Recalculating the global warming impact of italian livestock methane emissions with new metrics

Abstract The warming impact of methane (CH4) emissions calculated using the metrics proposed by the Intergovernmental Panel on Climate Change (IPCC), which measure its global warming potential in 100 years (GWP100) expressed as carbon dioxide equivalents (CO2e), accounts for the greatest impact in animal production chains. This work uses the new metrics, proposed to consider the difference between short living climate pollutants (SLCP), such as CH4, and long living climate pollutants (LLCP), such as carbon dioxide (CO2), which measure the warming equivalent (we) effect relative to that of CO2 in a given time frame (GWP*) and expressed as CO2we. The GWP* was applied to CH4 emissions from all Italian livestock supply chains and compared with GWP100 for annual and cumulative assessment from 2010 to 2020 of the impact of this gas on climate change. Using official data published by Istituto Superiore per la Protezione e la Ricerca Ambientale (ISPRA) from 1990 to 2020, almost all species, except for buffalo (+272.6% of emissions calculated with the new metrics), revealed lower CH4 emissions with the greatest re-dimensioning for non-dairy cattle (-53786 kt of CO2we of calculated with GWP* compared to +66437 kt of CO2e estimated with the GWP100 method). The total cumulative contribution of Italian livestock production to global warming over the past 10 years, including the nitrous oxide (N2O) emissions, has been greatly negative (-48759 kt of CO2we) compared to the data calculated using the GWP100 method (+206091 kt of CO2e). In conclusion, the application of GWP* metric to CH4 emissions of all Italian livestock supply chains allowed to better identify the role of Italian livestock on climate change. Over the 2010–2020 time frame, the Italian animal supply chains reduced the warming impact related to its CH4 emission, with the ruminants (expect buffaloes) being the major contributor to this positive effect. HIGHLIGHTS The application of GWP* metric reduced the warming impact of CH4 emissions of Italian dairy cattle, non-dairy cattle, sheep, goats, poultry and rabbits. The reduction of CH4 emission from the major ruminant species is the major contributor to the positive effect on climate change detected over 2010–2020 time frame. The application of GWP* metric to CH4 emissions of all Italian livestock supply chains allowed to better identify the role of Italian livestock on climate change.


