MMT CO 2-eq GHG emissions every year, and livestock CH 4 accounts for about 44% of this total amount. Saunois et al. (2019) summarized the outputs from inverse modeling of satellite-based observational data and reported a decadal mean CH 4 emission of 111 (106-116) MMT year −1 from global livestock production during 2008-2017. The Food and Agriculture Organization's (FAO) "bottom-up" inventory indicates that livestock contributed 103.5-109.9 MMT year −1 CH 4 globally during the same period (FAO 1997). In the U.S., CH 4 emissions from animal agriculture were 9.5 MMT in 2017 (EPA 2020).
Livestock, and in particular cattle, have been broadly branded as major emitters of GHGs and significant drivers of climate change (Steinfeld et al. 2006;Hyner 2015;Abbasi et al. 2016). Dairy and beef cattle account for 65% of global livestock's CH 4 emissions (Gerber et al. 2013). As a result, campaigns advocating for plant-based diets cite solving climate warming as one of the foremost reasons to forego meat (Orde 2016;McMahon 2019). But these opinions fail to distinguish the "flow gas" CH 4 from the "stock gas" CO 2 and the differences between biogenic and fossil fuel carbon. These arguments also overlook many other benefits of animal agriculture, including providing complete protein and utilizing non-arable land.
By using the examples of the U.S. cattle industry, the present paper intends to initiate a rethinking of CH 4 associated with animal agriculture, in respect to its comparatively short atmospheric lifetime, recycling in the biosystem, and the assessment of its climate impacts, with the objectives to clarify long-standing misunderstandings and uncover the potential role of animal agriculture in fighting climate change.

Calculation of CH 4 emission CH 4 emission from U.S. cattle
This paper looks at the CH 4 warming impacts of U.S. cattle production (dairy and beef ). The CH 4 emission data for U.S. cattle production (both dairy and nondairy) between 1961-2017 were downloaded from the FAOSTAT database (data source: FAO 1997).

CH 4 emission from California dairy cows
The population data of dairy cows in California between 1951 and 2017 were obtained from the Milk Pooling Branch and Milk and Dairy Foods Safety Branches of California Department of Food and Agriculture (CDFA 2017). The 2000-2017 enteric CH 4 annual emission factors of California dairy cows were from the Greenhouse Gas Inventory of California Air Resources Board (CARB 2019). Yearly CH 4 emissions from enteric fermentation were estimated as the product of dairy cow population and the emission factor (Eq. 6).
As the emission factors for the years before 2000 were not available from the CARB Greenhouse Gas Inventory, the emissions from the early years were estimated by using the emission factor of the year 2000.
Total CH 4 emissions from California dairies were the sum of CH 4 from enteric fermentation and manure management. Methane emissions from manure management were estimated as below (Eq. 7).
The Manure Management Practices (MMP i ) include anaerobic digester, anaerobic lagoon, dairy spread, deep pit, liquid slurry, pasture, and solid storage. The CH 4 emission factor for each MMP i and the yearly proportion of each MMP i in California manure management system, as listed in Table 3, were obtained from the CARB Greenhouse Gas Inventory (CARB 2019).

Calculation of CO 2 -equivalent
The CO 2 -equivalents of annual total CH 4 emissions from U.S. cattle production were obtained by multiplying the GWP 100 of CH 4 (Eq. 2) (6) E enteric = population × annual emission factor (7) Emission factor for MMP i × Proportion of MMP i E CO 2 -eq = E CH 4 × GWP CH 4 (100)  where E CH 4 is the annual total CH 4 emission and GWP CH 4 (100) is 28.

Calculation of CO 2 -warming equivalent
The CO 2 -warming equivalents of annual total CH 4 emissions from U.S. cattle production were obtained using the GWP* method (Eq. 5).

