1. Introduction
Cities are responsible for approximately 70% of all anthropogenic greenhouse gas (GHG) emissions and about 60% of all anthropogenic methane (CH
4) emissions [
1], [
2]. Solid waste disposal sites (including landfills and dumpsites), which are prevalent in global cities, emit CH
4 generated from the anaerobic biodegradation of municipal solid waste (MSW). Notably, the proportions of CH
4 emissions from disposal sites surpass 50% of the total CH
4 emissions in some megalopolises [
3]. CH
4 has a high global warming potential (GWP), being 28 times stronger than carbon dioxide (CO
2) over a 100-year period and 80 times stronger over a 20-year period [
4]. Understanding and mitigating CH
4 emissions from solid waste disposal sites is particularly pertinent and pressing, considering that the latest Synthesis Report from the Intergovernmental Panel on Climate Change (IPCC) emphasizes that the current pace of mitigation and adaptation policies and measures falls short of restraining global temperature rise to under 1.5 °C within the 21st century [
4]. More than 150 countries signed the Global Methane Pledge at the United Nations Climate Change Conference in Glasgow (COP26), which aims to reduce global annual CH
4 emissions by 30% by 2030, compared with emissions in 2020 [
5].
Landfilling will remain as the major waste treatment method in the near future, considering the slow pace of implementing source segregation of biowaste for separated treatments in anaerobic digesters or composting facilities. There are 300 000-500 000 operating and closed disposal sites around the world, which receive about 1.5 × 10
9 t of MSW annually and store approximately 1.0 × 10
11 t of MSW cumulatively [
6]. Global disposal sites have emitted an annual 30-50 Tg of CH
4 in recent years [
2]. Future emissions from disposal sites could exhibit continuous upward trajectories due to the relentless surge in waste generation that is concomitant with rapid urbanization, population growth, and economic development [
6]. In this comment, we highlight the two most critical problems related to such emissions. Firstly, the emission quantities are highly uncertain and are often underestimated, which will result in misguided mitigation strategies. Secondly, these emissions receive disproportionately low attention and priority in the various GHG mitigation plans as compared with other sources that have similar levels of emissions and even higher mitigation costs [
7]—most notably, fossil fuel production, intensive livestock farming, and transportation [
8].
2. Underestimated CH4 emissions from disposal sites
Establishing accurate site- and city-level inventories of CH
4 emissions from disposal sites is the prerequisite for understanding and mitigating such CH
4 emissions. However, bottom-up field measurements using current ground-based techniques, including flux chamber measurements [
9] and mobile analytical platforms [
10], [
11], require certain costs and labor. The mainstream estimation method adopted by the IPCC utilizes a first-order decay (FOD) model, which has been widely used in the 196 countries and regions under the United Nations Framework Convention on Climate Change (UNFCCC). The FOD model incorporates two critical parameters: waste decay rate (
k, time
−1) and methane generation potential (
L0, volume CH
4 per mass MSW). Waste managers are recommended to choose from a list of default emission parameters based on site conditions to calculate the corresponding annual emissions.
Inventory emission estimations based on FOD model are widely applicable and user-friendly but often have limited accuracy [
12]. In its model document, the IPCC acknowledges that the uncertainty range of the default
k values is between −40% and +300% [
13] and recommends that local emission adjustments be developed. Even using the same inventory estimation method, the annual national emissions in the United States estimated by two research institutes are approximately 150% different [
14]. Similarly, several research institutes reported the annual national emissions in China in the 2000s and 2010s, among which the highest and lowest estimations differ by 170% [
15], [
16], [
17], [
18], [
19], [
20]. Without exhaustively enumerating the disparities in the inventoried emission estimations in different countries, it suffices to conclude that the IPCC’s current model compromises accuracy in favor of applicability.