Introduction
Methane (CH 4 ) represents the second largest anthropogenic greenhouse-effect gas after carbon dioxide (CO 2 ) (IPCC 2021).That originated from livestock, either from enteric fermentations or from effluents, contributes about 1/3 of the global methane emissions (Saunois et al. 2020).The comparison among different gases for their warming effect was established by the Intergovernmental Panel for Climate Change (IPCC) in 1990 and updated continuously (IPCC 1990(IPCC , 2021)).The universally used IPCC metrics place 1 kg of CO 2 as the climate-changing unit and compare other gases with this over a given time horizon.The one chosen is 100 years for which all greenhouse gases (GHG) are assigned a global warming potential (GWP) over the horizon of a century (100) and the correspondence is expressed in units of CO 2 equivalents (CO 2 e).Briefly, GWP i considers the ability of each gas (i) to absorb energy (RF) and the length of its atmospheric residency (t) and CO 2 is the reference gas (r): Although, this metric is widely used, and is the de facto standard metric for a range of purposes, it is not suitable for any objective; as reported by the IPCC (1990), there is no universally accepted methodology for combining all the relevant factors into a single metric.In particular, GWP does not allow to highlight the different effects on the warming during the time between Long Living Climate Pollutant (LLCP) and Short Living Climate Pollutant (SLCP).The GWP considers the length of atmospheric residency of each gas, but the different accumulation pattern between LLCP and SLCP is neglected, because it compares the RF accumulated over a time-horizon resulting from a pulse-emission of a specific GHG to a pulse-emission of equal mass of CO 2 .This is of crucial importance because the warming effect depends also on the concentration of a GHG in the atmosphere, thus on its accumulation pattern, that is significantly different between LLCP and SLCP: actually, CH 4 has a half-life of 8.6 years, and it is almost completely removed (oxidized and absorbed) after 50 years, while CO 2 resides in the atmosphere for over a century (Saunois et al. 2020).The different accumulation pattern between LLCP and SLCP causes diverse effects on the warming during the time: a) when emissions increase, the warming caused by LLCP (CO 2 ) increases exponentially, whereas that caused by SLCP increases linearly (CH 4 ), following the different pattern of gas accumulation in atmosphere; b) under constant emissions, CO 2 causes a linear increase of warming, because it continues to accumulate, whereas that of CH 4 is constant, causing no further warming; c) with decreasing emissions, CO 2 continues to cause increasing warming (reducing the velocity of increase) due to the continue accumulation of the gas, reaching a plateau at zero emissions, whereas the stock of CH 4 starts to decrease when the reduction of emission takes place.A major concern related to calculating the GWP of an SLCP as well as an LLCP is the scenario of decreasing emissions: the temperature continues to rise also in response to decreasing emissions, until they reach and remain at zero emissions; in reality, the temperature begins to decrease at the same time as the former (SLCP) decreases.These differences make difficult to express the SLCP impact on global warming in term of equivalent impact of a LLCP which is the objective of the GWP.On this basis, a group of atmospheric physicists, as part of the Oxford Martin Program on Climate Pollutants project (Shine et al. 2005;Allen et al. 2016Allen et al. , 2018;;Cain et al. 2019;Collins et al. 2020), have developed new metrics that account for the different behaviour of different gases (fluxes, emissions, lifetime) to give more reliable values in decreasing or increasing GHG emission scenario, especially for SLCP as CH 4 .The IPCC (2021) has also begun to consider these new metrics, and it is expected that the next revisions of global warming potential equivalences among gases will be revised.
Below follows the comparison between the GWP and the new proposed metrics: where: E is the mass emission for a GHG in a given year, H is the forward time horizon and GWP H is the GWP for a GHG as according to IPCC (1990)  Compared with the traditional IPCC metric, the new metric rewards those who significantly reduce CH 4 emissions but penalises those who increase emissions much more.However, the GWP Ã metric does not represent a concession for further CH 4 emissions but more reliably shows the contribution of (declining) CH 4 emissions to a reduction of global warming (H€ ortenhuber et al. 2022).Using equations (2 and 4), Figure 1 shows the recalculated data, originally proposed by Cady (2020), displaying the trend of GWP and GWP Ã of 1 kt of CH 4 expressed respectively in CO 2 e and CO 2 we as the percentage of CH 4 emission reduction or increase over 20 years.Smith et al. (2021), considering all the average parameters suggested by Allen et al. (2018) scaled for a time of 20 years, refined the equation (4) into the following equation: Work is beginning to appear in the literature that uses these new metrics to estimate CH 4 emissions from livestock systems at country level.Liu et al. (2021) recalculated the CH 4 emissions from US cattle industry funding that it has not contributed additional warming since 1986.Place and Mitloehner (2021) analysing the US dairy industry by using the new metrics, forecast that a net zero GHG emission will be reached around 2040.As defined by the IPCC (2021), 'net zero GHG emission is the condition in which metricweighted anthropogenic GHG emissions are balanced by metric-weighted anthropogenic GHG removals over a specified period'; the quantification of net zero GHG emissions depends on the metric chosen, and it could be different, in term of temperature outcomes, from the quantification of net zero CO 2 (Schleussner et al. 2019).In this contest, the use of new metric to quantify the net zero GHG emission would allow to estimate similar temperature evolution as achieving net zero CO 2 (IPCC 2021).H€ ortenhuber et al. ( 2022), studying the case of CH 4 emissions from Austrian livestock farms, found a large reduction in emissions in dairy cattle and pigs, but not in other species.
This paper aimed to apply the new metrics GWP Ã to CH 4 emissions from the main animal production chains in Italy and to compare them with values obtained using IPCC standards GWP 100 over the 2010-2020 time frame, alone and including the N 2 O direct livestock emissions to obtain an alternative estimation of the cumulative impacts.

Calculation of methane emission
Data on CH 4 emission for dairy cattle, non-dairy cattle, buffalo, sheep, goat, swine, horses, mule and asses, poultry, and rabbits between 1990 and 2020 (Figure 2) were downloaded from the Italian government agency for environmental monitoring (ISPRA 2022), which   produces annual estimates of the environmental impacts of human activities in Italy according to international standards.Values of CH 4 emissions were obtained by the sum of the two emission categories indicated by the IPCC (2019): 'enteric fermentation' and 'anaerobic digestion of manure'.The methods used to produce the data are documented in the Italian Greenhouse Gas Inventory 1990-2019 (Romano et al. 2021) and were mainly based on the TIER2 approach.

