The latest National Surveys on Climate Change (UNDP and UCR, 2014; MINAE and UNDP, 2021) revealed that most of the Costa Rican population is aware that there are risks associated with climate change that can already be perceived and they agree to take action to fight against this global enemy. Accordingly, the government of Costa Rica has launched national policies and programs that seek to decarbonize its economy (Government of Costa Rica, 2019) and adapt to the climate change consequences (Government of Costa Rica, 2018), including the National Carbon Neutrality Program (PPCN, by its Spanish acronym) (DCC and PMR, 2020). Thanks to these efforts, Costa Rica was recognized with the Champions of Earth Policy Leadership Award (UNEP, 2019), but many challenges still remain.

With the publication of the latest version of ISO 14064-1 (2018), the PPCN included measurement uncertainty as a mandatory requirement for the reporting of greenhouse gas (GHG) inventories as an essential parameter to have precise and reliable data for the correct quantification of emissions and removals (DCC and PMR, 2020). Measurement uncertainty, formally defined as the doubt about the true value of a quantity that remains after making its measurement or estimation, is the best quality parameter of any measurement or estimation and reflects the impossibility of knowing exactly its value (JCGM, 2008a). Among the accepted methodologies for estimating uncertainty, the law of propagation of uncertainty included in the Guide to the Expression of Uncertainty in Measurement (GUM) and the Monte Carlo simulation method included in the Supplement 1 of the GUM (GUM-S1) stand out. These methodologies are based on modeling an output quantity y as a known function of several input quantities x i and handles the uncertainties associated with the input quantities by modeling them as random variables with defined probability distributions. These approaches use the on-hand information about the input quantities to produce an approximate evaluation of the uncertainty of the output quantity through a first-order Taylor series expansion or a propagation of probability distributions using simulation techniques (JCGM, 2008a; JCGM, 2008b).

In the context of GHG inventories, uncertainty estimation has been pointed as a key component to increase confidence in the reported results and help decision makers to better target areas for implementing mitigation strategies and policy development (El-Fadel et al., 2001; EPA, 2002; IIASA, 2007; Jonas et al., 2010a; Jonas et al., 2010b; Hergoualch et al., 2021). Several studies have been developed regarding this topic, including EPA (1996), Bharvirkar (1999), Frey (2007), Ritter et al. (2010), Pouliot et al. (2012), Milne et al. (2015), Quilcaille et al. (2018), and Solazzo et al. (2021), among others. With the publication of IPCC Guidelines for National Greenhouse Gas Inventories (IPCC, 2000; IPCC, 2006a; IPCC, 2019a), it was possible to establish a globally approved framework to estimate uncertainties in this field, based on both methodologies described in the GUM and GUM-S1. It has been pointed out that the first method may be easier to implement and suitable for calculating uncertainties from uncorrelated, normally distributed individual input estimates with variation ranges below ±30%, but it can lead to significant uncertainty underestimations when these restrictions are breached (Fauser et al., 2011; Wójcik-Gront and Gront, 2014). The Monte Carlo simulation method allows for different probability distribution functions, parameter correlations, complex models, and large uncertainties, making it more attractive for a wider range of cases. Applications of the Monte Carlo uncertainty estimation method in the field of GHG emission includes Monni et al. (2004), Ramírez et al. (2008), Fauser et al. (2011), Silva et al. (2011), Wójcik-Gront and Gront (2014), Caldwallader and VanBriesen (2017), and Cho et al. (2018), among others.

In Costa Rica, additional technical guidelines were developed to aid the implementation of uncertainty estimation in GHG inventories (DCC and LCM, 2020). Also, a study by Molina-Castro and Calderón-Jiménez (2021) served to complement and update the official database of Costa Rican emission factors (IMN, 2021), providing uncertainty values for the emission factors of the fuel sector and methodological guidance to approach uncertainty estimation of emission factors using asymmetric probability distributions. However, technical gaps still remain for an optimal implementation of uncertainty estimation in the GHG inventories of Costa Rica, including a lack of information regarding uncertainties of the national emission factors for the agricultural, waste treatment, livestock, and industrial sectors from the Costa Rican official database. Although studies carried out in other countries can be found to estimate emission factors and their uncertainties in the sectors mentioned above (for example Milne et al. (2014) and Monni et al. (2007) for agriculture, Zheng et al. (2004) for croplands, Caldwallader and VanBriesen (2017) for wastewater, Basset-Mens et al. (2009) for livestock, among others), no related works have been carried out in Costa Rica. These gaps are becoming serious limitations that prevent the reporting of complete, transparent, and reliable emission results that meet the nationally established requirements, urging the development of national-specific studies that complete the missing information.