Cattle production in the U.S.
From 1961 to 2017, the U.S. dairy cattle population has decreased by 46% (FAO 1997). At a similar time, the population of beef cattle peaked at 1.2 × 10 8 head in 1975, before declining. In 2017, the beef cattle population was 8.4 × 10 7 head-a reduction of 30% from 1975 (Fig. 2).
Total CH 4 emission from U.S. cattle production, including emissions from both enteric fermentation and manure management, was 7.4 MMT in 1961 and 6.2 MMT in 2017, with a peak emission of 8.5 MMT in 1975(FAO 1997. The CH 4 emission from U.S. cattle was 27% less in 2017 than the 1975 level.
Shown in Fig. 3, CH 4 from U.S. cattle was contributing negative CO 2-we to the climate each year since 1986, except for the period of 2008-2012. Between 1986 and 2017, the decrease in average annual CO 2-we from U.S. cattle CH 4 emission is equivalent to decreased warming from 50 MMT atmospheric CO 2 (Fig. 3, top), which is approximately 1% of the emission from nationwide fossil fuel combustion (EPA 2020). However, the GWP results suggested that the CH 4 emissions from U.S. cattle production led to a "net carbon" (CO 2-eq) gain of 165-196 MMT annually during the same period.
For the cumulative climate impacts assessed by GWP, it was aggregating all the past impacts throughout 1981-2017 without acknowledging decreases in warming during those years. It only showed positive warming from a decreasing herd, even though less cattle resulted in less CH 4 , and thus less warming. As a result, it did not accurately calculate the warming caused by CH 4 from the U.S. cattle herd during that period (Fig. 3, bottom). Conversely, the GWP* fluctuated between 55 and 200 MMT CO 2-we in the 1980s, and since 1990, it has become increasingly negative in response to factoring in the reduced emissions of the gradually decreasing herd.

Dairy production in California
California leads the United States in agriculture production and is the largest producer of milk and dairy products (USDA 2020). To further investigate how the development of the U.S. livestock industry affects climate change, and how GWP versus GWP* provide different indications to mitigation priorities, we focused on California dairies, applying the two metrics to their CH 4 emissions. From 1950 to 1970, the California dairy industry was contracting by farm but not animal numbers: The number of farms decreased by 75% (from 19428 to 4473) while the total herd in the state remained stable, between 7.5 × 10 5 and 8.5 × 10 5 head (CDFA 2017).
Between 1970 and 2008, the California dairy industry boosted its production and the total herd doubled from 8.5 × 10 5 to 1.9 × 10 6 head (Fig. 4). The special concentration continued with the number of farms decreasing from 4473 in 1970 to 1852 in 2008. During this time, from 1970 to 2008, the warming impact of California dairies increased using both GWP and GWP*. But GWP* showed warming increasing three times quicker than the traditional method, GWP. Noticeably, the state implemented its first climate policy, California Global Warming Solutions Act, in 2006 and set its goal for a sustainable development outlook.
Between 2008 and 2016, the number of California dairy cows has been decreasing by about 1% annually, and GWP* and GWP characterize climate contributions of California dairies drastically different. GWP* calculations show warming peaking in 2008, and then rapidly decreasing to 50% of its peak value in 2017, while the Grey and black solid lines represent GWP and GWP* of the methane emissions from California dairy cows, respectively; grey and black dotted lines represent the GWP and GWP* results, respectively, when the herd is constant; grey and black dashed lines represent the GWP and GWP* results, respectively, when the herd decreases 1% every year; grey and black dash-dotted lines represent the GWP and GWP* results, respectively, when the methane emissions meet California's mitigation target GWP results hit a plateau in 2008 and held at elevated levels from then on (Fig. 5, top). Because the GWP* results showed that climate warming effects of animal agriculture could be significantly reduced by lowering emissions slightly, we continued our study with a 20-year projection, starting from 2018, in three assumed scenarios. The first scenario simulates when the California dairy industry continues current production practices and the emissions of CH 4 remain constant with the 2017 level. The second scenario simulates the emissions of CH 4 continue to decrease by 1% per year to approximate the results of the 1% annual decrease in population between 2008 and 2016. The third scenario simulates the emission of CH 4 meets the target mitigation goal of California to reduce 40% of CH 4 emissions from livestock by 2030 (below 2013 level).
If CH 4 emissions from the California dairy industry remain constant every year, GWP* suggests that annual warming will decrease to less than 20% of the 2008 peak level in ten years and stabilize around 13% of the peak level in twenty years (Fig. 5, top).
If CH 4 emissions continue to decrease by 1% every year, GWP* calculations indicate that the dairy industry will no longer contribute additional warming after ten years of reduction.
If CH 4 emissions follow California's ambitious mitigation goal established in Senate Bill 1383, which calls for a 40% reduction of 2013 emission by 2030, the CH 4 from the dairy industry will be contributing negative GWP* after five years of reduction (Fig. 5, top). Assuming the reduction has started in 2018, the annual reduction is equivalent to removing the warming caused by 9 and 25 MMT CO 2 every year during the 2020s and the 2030s, respectively. The projection indicates that it is an efficient short-term solution. In contrast, the GWP results continued to show the warming contribution by the dairy industry every year in all three projected scenarios (Fig. 5, bottom).