Calculation of CO 2 -equivalent
The CO 2 e of annual CH 4 emissions for each species were calculated following the equation ( 2) of the IPCC (1990), where E is the annual CH 4 emission and GWP H is the global warming potential of one ponderal unit CH 4 in a time horizon of 100 years corresponding to 28 units of CO 2 e (IPCC 2019).The impact values obtained are obviously the same as those calculated by ISPRA for the different species.

Calculation of CO 2 -warming equivalent
The CO 2 we of annual CH 4 emissions for each species were calculated following the equation ( 5) of Smith et al. (2021), where E SLCP (t) represents the annual CH 4 emission for a considered year, and E SLCP (t-20) is the annual CH 4 emission relative to the previous 20 years.
Because the available official annual data range from 1990 to 2020, the CO 2 we of annual CH 4 emissions were calculated for the decade 2010-2020, a period deemed sufficient for a comparative time series between the two metrics cumulative CH 4 emissions in 11 years (2010-2020) were calculated following the equations (2 and 5).

Results
The CH 4 climate annual impacts of Italian livestock for dairy cattle, non-dairy cattle and buffalo, from 2010 to 2020, calculated by using GWP Ã showed a value always below zero for the former, but with a trend towards zero in recent years for the second, and values above zero and increasing for buffaloes (Figure 3).Regarding the cumulative climate impact, dairy and non-dairy cattle showed linear increasing when assessed by official IPCC metric (GWP 100 ), whereases it assumed increasingly negative values when assessed using GWP Ã .Conversely, cumulative impact of buffalo presented increasingly positive values using the two metrics, with the GWP Ã values having a higher rate of increase than GWP.Sheep and goats showed a trend of GWP Ã values consistently below zero (except for the first year in sheep and the last year in goats) so that the cumulative values at the end of the period were also strongly negative (Figure 4).For the three largest monogastric species, pigs, horses, and muleasses, the annual climate-altering values calculated by the two methods were always positive, except for the mules and asses showing negative values for GWP Ã in the first two years.Cumulative climate impact evidenced higher value calculated by GWP Ã for swine, an interesting overlap of estimates for horses and the greatest impact for the last category when calculated with GWP Ã , especially from 2015 (Figure 5).The climate-altering impacts of poultry and rabbits showed, when calculated with the new metric, decreasing annual values moving from positive to negative, with 2014 having values near zero.Cumulative impacts calculated with GWP Ã (constantly lower than GWP) initially increased, with positive values until 2015, and then started to decrease with increasing negative values (Figure 6).The Figure 7 shows the climate-changing impacts of Italian whole-farming for the years 2010-2020.Values were consistently below zero when calculated with the GWP Ã metrics, and positive and almost constant when calculated with the GWP.Cumulative impact increased linearly when calculated by GWP, whereases it was increasingly negative when assessed by new metric.
Table 1 shows the quantitative and proportional variation of methane emission from 1991 to 2020, and the cumulative impact calculated with GWP and GWP Ã of the livestock sector in the 11 years under consideration.Except for buffalo, that increased the methane emissions by the 372% (from 7.8 to 36.9 kt CH 4 , from 1991 to 2020, respectively), all ruminant species showed decreased values of their emissions (mean of À18%).Among monogastric animals, poultry and rabbits decreases the CH 4 emissions (mean of À31.7%) whereases, swine, horses, and mules and asses increase their emission by 1.6%, 17.3% and 8.8%, respectively.Overall, the total livestock sector reduced the CH 4 emissions by 14.4%, from 821 to 702 kt CH 4 , from 1990 to 2020.In term of cumulative impact, cattle (dairy and non-dairy), sheep, goat, poultry and rabbits evidenced negative values of CO 2 we, whereases, buffaloes, horses, and mules and asses had positive values.
Considering the nitrous oxide (N 2 O) (direct) emissions over the 11 years as calculated by Romano et al. (2021) for the Italian livestock species, the accumulated climate change value of the GHG emitted, calculated as the sum of these data and those of CH 4 evaluated with the GWP Ã , is still largely negative and on average equal to À4.43 Mt/year compared with the official figure, in which both emissions are calculated with the GWP metrics, which is equal to þ18.73 Mt/year (Table 2).