This article sought to fill the gap of uncertainty information that the official database of Costa Rican emission factors currently has, providing values for the missing uncertainties of emission factors through the collection of information and application of the Monte Carlo simulation method where needed. It should be remembered that emission factors are key elements for indirect quantification of emissions, where emissions ( E ) are not measured directly as an amount of gas released into the atmosphere but estimated from other data values associated with the activity that cause the emission ( d ) and emission factors ( f ) that relate these data to the amount of gas emitted, as shown in Eq. 1. E = d ⋅ f (1) Therefore, according to the GUM uncertainty estimation principles mentioned previously, emission factor uncertainties are necessary for the estimation of the emission uncertainty. As a consequence of the process followed to achieve the proposed objective, this study also includes suggested updates for some of the emission factors considered. It is expected that this study, together with the one previously published by Molina-Castro and Calderón-Jiménez (2021), will ease compliance with the requirements for reporting GHG emission results and will serve as a guide for uncertainty estimation and interpretation in GHG inventories in Costa Rica and other countries around the world.

Table 2 shows the complete results obtained from the simulations processes used for each emission factor considered. The results correspond to the estimated value of the emission factor, its standard uncertainty u, and the limits of its 95% confidence interval calculated from the simulated data population for each emission factor. All estimated standard uncertainties and limits of 95% confidence intervals are reported as absolute values. However, due to their widespread use in the GHG sector, the corresponding relative standard uncertainties and interval limits are shown in Supplementary Table S4 of the supplementary material.

When comparing the values of the official emission factors shown in Table 1 with the estimated values shown in Table 2, differences are obtained for buffalo enteric fermentation, composting of solid waste (N₂O), river discharge of industrial wastewater, non-energy use of lubricants, and solid waste disposal in landfills. The reason for these differences is due to the use of updated information from the latest 2019 versions of the IPCC guidelines or other references in the present study, while the official values are based on outdated values included in the 2006 or earlier versions of these guidelines. The specific variables or input quantities updated this way are specified in Supplementary Tables S1–S3 of the supplementary material. For this reason, users and those responsible for the official list of emission factors of Costa Rica are suggested to update the emission factors according to the latest versions of the references used. The information included in Table 2 can be used for this purpose.

In this same context, it should be noted that 2019 IPCC guidelines (IPCC, 2019b) established a new methodology to estimate emissions from livestock manure management instead of the default use of recommended emission factors (Tier 1 method from IPCC, 2006d). Therefore, it is also suggested to those responsible for the official list of emission factors of Costa Rica to consider updating the values for these sources consistently with the new methodologies indicated by the updated references and evaluate their corresponding uncertainty estimations. This update could not be carried out in the present study due to the lack of data required to apply the new methodologies (IPCC, 2019b). Additionally, an update in the data used to estimate the emission factor associated with landfills is suggested. As evidenced in this study, the current official factor continues to use values of mass fractions in the bulk waste from a study conducted 20 years ago (FEDEMUR, 2002). Since then, several municipal waste composition studies have been developed in Costa Rica, including Campos-Rodríguez and Soto-Córdoba (2014), Herrera-Murillo et al. (2016), and Soto-Córdoba and González-Buitrago (2019), among others. Results from these and other studies could be used to estimate more accurate mass compositions of the bulk waste and improve the national emission factor.

The uncertainties included in Table 2 for each of the emission factors are highly relevant considering that an estimate “is complete only when accompanied by a statement of the uncertainty of that estimate” (JCGM, 2008a). Therefore, with the standard uncertainties and confidence intervals shown in Table 2, now it can be considered that these emission factors are complete estimates, useful for users of this official list who seek to estimate the uncertainty associated with their emission inventories.

It is important to note that the limits of the 95% confidence interval are usually used for expressing uncertainty in a condensed way known as expanded uncertainty ( U ) in metrology, specifically when the variation interval is symmetric to the value of the emission factor and the interval can be expressed as f ± U . For asymmetric intervals, the reported limits are maintained due to their capacity to clearly express the magnitude of the variability associated with emission factors, even though they cannot be expressed in the condensed notation (IPCC, 2019a). In Supplementary Table S4 of the supplementary material, where expanded uncertainties U can be easily identified, symmetrical cases in which the ± U notation could be applied include emission factors from livestock enteric fermentation ( f ± 50%), manure management ( f ± 30%), and industrial wastewater treatment with anaerobic reactors and shallow lagoons ( f ± 31%).

It should be remembered that the standard uncertainties shown in Table 2 must be interpreted as standard deviations associated with the emission factors since the latter are considered random variables. For this reason, these uncertainties are of special interest for users since they can be combined with uncertainties of other variables (JCGM, 2008a; JCGM, 2008b). This is the case for the indirect quantification of emissions, which combines emission factors ( f ) with activity data ( d ). With the standard uncertainties shown in Table 2 available, users only need to know the uncertainties associated with their activity data (typically taken from their measurement instruments or certificates) to estimate their emissions’ uncertainties. For the estimation of relative uncertainties according to DCC and LCM (2020), values shown in Supplementary Table S4 of the supplementary material are recommended.