Recycled carbon in animal agriculture
To fully understand livestock's contributions to the climate, it must be understood that CH 4 emissions from biogenic-versus fossil sources do not equally correspond to warming. This is because biogenic CH 4 is not new carbon in the atmosphere. It is a constituent of the natural biogenic carbon cycle, which has been an essential part of life since it began.
In the natural biogenic carbon cycle, plants assimilate CO 2 from the atmosphere during photosynthesis and store it as carbohydrates (e.g., cellulose or starch). Ruminant animals consume the plants and convert some of the carbon contained in plant carbohydrates into CH 4, which is then exhaled or belched out into the atmosphere. The CH 4 remains in the atmosphere for about 12 years, before it is converted back into CO 2 through hydroxyl oxidation (Levy 1971;Badr et al. 1992). Therefore, biogenic carbon is "recycled carbon" and not new and additional to our atmosphere, though the warming effects during its atmospheric presence should still be recognized. Biogenic carbon is markedly different from fossil fuel carbon, the latter which was stored underground for millions of years, and then added to the atmosphere. The combustion of fossil fuel frees this carbon at a speed much faster than it can be removed, resulting in "net additional carbon" added to the atmosphere. Therefore, the carbon in biogenic and fossil CH 4 are different in respect to their originations and atmospheric behaviors. Biogenic carbon keeps recycling among bio-system and the atmosphere, while the carbon from fossil fuel is a "net" addition to the atmosphere.
In the case of a stable herd with decreasing CH 4 emission, the availability of cattle emitted carbon is reduced in the atmosphere. Yet, the biogenic carbon cycle will still require carbon as the herd's feed demand will remain unchanged. Atmospheric carbon-from biogenic or fossil sources-will be incorporated into the cycle, eating into the abundance of CO 2 that has accumulated in the atmosphere. If the CH 4 emission from a herd decreases due to improved technologies and farm management, the biogenic carbon cycle can continuously absorb the airborne net carbon in the air, serving as a temporary "sink" to reduce the current atmospheric carbon burden, providing a short-term solution to climate warming.

Quantification of methane's climate effects Limitations with applying GWP to biogenic methane
Albeit the wide international application of GWP 100 as a quantitative basis for GHG trading, there is no shortage of discussion on its limitations, especially its applicability to SLCPs like CH 4 (Harvey 1993;Manne and Richels 2001;Alvareza et al. 2012).
First, though GWP does account for the different lifetime of GHGs, the physical interpretation of a SLCP's GWP becomes increasingly ambiguous as the selected time horizon extends. When the integration in Eq. 1 proceeds over the 100-year horizon, the numerator approaches a constant quickly because the emitted CH 4 will be oxidized in about a decade, but in the meantime, the denominator keeps increasing. Therefore, the magnitudes of GWPs are strongly dependent on the selection of the target time horizon for assessment (Manne and Richels 2001).
Second, GWP does not accurately reflect the actual climate impacts of CH 4 . Defined as the ratio of time integrals of RF, GWP can only be positive and always indicates a "warming" effect on the climate. It cannot reflect the potential "cooling" caused by a decline of the Page 10 of 13 Liu et al. CABI Agric Biosci (2021) 2:22 CH 4 emission (Fig. 1, right). Therefore, using GWP as the quantification tool overestimates the climate impacts caused by constant or decreasing emissions of SLCPs, and therefore could overlook opportunities for climate mitigation.

GWP* for evaluating the climate effects of SLCPs
GWP* is designed to characterize the short-lived nature of SLCPs. Cain et al. (2019) and Lynch et al. (2020) explained the application of GWP* with different emission scenarios and demonstrated that GWP* was able to accurately assess the "cooling" of the temperature compared with 20 years ago when the sinks of CH 4 outweigh the emissions, while GWP still ended up with "warming" effects under the same scenario. Also, GWP* can be directly linked to the temperature change by using a transient climate response to cumulative carbon emissions (TCRE) coefficient. But this method tends to underestimate the temperature response and is less accurate compared to more comprehensive methods (Lynch et al. 2020).
However, the practical application of GWP* will inevitably encounter challenges. Developers still need to provide different SLCP-specific parameter sets for GWP*, which may complicate its application. More case studies are necessary to further comprehension of the new GWP*, as well as the conceptional differences between E CO 2 -eq and E CO 2 −we . Future investigations are needed regarding how to incorporate the climate information provided by GWP* into carbon footprint studies and GHG mitigation policies (Schleussner et al. 2019).