Discussion
In this work, the environmental impact of the CH 4 emissions from the main animal production chains in Italy over the 2010-2020-time frame was assessed using IPCC standards GWP 100 and the new metric GWP Ã .The use of the new metric allows to better account for the different physical behaviours of shortand long-lived gases (Forster et al. 2021), that lead to different warming effects.This is of crucial importance for the livestock sector, considering that a large part of the environmental impact is due to the emissions of the SLCP CH 4 (Saunois et al. 2020).It should be stressed that the different metrics (i.e.GWP and GWP Ã ) do not provide the same answer, and the appropriateness of the choice of a specific metric depends on the reasons for which gases are being compared (IPCC 2021).We estimated different pathways of CH 4 emissions for different livestock specie, either looking to the annual emissions and to the cumulative emission framework, for the decade 2010-  Annual (left panel) and cumulative (rigth panel) methane emissions estimated as CO 2 equivalents (ECO 2 e; blue solid lines) using the global warming potential (GWP), and as CO 2 warming equivalents (ECO 2 we; orange dotted lines), calculated by global warming potential star (GWP Ã ).
2020.It should be emphasised that the GWP 100 is not indicated for use in GHG cumulative frameworks and, in this paper, its comparison with GWP Ã is only intended to highlight wherever the new metrics can be more suitable, and to compare our results with those of recent papers using a similar approach (Del Prado et al. 2021;Liu et al. 2021;Place and Mitloehner 2021).According to the method followed in these last cited works, and to the discussion on the applicability of GWP Ã of the FAO report (2022), in the present work the CO 2 we for a time series (a decade) was calculated, in order to consider the dependence of GWP Ã from the reference baseline year.Indeed, step/pulse metrics like GWP Ã depend on the emission in the considered year and 20 years ago (reference baseline year); if applied only from the present day relative to 20 years prior, it will only indicate the additional effect of the emissions on the temperature trend at the present but it will not reveal the absolute level of ing caused by the methane emissions (FAO 2022).

Climate impact of italian livestock
The trend of climate impact evidenced by the large ruminants of implies that the cumulative impacts calculated with the two metrics, were of opposite sign for the first two categories, while for buffaloes the cumulative impact calculated with GWP Ã was higher than that calculated with GWP.The results observed for the two cattle categories (dairy and non-dairy) agree with a recent report on the cattle CH 4 climate impact in US (Liu et al. 2021), although the impact is of different magnitude, considering the huge difference in the number of animals.
Regarding the cumulative impact of sheep, the result of the present work agrees with a recent work investigating on the global warming of small ruminant dairy sector in European regions (Del Prado et al. 2021); however, the result for goat were contrasting with our results, being the trend of accumulative impact constantly of positive sign.The authors explain the differences among species with a larger expansion of the goat dairy system compared to sheep ones; on the other hand, in Italy the consistency of both species has decreased in the last decades.
Regarding the three largest monogastric species, pigs, horses, and mule-asses, the absolute values for these classes of livestock are far lower than those for ruminant species, due to both the livestock  consistencies discussed below and the lower enteric emissions.In addition to the very limited contribution of poultry and rabbits to the CH 4 emissions of Italian livestock, the cumulative impact, calculated by GWP Ã , showed negative contribution of these two categories to the global warming.
The trend of the annual and cumulative impacts of the whole Italian livestock sector was similar to that observed for the dairy and non-dairy cattle, consistently with the huge contribute of these two categories to the total CH4 emission.Obviously, the cumulative results for the 11 years calculated with the two different metrics diverge and show that for the official statistics Italian animal farming has contributed, albeit limitedly compared to other sectors, to global warming, while it has decelerated the phenomenon thanks to the reduction of the heads reared for the species with higher emission of the GHG (Figure 7).
The explanation for the trends in the calculation of GWP Ã values can be attributed in large part to the to the variation in the number of the various categories of Italian livestock over the 1991-2020 time frame (Table 3).Dairy cows are constantly decreasing, as well as other cattle, while sheep and pigs have increased at the turn of the millennium and then a numerical fall that influenced the reduction of the impacts calculated with the GWP Ã metrics; on the contrary, pigs and above all buffaloes showed a numerical increase in the period 2010-2020 compared to the previous twenty years, so their  .Methane (CH 4 ) climate impact of Italian livestock for swine, horses, and mules and asses, from 2010 to 2020.Annual (left panel) and cumulative (rigth panel) methane emissions estimated as CO 2 equivalents (ECO 2 e; blue solid lines) using the global warming potential (GWP), and as CO 2 warming equivalents (ECO 2 we; orange dotted lines), calculated by global warming potential star (GWP Ã ).emissions were positive and even higher if calculated with the GWP Ã metrics than those obtained with GWP.
Interestingly, a huge variability can be observed in the annual trend of CO 2 we reported for almost all the species, compared to the trend of CO 2 e; this aspect can be explained by the previous mentioned dependence of GWP Ã on the emissions in the present and 20 years ago.Indeed, when methane emissions show a considerable year to year variability, GWP Ã results are more variable compared to those of GWP (Meinshausen and Nicholls 2022).
The reduction of climate impact observed using the GWP Ã only reflects the lowering of number of animals.However, it should be stressed that in general a reduction of GHG emissions is related to the improvement of farm efficiency.For example in Italy, cow's milk production has increased in the last two decades      1), the cumulative impact of the livestock sector in the 11 years under consideration would have affected more than 194 Mt if calculated using GWP metric, while the value turns out to be almost À60 Mt if calculated using GWP Ã metric, a big difference.In terms of individual categories, the contribution of the cattle and sheep sector stands out, while the buffalo and pig sectors have accumulated positive impacts removing a share of the virtuous trend of the first two.
Notwithstanding the use of the new metrics is relatively recent, some criticism arose in the scientific debate.Rogelj and Schleussner (2019) warn against the indiscriminate use of these new metrics when applied at the country level.They propose that the GWP Ã be adjusted for the time series of emissions, so as not to reward countries that have polluted heavily in the past and penalise developing countries that increase them to further their economic growth having low emissions in the past.These authors also warn against the abuse of the negative emissions value that results from applying the new metrics to LLCP reduction cases, which if applied out of scientific context and for policy purposes can be misleading.However, these objections can be applied both among countries and among breeding sectors and can lead to misleading conclusions if the exact purpose for which the measurements were carried out is not specified.The aim of this work was exclusively to provide measurements using the new metrics regardless its application which would require, as observed by Rogelj and Schleussner (2019), the choice of economic contexts and the related parameters that are arbitrary, and which border on speculative reasoning, typical of economic and social sciences.

