1. Introduction
Globally, more than two billion tons of municipal solid waste (MSW) are generated every year. Waste generation tends to rise as countries become wealthier, with higher levels of industrialization and urbanization, which lead to a shift in housing and consumption patterns [1]. Substantial greenhouse gas (GHG) emissions can occur during the life cycle of materials, including extraction, production, transportation, and waste disposal treatment [2]. It was reported that the waste sector is responsible for approximately 5% of global GHG emissions [3]. Disposal of MSW in landfills can emit GHG emissions, particularly methane (CH4) gas from organic waste decomposition [4]. In addition, carbon dioxide (CO2) gas and a smaller amount of nitrous oxide (N2O) gas can also be emitted into the atmosphere by waste incineration [5]. The rates and amounts of GHG emissions from MSW largely depend on the types of waste and treatment methods. Adopting effective waste management strategies such as waste reduction at source, reuse, recycling, and waste-to-energy can significantly reduce such emissions [6].
In South Korea, public concerns over environmental problems from MSW have been raised since the early 1990s. This phenomenon increased pressure on the Korean government to develop integrated and sustainable waste management [7]. MSW in South Korea is commonly classified into three major types: recyclables, food waste, and disposable mixed waste. Recyclables include various waste materials such as plastics, metal, glass, paper, and other recoverable items that are separately discharged for recycling in households and commercial sectors. Food waste consists of various residues such as vegetables, meat, fish, and other leftover foods generated from cooking processes and dining [8]. Disposable mixed waste contains non-separated waste at households and other non-recyclable wastes that are commonly disposed of in prepaid-disposable bags, which are designated for incineration and landfilling. This disposable bag system, commonly known as the pay-as-you-throw scheme (or volume-based waste disposal fee system), has been implemented as part of waste minimization efforts since 1995. Such pay-as-you-throw scheme requires every household to discard non-recyclables into prepaid-disposable bags [9]. By encouraging households to reduce MSW at the source, this scheme can positively promote waste segregation and recycling [7], which are key opportunities for GHG reduction potentials.
Seoul, the capital city of South Korea, with a population of approximately 9.4 million in 2022, generates a significant amount of MSW every year, which contributes to GHG emissions. According to the data released by the National Waste Generation and Treatment Statistics in 2022, Seoul generated a total of 3,983 kt/yr of MSW. In accordance with the 2030 Nationally Determined Contributions (NDCs) by Korea, it aims to reduce total national GHG emissions by 40% from the level in 2018. Therefore, immediate actions are needed to reduce GHG emissions. Seoul is facing a challenge in managing a large amount of MSW streams while minimizing environmental impacts. Various MSW policies have been introduced to reduce waste at source, promote recycling, and ensure safe treatment and disposal. Under Enforcement Rules of the Waste Management Act by Korea Ministry of Environment, a landfill-ban policy (i.e., zero-landfill policy) for MSW will take effect at the beginning of 2026 in Seoul and its surrounding cities. By facing the landfill-ban policy along with the need for carbon neutrality, more effective and sustainable MSW management practices should be developed and implemented.
Several studies have evaluated the GHG emissions and reduction potentials of different waste management strategies across different regions. A case study in Serbia [10] with a population of 7.2 million and daily waste generation of 1.2 kg/capita found that landfilling 80% of waste would result in almost 3,000 kt CO2 eq/yr of GHG emissions. Engelmann et al. [11] further supports this finding by observing that a higher dependence on landfills leads to worse environmental and energy performance. In contrast, a study in Brazil [12] estimated potential GHG savings of −300 kt CO2 eq/yr from the integrated system with composting and recycling options. According to Vergara et al. [13], applying a 40% source reduction practice for the disposal of 28,800 kt of California’s waste could lead to GHG savings of up to −30,000 kt CO2 eq/yr. Similarly, Boston City, which currently diverts only 25% of its waste, by adopting zero-waste strategy that emphasizes recycling practices and organic waste diversion can avoid approximately −1,300 kt CO2 eq/yr of GHG emissions [14]. These findings demonstrate GHG emissions reduction potentials through the modification of MSW management methods. However, there remains a significant research gap regarding scenario-based studies that evaluate GHG reduction potentials from MSW management practices by Seoul. Additionally, existing studies often overlook the influence of local policies and future MSW management targets. Further study that models GHG reduction potentials under locally relevant policy scenarios is needed to better support decision-making in MSW management towards a carbon-neutral society.