To compare and better understand the magnitudes of the estimated uncertainties, their relative values shown in Supplementary Table S4 are used. Most of the standard uncertainties are between 15% and 50% of the value of the emission factor. This behavior was expected since the emission factors usually show uncertainties greater than 15% (IPCC, 2006a; DCC and LCM, 2020). Also, their confidence intervals tend to asymmetry as their uncertainties or the number of multiplicative elements in their estimation increases (IPCC, 2006a; IPCC, 2019a). When comparing these results with other studies, a general high consistency was found with the values reported by Solazzo et al. (2021) and Milne et al. (2014). The confidence intervals associated with livestock (enteric fermentation and manure management) are practically identical in all cases. It should be highlighted that Milne et al. (2014) suggest fitting a lognormal distribution for ±50% intervals. Under this assumption, the standard uncertainties for enteric fermentation emission factors estimated in the present study could increase from 25% to 28.3%. However, fitting a normal distribution for a symmetrical interval of ±50% is consistent with Wójcik-Gront and Gront (2014), who suggest this value as an upper limit for this assumption. In the case of wastewater treatment, Solazzo et al. (2021) indicate that the uncertainty for wastewater treatment emissions highly depends on the technology and that the confidence intervals for these emission factors can vary between -33% and +78%. In the present study, a minimum lower limit of -56% (septic tank) and a maximum upper limit of +74% (river discharge) were obtained. For landfills, Solazzo et al. (2021) indicate that the global confidence intervals of uncertainty for CH₄ can vary between 35% and 134%, the first value being consistent with the interval of [-34%, +31%] obtained in the present study. Finally, for the emission factor associated with the non-energy use of lubricant, Solazzo et al. (2021) suggest a confidence interval of ±100%, practically double that obtained in the present study. This difference can be justified because the former corresponds to a generalized value and the latter responds to a localized national study.

Although meeting the results’ expectations, it should be noted that the present study used expert criteria in a novel way to establish the expected variabilities for some input variables in the absence of this information. Additionally, the present study did not only map the different strategies to define the emission factors and apply the Monte Carlo method as a flexible and technically sound methodology for the quantification of their uncertainties but also reported the standard uncertainties associated with the emission factors, critical information to complement the official database and to help users obtain reliable results more easily.

Attention is drawn to the large values of standard uncertainties and their confidence interval limits estimated for the emission factors associated with the biological treatment of solid waste. These factors correspond to default emission factors taken from the IPCC guidelines (group 1 in this study). As such, they can present very high uncertainties (standard uncertainties ≥50%, confidence interval limits ≥100%) because they describe the behavior of emissions under a wide range of conditions evaluated in different studies compiled by the IPCC. These cases are clear examples of possible opportunities to focus national and regional environmental efforts towards the quantification of specific emission factors for tropical countries like Costa Rica. These efforts may include studies that consider the specific characteristics of the tropical region, such as its climate, topography, available technologies, treatment conditions, among others. In this way, the estimation of national emission factors more suitable for users could be achieved, with smaller uncertainties than those currently estimated.

With the present study, it was possible to fill the gap on the information of uncertainties associated with the emission factors considered from the official database of Costa Rica thanks to the use of probability distributions’ fitting and the Monte Carlo simulation method. This information included both standard uncertainty values and 95% confidence intervals for each of the emission factors addressed. These results are critical to complement the official database of Costa Rican emission factors and for national users to estimate the uncertainties of their GHG inventories. A higher level of confidence in the results of GHG inventories is expected at the national level through the implementation of the results generated in this study. In turn, this will ease compliance with the national environmental policies and commitments by adapting to international requirements in the fight against climate change.

One of the main limitations of this study was the decentralization of the information since different national actors handle the data required to make these estimations. For this reason, significant time was invested in tracking information and coordinating meetings between different professionals involved in this field of study. Also, the information found could be incomplete or outdated in some cases. This situation made it necessary to use expert consensus or the search for complementary sources such as normative specifications as strategies to fill in the information gaps. Furthermore, the absence of published studies on the subject of uncertainty estimation for GHG inventories in Costa Rica is mentioned, so these pioneering works do not have national references to contrast the obtained results and it is necessary to rely on studies in other latitudes to corroborate their technical consistency and rationality.

Improvement opportunities were identified to update the estimates of some national emission factors based on outdated references and methodologies, including factors from livestock enteric fermentation, manure management, waste treatments, and non-energy use of lubricants. It was also possible to identify remarkably high uncertainties for three emission factors associated with the biological treatment of solid waste (confidence interval limits ≥100%). The accuracy of these factors could be improved and their uncertainties may be reduced through national studies adapted to the specific characteristics of tropical countries like Costa Rica instead of using generalized international references.

Finally, it is considered that the present study provides the expected guidance for the interpretation and manipulation of emission factor uncertainties. This study will hopefully ease the process of implementing uncertainty estimation in GHG inventories, obtaining a more accurate, transparent, and reliable quantification of GHG emissions in Costa Rica and other countries around the world.