Conclusions
The application of the GWP Ã new metrics on the CH 4 emissions of Italian livestock sector compared to the values obtained applying the GWP 100 standard metric, over 2010-2020 time frame, evidences that the Italian animal supply chains reduced the warming impact of CH 4 emissions.
The result of the work allows to identify differences among species, being ruminants (except buffaloes) major contributors to this positive effect, balancing the increase of the emissions registered by the lowest CH 4 impacting species (buffalo, swine, horses, mules and assess) at national level (18% of total methane emissions).
The entity of the decrease in CH 4 emissions over the 2010-2020 time frame assessed using GWP Ã was also able to compensate the warming effect of LLCP N 2 O of all Italian livestock sector, allowing to better identify its role on climate change.
caƩle Sheep Swine Buffalo Goats Horses Mules and Asses Poultry Rabbit

Figure 3 .
Figure3.Methane (CH 4 ) climate impact of Italian livestock for dairy cattle, non-dairy cattle and buffalo, from 2010 to 2020.Annual (left panel) and cumulative (rigth panel) methane emissions estimated as CO 2 equivalents (ECO 2 e; blue solid lines) using the global warming potential (GWP), and as CO 2 warming equivalents (ECO 2 we; orange dotted lines), calculated by global warming potential star (GWP Ã ).

Figure 4 .
Figure 4. Methane (CH 4 ) climate impact of Italian livestock for sheep and goat, from 2010 to 2020.Annual (left panel) and cumulative (rigth panel) methane emissions estimated as CO 2 equivalents (ECO 2 e; blue solid lines) using the global warming potential (GWP), and as CO 2 warming equivalents (ECO 2 we; orange dotted lines), calculated by global warming potential star (GWP Ã ).

Figure 5
Figure5.Methane (CH 4 ) climate impact of Italian livestock for swine, horses, and mules and asses, from 2010 to 2020.Annual (left panel) and cumulative (rigth panel) methane emissions estimated as CO 2 equivalents (ECO 2 e; blue solid lines) using the global warming potential (GWP), and as CO 2 warming equivalents (ECO 2 we; orange dotted lines), calculated by global warming potential star (GWP Ã ).

Figure 6 .
Figure6.Methane (CH 4 ) climate impact of Italian livestock for poultry and rabbits, from 2010 to 2020.Annual (left panel) and cumulative (rigth panel) methane emissions estimated as CO 2 equivalents (ECO 2 e; blue solid lines) using the global warming potential (GWP), and as CO 2 warming equivalents (ECO 2 we; orange dotted lines), calculated by global warming potential star (GWP Ã ).