In fact, the conventional inventory bottom-up methodology is being challenged by a top-down methodology that uses atmospheric inversion modeling. This new methodology utilizes high-resolution satellites [
21] and drones [
22] to measure atmospheric CH
4 concentrations and back-calculate site-specific CH
4 emissions, offering potentially more accurate and timely emission estimations than the inventory method [
23]. The reported atmospheric inversion results have revealed consistent underestimations of the current inventory values (30 out of 31 sites in four countries), ranging from +4% to +737% (
Fig. 1(a)) [
3], [
24], [
25], [
26]. Extrapolating the 10th (+17%) and 90th percentile (+377%) values of the percentage of underestimation in
Fig. 1(a) to global disposal sites shows that the underestimated emissions reach 10-150 Tg∙a
−1 globally.
The top five emitters are responsible for about 50% of all underestimated emissions (
Fig. 1(b) [
27]). As the two leading countries in MSW generation, the United States and China are also the largest two emitters of landfill CH
4. Landfills in the United States are commonly well managed, with a comprehensive reporting scheme known as the Landfill Methane Outreach Program (LMOP) for site-scale emissions. The Environmental Protection Agency (EPA) is making ongoing progress as well in upgrading estimation methods and incorporating remote sensing [
28]. In November 2023, the Ministry of Ecology and Environment of the People's Republic of China also proposed systematic enhancements in monitoring and managing landfill CH
4 emissions for the first time [
29]. It should be noted that India-despite having the largest population in the world—is not among the top five emitters in either the EDGAR database (where India ranks 7th) [
27] or the EPA Non-CO
2 GHG database (where it ranks 9th) [
30]. This may seem counterintuitive, considering India’s amount of waste generation, which is second only to those of the United States and China. A potential reason is the high proportion of food waste in India’s MSW, as such waste has a low degradable organic carbon content (DOC
food waste = 0.15). In addition, much of India’s waste is not centrally disposed of in landfills, so it is not included in landfill emission estimates. However, additional
in situ measurement data and rigorous assessments are needed in the future to validate India’s current emission estimates.
The substantial disparities between bottom-up and top-down emission estimations necessitate a reevaluation—and possibly a revamp—of the IPCC’s model [
31]. Firstly, the IPCC’s model needs refinement, and its recommended
k and
L0 values need localization. Both the
k and
L0 values have been demonstrated to vary widely based on site-specific temperature, precipitation, waste composition, infrastructure, operations, and waste management policies. These variations can exceed the ranges suggested by the IPCC [
32]. One exemplar attempt at a refined model is the establishment of the California Landfill Methane Inventory Model (CALMIM) in the United States [
12]. Secondly, utilization of the IPCC’s model is often hampered by inaccurate, incomplete, or outdated site-specific information. Thus, it is recommended to establish city- and country-level databases, such as the LMOP database, to record the time-dependent environmental and operational conditions of each site. Thirdly, given current technological availability and economic constraints, it is only viable to integrate the bottom-up and top-down approaches, rather than focusing exclusively on a singular approach. The bottom−up methodology can be improved according to top-down results [
33] and vice versa. While the emerging satellite-based technique offers an efficient way to identify emission profiles for so-called “super-emitters” [
34], mainstream ground-based measurements [
35], [
36] should also be enhanced in order to solve the whole puzzle. Future work should thus focus on streamlining and optimizing a merged and coordinated estimation methodology.