To fill this research gap, this study was conducted to estimate the potential reduction in GHG emissions under the current and alternative waste management systems in Seoul. The Waste Reduction Model (WARM), developed by the US Environmental Protection Agency (EPA) was used as a tool designed to determine GHG reduction potentials [2]. By examining existing and alternative solutions, this study analyzed Seoul’s waste generation trend by 2040, conduct GHG reduction potentials analysis, and identified key waste management options that could contribute to GHG reduction. This study contributes to the field as one of the first scenario-based analyses of GHG reduction potentials from MSW management in Seoul, incorporating its unique policy measures, such as landfill-ban policy, MSW reduction targets, and recycling rate goals. As Korea moves toward 2050 carbon neutrality goal, understanding the impacts of different MSW management scenarios becomes increasingly critical. Scenario-based assessment could help to visualize the trade-off impacts of each potential pathway. Furthermore, the results from this study can be used to support the development of future sustainable waste management plans toward a carbon-neutral city. Since Seoul already has a well-established waste separation and recycling system, a key assumption underpinning this research is the prioritization of disposable mixed waste and food waste over recyclable waste. Disposable mixed waste is typically sent to incineration facilities and landfills, which generate significant GHG emissions. Reducing and diverting valuable materials from disposable mixed waste is assumed to be more impactful in reducing GHG emissions than implementing it for recyclable waste.
2. Methodology
2.1. Data Acquisition
In this study, the term ‘MSW’ refers primarily to waste generated by households, and also includes similar waste from commercial and institutional sources. Historical waste generation data were collected from the ‘National Waste Generation and Treatment Statistics’ [15]. For the analysis of the generation trend of MSW in Seoul (Fig. 1), we used data covering the years 2000–2022. However, for the purpose of predicting MSW generation trend by 2040 in Seoul, we focused on the more recent data from 2010–2022. Seven mathematical models were applied to this dataset: linear, arithmetic, geometric, exponential, least-square, logistic curves, and Gompertz model [16–18]. Such models were chosen to capture and represent different approaches to trend analysis, each suitable for different types of data behavior. The future waste generation trend was estimated using the average of their results.
2.2. Waste Reduction Model
The Waste Reduction Model (WARM) was developed by the US Environmental Protection Agency (EPA). It is a software tool designed to help solid waste planners and organizations estimate GHG emission reductions associated with different waste management practices [2]. WARM uses a life-cycle approach, covering upstream and downstream emissions to estimate GHG emissions for baseline and alternative waste management [11, 14]. By comparing management scenarios, WARM aims to identify the net GHG emissions through multiple waste management practices, such as source reduction, recycling, composting, anaerobic digestion, combustion, and landfilling. Furthermore, the model also estimates the energy consumption related to each waste management option. In this study, we used the WARM to estimate GHG reduction and energy consumption from both current and future waste management practices in Seoul. To input data into WARM, the waste composition was recalculated and reclassified to match the material classification required by the model as shown in Table S1 (supplementary material). Several assumptions are defined to better reflect the condition of Seoul’s waste management system (Table S2).
2.2. Scenario Analysis
Scenario analysis was conducted to predict the changes in waste generation and the potential for GHG reduction. According to the Second Seoul Resource Circulation Plans (2023–2027), Seoul aims to achieve 26% waste reduction and 67% recycling rate by 2027. Nationally, the Korea Resource Circulation Plans sets a goal to gradually expand the biogas production by anaerobic digestion up to 36% by 2027. Another important policy is the landfill-ban, as Seoul is one of three major areas (Seoul, Incheon, and Gyeonggi-do), where the landfilling of MSW without pre-treatment will be prohibited in 2026 (Enforcement Rules of the Waste Management Act). Therefore, by considering the future possible or desirable situations of MSW management, four scenarios (BAU, S1, S2, S3) were set by reflecting such policy changes and goals for this study. The detailed assumptions for each scenario are described in Table 1.