The climate impacts of U.S. cattle production when considering GWP*
According to the long-term projection of the International Farm Comparison Network (IFCN), the worldwide milk demand will increase by 35% by the end of 2030 as both the global human population and dairy consumption per capita increases by 16% (Wyrzykowski et al. 2018). However, increasing demand for production does not necessarily result in the proportional increase of CH 4 production.
Methane emission from U.S. cattle was decreasing and contributing negative CO 2-we to the climate each year since 1986, except for the period of 2008-2012, indicating decreasing of the temperature rather than "warming". As a result of a decreasing CH 4 emission, the biogenic carbon cycle consumed more carbon than it emitted and offset "net carbon" in the atmosphere, contributing to a "cooling" of the temperature compared to 20 years ago.
According to Cain et al. (2019), it will not add additional warming to the climate in twenty years when the CH 4 emission is reduced by 0.3% every year. The 20-year projections in our study indicates that the dairy industry in California can effectively help limit warming in ten years with an annual CH 4 reduction of 1%, which is achievable by further utilizing production efficiencies and optimizing waste management. It is an example of a shortterm solution to climate warming that the global community can leverage while developing long-term solutions.

Approaches of California dairy industry to climate neutrality
The projections demonstrate that portions of animal agriculture are already part of a climate solution in some regions. With genetic optimization, better nutrition and animal care, and farm management improvements, less emissions are generated today while still meeting the increased demand for dairy products. For example, the advancements in genetic evaluation and artificial insemination in the late 1960s increased the availability of the high-yielding dairy cows for producers, which promoted the annual yield of milk by 55% since 1980 (Shook 2006;Bauman and Capper 2010). The introduction of a total mixed ration in the 1970s and the diet formulation program enabled feeding a nutritionally well-balanced diet to ensure the performance and productivity of dairy cows (Kolver and Muller 1998;Bauman and Capper 2010). From 1950 to 2016, the dairy industry in California has tripled its milk production efficiency from 3.3 × 10 3 to 10.6 × 10 3 kg per cow, while the CH 4 emitted per unit milk production (kg CH 4 / kg milk) decreased from 0.102 to 0.035.
Continued progress of farm management practices significantly reduce GHGs and other gas pollutants from dairy farms (Boadi et al. 2004;Newbold and Rode 2006). For example, anaerobic digesters have gained growing popularity due to their capacity to reduce GHGs and recover energy. As of March 2020, there were a total of 127 anaerobic digesters on dairy farms throughout California, and 108 of them were granted by the Dairy Digester Research and Development Program (DDRDP) between 2015 and 2019 (CDFA 2020). In conjunction, a total of 106 dairies were funded to install alternative manure management practices (AMMPs), including separators, weeping walls, scrapers, alternative manure treatment and storage, etc. According to CDFA, these measures will provide an annual reduction of 2.2 × 10 6 tons CO 2-eq GHGs, which equals 25% of the manure CH 4 emissions in the state's 2013 inventory, over the next five years (CDFA 2019).
California dairy farms are also taking additional steps to mitigate their total GHGs emissions via various measures, such as the adoption of solar energy and electrified farm practices. According to the life cycle assessment of  Naranjo et al. (2020), California dairies emitted 1.12 to 1.16 kg CO 2-eq GHG emissions to produce 1 kg energyand protein-corrected milk (ECM) in 2014, which is a reduction of 45-46.9% compared to its 1964 level.

Conclusions
Methane is a short-lived climate pollutant and it is fundamentally incorrect to assess the climate contribution of the "flow gas" CH 4 in the same way as the "stock gas" CO 2 . The widely used metric GWP overestimates the CH 4 induced "warming" and fails to reflect the relative "cooling" when the emission is decreasing. Therefore, applying GWP to biogenic CH 4 from animal agriculture may result in misguided mitigation strategies and targets. GWP* should be used in combination with GWP to provide feasible strategies on fighting SLCPs-induced climate change. The GWP* results in the present study showed that U.S. cattle production did not contribute additional climate warming between 1986 and 2017. It also suggest that California dairy farms are on the path to climate neutrality. By continuously improving production efficiency and management practices, animal agriculture can be a short-term solution to fight climate warming that the global community can leverage while developing long-term solutions for fossil fuel carbon emissions.