3. Implications for mitigation strategies
Underestimated CH
4 emissions have profound implications for policymakers in devising cost-effective strategies to mitigate GHG emissions. In the context of developed countries (mostly Organisation for Economic Co-operation and Development (OECD) countries), it has been conventionally posited that the amount of technically feasible CH
4 mitigation at disposal sites, at an abatement cost lower than approximately 100 USD∙t
−1 CO
2 equivalent (CO
2-eq, the amount of CO
2 equivalent to CH
4 emissions based on GWP) is limited [
37]. This is largely due to the assumed well-engineered and well-managed nature of the disposal sites in these countries, with about 50% of the CH
4 emissions being deemed as “residual emissions,” which are very difficult to eliminate. However, once underestimated CH
4 emissions—which may not be “residual”—are taken into account, the potential for emission mitigation at these sites becomes notably higher. For example, landfills in a country that were previously estimated to emit 4 Gg of CH
4 per year with 50% residual emissions may actually emit 6 Gg, as revealed by more accurate measurements. If the additional 2 Gg of emissions could be mitigated at low or net-zero abatement costs via measures such as flexible gas-collection plans, improved gas-collection systems, effective emission monitoring, and routine infrastructure maintenance, the technically feasible abatement potential would increase from 50% to 67% of the total emissions. Under this likely scenario, the marginal abatement cost curve would be altered (
Fig. 1(c)), suggesting that increased CH
4 mitigation can be achieved without incurring additional marginal costs.
The disposal infrastructures in developing countries are generally poorly managed, resulting in higher mitigation potential-that is, less current mitigation—in comparison with those in developed countries (
Fig. 1(c)). Presuming that the efficacies of mitigation measures remain unchanged, then adding the underestimated CH
4 emissions to regions with developing countries would shift their marginal abatement cost curves to the right. These shifted curves demonstrate that more CH
4 emissions can be mitigated than currently expected at the same cost. As a result, developing countries with various economic and technical constraints would be justified in elevating the priority of mitigating emissions from disposal sites as a cost-effective strategy to bridge the gap between current emissions and future mitigation targets. The anticipated payback period of a landfill CH
4-recovery project is on the order of a few years to a decade [
38], [
39]. Developing countries also need to consider the entire life cycle of carbon emissions. Beyond improvements at the site scale, enhancements in local waste management systems, such as the introduction of waste sorting and “waste-free city” campaigns, can effectively reduce GHG emissions from disposal sites [
40].
Technical feasibility and economic capability should be further considered when determining site-specific CH
4 mitigation measures at disposal sites with different scales and local conditions. It is necessary to derive more accurate marginal abatement cost curves based on the achievable cumulative CH
4 emission mitigation over the whole lifespans of disposal sites. Information concerning CH
4 mitigation technologies for future use should also be identified and analyzed [
41]. The assessment process consists of the following steps: ① calculate the total costs and abatement potential for candidate technologies; ② identify possible combinations and incompatibilities; ③ manipulate and standardize data; and ④ derive the corresponding abatement cost curves.
It is vital to realize that disposal sites are huge carbon reservoirs, while CH
4 emissions constitute only one carbon transformation and transport pathway. The embodied carbon in disposed MSW also exists in solid (waste) and liquid (leachate) phases, which are prone to different transformation and transport pathways in the long term [
42]. For example, solid waste undergoes offsite transport (via wind, surface water, waste landslide, scavenging, and waste mining) and transformation (through burning, leaching, erosion, and biodegradation), while leachate undergoes physico-, chemo-, and bio-degradation, leakage, and pump-and-treat processing [
43]. It is necessary to expand the boundary of interest from the waste-atmosphere interface to the whole site and its vicinity. The notion of calculating carbon emissions from disposal sites should gradually evolve into understanding and managing site-specific carbon budgets.
4. Conclusions
In this paper, we firstly call for an urgent reevaluation and enhancement of current landfill CH4 emission quantification methodologies. We propose the incorporation of updated emission parameters and landfill information into the widely used IPCC FOD model. Satellite-based and local ground-based measurements are urgently needed to improve inaccurate estimations. Secondly, we underscore that incorporating the underestimated CH4 into estimations will shift current GHG marginal abatement costs to reveal new mitigation opportunities. In developed countries, improved landfill management could significantly curtail CH4 leakages at low cost. In developing countries, prioritizing the reduction of landfill CH4 emissions could effectively contribute to meeting future mitigation targets due to the high mitigation potential and economies of scale. It is essential for policymakers and administrators to adopt local- and region-specific measures to mitigate waste-related GHG emissions.
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