3. Results and Discussion
3.1. Analysis of Waste Generation Prediction
3.1.1. Current trend of municipal solid waste
Fig. 1 reveals the amount of annual MSW generation in Seoul between 2000 and 2022, which showed a fluctuating trend. The trend declined noticeably between 2000 and 2013, but after 2014 it began to increase steadily until 2022. This increase was partly due to the change in consumption patterns, dominated by online shopping and food-delivery services [19]. A key trend among all wastes is the large fraction of recyclables. This shift highlights the growing adoption of source separation practices for recyclables. According to Park & Lah [7], the pay-as-you-throw scheme has a positive impact on boosting the recycling rate. This scheme encourages households to separate waste materials at the source and reduce the expense of purchasing prepaid disposable bags. Such practices would keep maximizing waste diversion and also reduce GHG emissions by conserving energy and materials.
Fig. 2 represents the waste streams and treatment methods of Seoul’s waste management system in 2022. The amount of MSW generated in 2022 was 3,983 kt/yr, which is categorized into three main waste types: recyclables (1,644 kt/yr), disposable mixed waste (1,374 kt/yr), and food waste (965 kt/yr). Out of the total recyclable waste, others, plastics, and paper were the main waste components, accounting for approximately 83%. Jung et al. [20] studied 17 major regions in South Korea and found that during 20 years (1998–2017), the amount of plastic waste was increasing while the discharge of paper showed a downward trend. A similar tendency can also be observed in European Union. According to Eurostat [21], the plastic waste generation has increased from 7.5 million tons in 2004 to 12.4 million tons in 2022. This shift led to the recycling priority on plastic waste that reflected in both Korea and EU policies (e.g., Korea’s Master Plan for Resource Circulation and EU Circular Economy Action Plan). Similarly, plastic, others, and paper dominated the mass composition of disposable mixed waste, suggesting additional opportunities to divert these wastes from being burned and landfilled. However, food waste followed diverse treatment pathways: 50% animal feed manufacturing (482 kt/yr), 25% composting (241 kt/yr), 15% anaerobic digestion (145 kt/yr), and 10% incineration (96 kt/yr).
3.1.2. Predicted generation of municipal solid waste in Seoul
As shown in Fig. 3 and Table S3, the prediction of waste generation trend in Seoul based on historical data (2010–2022) by the mathematical models indicates an increasing trend. By 2040, the generation of MSW is estimated to reach 4,412 kt/yr or equal to 12.1 kt/day. This reflects an 11% increase compared to 3,983 kt/yr in 2022. This growth underscores the urgent need for proactive waste management strategies to effectively manage the increasing amount of MSW.
The material flows of waste generation in Seoul in 2040 under BAU (a) and S3 (b) are shown in Fig. 4, which illustrates the key differences in waste treatment methods. Based on BAU scenario, all disposable mixed waste, which was previously managed through both landfilling and incineration is assumed to be treated by incineration method only. This shift could reflect Seoul’s efforts and commitments to reducing dependency on landfilling. Besides that, the amount of MSW that will be directed to incineration facilities is projected to increase without reduction efforts. Thus, it has the potential to exceed the incineration capacity of Seoul at 2,865 tons/day or equal to 1,046 kt/yr [22].
Scenario 3, however, highlights a more sustainable alternative scenario by integrating source reduction efforts. It also adopts the implementation of mechanical pre-treatment facility for SRF (Solid Refuse Fuel) production, which reduces the amount of MSW to be sent to incineration facilities and landfills [23]. Under this scenario, 747 kt/yr of MSW will be reduced, while approximately 443 kt/yr of disposable mixed waste will be recovered by the mechanical pre-treatment facility.
3.2. GHG Reduction Potentials by Scenario Analysis
As shown in the BAU scenario (Fig. 5, Table S4, and Table S5), Seoul’s GHG reduction potentials from 2010 to 2040 was predicted using the WARM. Overall, Seoul has maintained negative values of GHG emissions, indicating GHG savings instead of generating GHG emissions. This finding aligns with a previous study, which found that Seoul is already a net-negative GHG producer due to the high levels of source-separated collection and recycling system [24]. However, after the implementation of the landfill-ban policy in 2026, the GHG reduction is predicted to decrease by −3,256 kt CO2 eq/yr as a result of more waste being incinerated. This finding is relevant to a study by Liu et al. [25], which noted that the increase in CO2 emissions from the MSW management is strongly associated with the accelerated adoption of incineration as a waste disposal method. This result reflects an increasing pattern of GHG emissions, mainly emitted from burning MSW in incineration facilities.
In order to contribute to carbon neutrality by 2050, Seoul must act swiftly to implement effective waste management strategies, since it is crucial for minimizing waste-related emissions. The impact of GHG reduction on alternative scenarios varies depending on the approach taken. In 2040, the implementation of Scenario 1(S1), which introduces low levels of source reduction, recycling efforts, and waste diversion, leads to a 36% GHG reduction (−4,842 kt CO2 eq/yr) compared to BAU (−3,554 kt CO2 eq/yr). Scenario 2(S2) enhances these efforts to moderate levels, leading to a 59% GHG reduction (−5,649 kt CO2 eq/yr). Scenario 3(S3) delivers the highest GHG reduction potentials at 76% (−6,258 kt CO2 eq/yr) by implementing ambitious levels in MSW management. These findings clearly illustrate the higher GHG reduction achievable through progressive and continuous implementation of MSW management targets and policies adaptation.
Chen et al. [26] stated that different types of MSW can have varying environmental impacts depending on the disposal method. Fig. 6 shows how MSW will be treated, which helps us to understand why GHG saving would increase. Among all waste types, mixed paper contributed the most to GHG reduction potentials, followed by mixed recyclables and food waste. Similarly, a study in Surabaya City, Indonesia [27] emphasized the importance of securing paper recycling due to its high amount in the waste stream and its avoided emissions potential. Reducing the quantity of paper products by source reduction or recycling can lead to the rise of carbon storage from decreasing demand for virgin pulpwood. Additionally, since paper is partially biogenic when it is burned, it is considered as a part of the natural carbon cycle and does not count as GHG emissions [28]. Mixed recyclables contribute significantly to GHG reduction, mainly due to their treatment process through recycling. This activity further leads to GHG emissions offsets from virgin-source material production [4]. Food waste, on the other hand, results in substantial GHG reduction primarily from source reduction practices, as food production (upstream process) is a highly carbon-intensive process (e.g., land-use change, fertilizer application) [29]. This result also emphasizes the importance of material-specific strategies in MSW management, as GHG reduction potentials can vary widely on the type of waste and treatment methods. This is further supported by a recent study in the U.S [30], which found that diverting highly degradable organic waste (e.g., food waste, yard waste) away from landfills can dramatically reduce CH4 emissions.
Source reduction and recycling efforts effectively lower GHG emissions by tackling the GHG emissions from the origin. Source reduction eliminates GHG emissions throughout the entire life-cycle of the material, while recycling reduces upstream emissions by avoiding the need for extracting virgin material [4]. Under S3, source reduction efforts could save GHG emissions of approximately −2,220 kt CO2 eq/yr and −4,063 kt CO2 eq/yr from recycling practice. This result is also supported by the Engelmann et al. [11] study, which showed that higher recycling rates lead to better environmental performance. Additionally, Bian et al. [31] found that the emissions reduction will increase linearly with the improvements in waste recycling efficiency.
Energy recovery from incineration can also help reduce GHG emissions, but burning waste in incinerators, even in technologically advanced facilities, still release toxic emissions that harm the environment [32]. The GHG emissions from incineration in BAU was observed to be 261 kt CO2 eq/yr in 2040, with mixed plastics being the largest contributor. This is consistent with the prior studies showing that a greater proportion of plastics in the incineration waste stream increases GHG emissions due to its high fossil-based carbon content [33–34]. In accordance with Seoul Metropolitan Government, Seoul planned to expand incineration facilities to handle the rising amount of MSW, especially after the landfill-ban policy. However, incineration facilities have faced persistent opposition from local communities. Nevertheless, Seoul has great potential to reduce reliance on incineration by adopting mechanical pre-treatment facilities. Such facilities can mechanically sort mixed waste to recover materials suitable for energy recovery and recycling (e.g., solid refuse fuel or feedstocks for plastic pyrolysis), thereby diverting a substantial portion of waste from incineration. As a result of lowering the amount of waste to incineration, GHG emissions by 2040 for S1, S2, and S3 are projected to decrease to 176 kt CO2 eq/yr, 147 kt CO2 eq/yr, and 131 kt CO2 eq/yr, respectively.
Salemdeeb et al. [35] found that processing food waste into animal feed has a lower environmental impact than anaerobic digestion or composting, due to minimal energy input. Recently, food waste recycling through animal feed processing has shown a declining trend due to the outbreak of African Swine Fever [36–37]. In contrast, Moult et al. [38] highlighted that anaerobic digestion is gaining popularity over composting for treating food waste because of its dual benefits to producing methane-rich biogas and nutrient-rich digestate. Similarly, a recent study also found that the introduction of composting and anaerobic digestion provides substantial GHG reduction, emphasizing the shift to treat food waste from landfills [30;39]. Nationally, the Korea Resource Circulation Plans have established a strategic goal to expand biogas production through anaerobic digestion. This strategy aims to achieve not only significant GHG reduction but also enhanced resource recovery and circularity within the waste sector. Therefore, to reflect these trends, alternative scenarios were set to gradually increase the food waste recycling through anaerobic digestion. This shift will give an opportunity to gradually reducing reliance on traditional treatment methods, including wet and dry feed processing, composting, and incineration. However, the result did not show significant GHG reduction. GHG emissions from food waste recycling (anaerobic digestion) are projected to be reduced by −17 kt CO2 eq/yr for S1, −22 kt CO2 eq/yr for S2, and −30 kt CO2 eq/yr for S3.
Evaluating energy consumption associated with waste management practices is an essential issue to uncover opportunities in energy use. Energy recovery from incineration is the potential energy captured from the processing of burning waste [40]. The recovered energy from burning MSW through incineration can be used in different forms, including thermal energy (e.g., steam, hot water), or as electrical energy that is typically produced by directing steam to turbines [41]. This positive energy replacement effect could also contribute to the reduction in GHG emissions because it can replace other sources of energy (e.g., coal, natural gas, diesel) [42–44]. As illustrated in Fig. S1 and Table S6 (supplementary material), S1 would save energy around −60.1 of TBTU (trillion BTU), represents a 15% increase in energy saving compared to the BAU. S3 resulted in the highest energy savings of −69.3 TBTU (saving 33% energy). In terms of energy savings, mixed plastics is the main contributor to energy recovery, followed by mixed paper. This is because burning plastic waste, which has a high energy content and utilizing it for energy generation can help reduce the need for conventional fossil fuels [4]. Paper recycling also offers greater environmental benefits in comparison with recycling metals and glass [45]. On the other hand, increasing biogas production through anaerobic digestion would improve energy savings, as biodegradable materials in the absence of oxygen can produce biogas between 60–70% CH4 that is typically burned for electricity generation [4]. Composting can also help reduce GHG emissions but does not recover energy, whereas it requires energy [12]. Improving waste management efficiency, through source reduction, recycling, or energy recovery, can directly reduce GHG emissions by decreasing the demand for energy-intensive raw material extraction and production.
3.3. Comparison of GHG Reduction Potentials with Other Studies
The reduction of GHG emissions from MSW management is a crucial issue for cities to achieve a carbon neutral society. Different regions have adopted diverse waste management strategies, which are contributing to varying degrees of GHG mitigation. Comparative case studies provide valuable insights into how different cities and countries address GHG emissions from MSW, offering lessons that can be applied to enhance overall sustainability. The waste management practices and GHG reduction potentials in previous case studies are described in Table 2.
Despite being a densely populated city, Seoul has built a structured waste management system by implementing pay-as-you-throw scheme and high recycling rates, which makes BAU scenario a net-negative GHG emitter. Although the GHG saving was greater than the emissions generated, there is still a great opportunity to reduce GHG emissions even more. Seoul’s strategy based on this study (S3) results in an estimated GHG reduction of −6,258 kt CO2 eq/yr. Stronger efforts in source reduction and recycling, with less reliance on incineration, showed an optimal path for maximizing environmental benefits in terms of GHG emissions compared to other scenarios.
In comparison, Serbia relied heavily on landfilling, leading to GHG emissions of approximately 2,800 kt CO2 eq/yr [10]. Similarly, Hong Kong generated 4,062 kt/yr of MSW, the majority was managed through landfilling (68%), leading to estimated GHG emissions of 2,376 kt CO2 eq/yr [46]. These cases clearly illustrate the direct correlation between high landfill dependency can influence GHG emissions. A comparable case study in California by Vergara et al. [13] found that sending 28,800 kt of waste to landfill with low landfill gas (LFG) collection resulted in more than 10,000 kt CO2 eq/yr of GHG emissions. The LFG collection rate was responsible for the variation of life-cycle GHG emissions [47], as lower rates led to more methane escaping. On the other hand, when California implements a 40% source reduction effort by diverting 11,500 kt of waste from the disposal stream, the GHG saving will reach −30,000 kt CO2 eq/yr. Hosseini et al. [48] also suggested that an integrated waste management approach, including waste prevention, effectively mitigates GHG emissions. Castigliego et al. [14] reported that Boston’s zero-waste strategy, which aims for a future without burning or burying waste, would divert 90% of recyclable waste such as paper, glass, and organics. The remaining 10% of difficult-to-divert materials would be sent for combustion. This scenario led to over −1,300 kt CO2 eq/yr of avoided emissions. However, achieving a 90% diversion rate requires effective waste separation, which is the key challenge in ensuring high recycling practice. As a city with high-diversion efforts, Seoul has strong policy frameworks (e.g., pay-as-you-throw scheme, landfill-ban policy) and high public awareness. According to Lee-Geiller & Kütting [49], citizens in Seoul are legally obligated to contribute to waste governance by paying to dispose of their commercial, residential non-recyclable, and food waste. Therefore, Boston’s approach requires continuous policy refinement and high residents’ participation in waste practices. By examining multiple cases, this analysis aims to provide a clearer understanding of the most effective pathways to reduce GHG emissions from MSW.
4. Conclusions
This study analyzed the historical waste generation trends in Seoul from 2000 to 2022 and estimated GHG reduction potentials from MSW management between 2010 and 2022. In addition, projections for future waste generation and GHG reduction potentials were also made through 2040. The findings highlight the urgency of waste minimization and improvement in waste management in Seoul. Such approach is crucial as the upcoming landfill-ban policy will take effect in 2026, while waste generation may continue to increase.
The existing MSW management system already contributes to net-negative GHG emissions, primarily due to its strong recycling practices. However, further GHG reductions can be enhanced by following the national policy and strengthening waste minimization strategies. This study demonstrates that combining higher source reduction, recycling, and diversion of disposable mixed waste from incineration by expanding mechanical pre-treatment facilities yielded the highest potential for GHG reduction. Key areas that offer the highest GHG savings result from efficient management of mixed paper, mixed recyclable, and food waste. By 2040, S3 is predicted to reduce −6,258 kt CO2 eq/yr of GHG emissions or equal to 76% lower than BAU scenario. These findings emphasize the need to prioritize reduction at the source and sustainable management practices towards a carbon-neutral city. Delaying moving toward more effective waste management practices would result in higher cumulative GHG emissions by keeping reliance on incineration and also miss the opportunities for GHG and energy savings.
As Seoul moves toward 2050 carbon neutrality goals, this research serves as a foundation for future waste policy decisions and strategies and supports the refinement of waste reduction plans to mitigate GHG emissions from MSW management. Furthermore, the study underscores the necessity of integrating waste facilities and infrastructures such as mechanical pre-treatment facility for resource recovery to enhance efficiency and sustainability in waste management. These insights highlight the need to align waste management strategies with broader policy actions, ensuring that reduction efforts effectively contribute to Seoul’s long-term emission reduction targets.
While this study provides valuable insights into the GHG reduction potentials of alternative waste management scenarios in Seoul, several limitations and uncertainties should be acknowledged. First, it should be noted that WARM uses emissions factors and electricity grid mix factors based on the United States conditions. Thus, they may differ from the values and conditions in South Korea. To enhance the reliability of future analyses, it is recommended to develop country-specific emission factors that can reflect local conditions. Although this introduces uncertainty into the GHG reduction potentials, WARM still remains a valuable comparative tool for scenario analysis when localized data is limited. Second, the alternative scenario assumptions (e.g., source reduction rate, recycling rate) are based on policy target defined in the local and national plans. In practice, these rates are subject to uncertainty due to several factors, such as citizen participation, delays in infrastructure development, and a lack of enforcement mechanisms. Future studies could incorporate dynamic modeling approaches to better reflect behavioral uncertainties. Additionally, this study does not account for the economic feasibility associated with the alternative scenarios. In reality, even if the MSW management scenario provides significant environmental benefits, it may be financially impractical to implement. Therefore, the methodology could be expanded to include the cost-effective evaluation since it is crucial for actionable policy guidance.
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