Big disparities in CH₄ emission patterns from landfills between the United States and China and their behind driving forces

Highlights

• •The United States CH₄ emissions from municipal solid waste (MSW) landfills were 2.5 and 2.3 times more in total and per tonne, respectively than those in China in 2012.
• •The different historical annual CH₄ emissions from MSW landfill depended on their different stages of industrialization and waste management.
• •The secondary industry contributed more to CH₄ emissions from MSW landfills in China, while the tertiary industry in the United States.
• •The greater the tertiary industry GDP proportion, the lower the CH₄ emissions from landfills.
• •Science for society.
• •To clarify the difference of CH₄ emissions from landfill in the US and china based on the US-China joint glasgow Declaration, this study provides a deep insight into the CH₄ emissions from MSW landfills in the first two leading economies of the united states and China, and the corresponding emission patterns were compared from the country, province, city levels and the typical agglomeration scales, to identify the disparities of CH₄ emission with the socio-economic conditions, the relationship were established from the industry structure, the city scale and the locations, and regional socio-economic factors are discovered to estimate the local CH₄ emissions from MSW landfills through regression models. The strategies for emission reduction are put forward from the perspective of industry structure. The article provides a new insight and perspective for GHG emissions, the industry structure and the urbanization processes, as waste is the shadow of our human beings.

Abstract

Waste is the bridge linking resource consumption and greenhouse gas generation, and waste landfills are the main anthropogenic source of methane (CH₄). The United States (US)–China Joint Glasgow Declaration and the Global Methane Pledge are committed to reducing tractable CH₄ emissions; however, differences between the involved countries as well as their generation forecast processes have hampered cooperation. In this study, we provide a deep insight into CH₄ emissions from municipal solid waste (MSW) landfills and identify the disparities in CH₄ emissions with local socio-economic conditions. The US and China, the world's two largest economies, generated approximately 3.73 and 1.48 million tonnes of CH₄ from 1248 to 1955 landfills in 2012 using the FOD/bottom-up method, with corresponding 26.93 and 11.94 kg per tonne waste and emission value from each landfill ranging between 10⁰ and 10⁵ and 10⁻⁵–10⁵ tonnes. The spatial distribution was also quantified and compared with national, state/province, and urban agglomeration perspectives based on historical MSW variations (1990–2015) to clarify the triangular relationship between the economic situation, waste properties, and landfill CH₄ emissions. High-density CH₄ emission regions spatially overlapped with highly developed urban agglomerations, positively correlated with the local gross domestic product (GDP) and population (p < 0.01), with more emissions generated per thousand US dollars in the US (0.25 tonnes) than in China (0.16 tonnes). The US tertiary industry and China's secondary industry contributed to high CH₄ emissions from the waste sector. The increase in tertiary industry might reduce the waste sector's CH₄ emissions. This study will help to understand this new triangular relationship and predict future patterns of CH₄ emissions.

Graphical abstract

1. Introduction

Elevated greenhouse gas (GHG) emissions continue to alter global temperature patterns and pose an increasing threat to human health, the environment, and economies [1], [2], [3], [4]. Generated 27.8 times more than CO₂ over the last 100 years, methane (CH₄) is one of the most important heterogeneous contributors, mostly as a result of the biochemical degradation of the organic fraction of municipal solid waste (MSW) emitted from landfills [5,6]. To reduce CH₄ emissions, the United States (US)–China Joint Glasgow Declaration and the Global Methane Pledge have been signed to recall their firm commitment to work together to strengthen the implementation of the Paris Agreement [7]. However, a detailed understanding of the patterns of CH₄ generation remains unclear, which hampers the implementation of the principle of ‘common but differentiated responsibility’ and the Intended Nationally Determined Contributions.

The US and China, the world's two largest economies, are the largest GHG emitters and generate the most waste [2,7]. The US is the largest developed country, and China is the largest developing country; nonetheless, their MSW generation trends and disposal patterns are similar. The annual growth rate of MSW in the US and China was 0.4%–3.7% and 0.5%–8.8% during 1990–2015, respectively, and most of these waste were landfilled, with corresponding ratios of 52.5%–68.7% and 60.3%–88.1% [8], [9], [10]. In 2012, the number of landfill sites in the US and China reached 1248 and 1955, with disposal capacities of 138.1 and 124.0 million tonnes (Mt), respectively [5,11].

GHG emissions from MSW greatly rely on waste management systems, local economic situations, urbanization rates, and living habits [3,12,13]. Residents in the US and China have different dietary habits. For instance, the amount of GHG emissions from MSW varies according to the composition of food scraps in the municipal waste stream [8,14,15]. In the US and China, urban agglomerations and metropolitan regions are core areas of high levels of urbanization and intensive economic activity. Urban agglomerations are a typical feature in socio-economic research and a well-known source of GHG emissions [16], [17], [18], [19]. Managing their waste is of particular importance, with MSW being the dominant contributors to anthropogenic CH₄ emissions, especially from landfills [6,10,20]. Local industrial conditions also influence the population, consumption habits, and economic situation, which may result in a change in the waste sector [21]. The transformation of an industry's structure from being a heavy industry to a service and technological industry can lead to a substantial reduction in GHG emissions [22,23], particularly in highly developed regions [24]. CH₄ emissions from the waste sector may be related to local socio-economic conditions.

China intends to develop a comprehensive and ambitious National Action Plan on methane, and the US has announced the US Methane Emissions Reduction Action Plan [4,7]. Both of them intend to convene a meeting, which focuses on the specifics of enhancing measurements and CH₄ mitigation through the standards development to reduce CH₄ from waste sectors [7]. Therefore, it's necessary to identify differences in CH₄ emission patterns from MSW landfills. With 2012 as the baseline year, the detailed CH₄ emissions were evaluated and compared based on their spatial distribution at the national, regional, state, and provincial scales in these two countries during different development stages. Socio-economic factors, including the gross domestic product (GDP), population, industrial structure, urbanization rate, and income, were considered and correlated with CH₄ emissions in different urban agglomerations and metropolitan regions. Finally, a policy recommendation was proposed to identify feasible measures for CH₄ emission reduction.

2. Methods

2.1. Emission calculation

Landfill CH₄ emissions were estimated using the first order decay (FOD) model as follows:

$$\begin{array}{ccc}& & C{H}_4=M·\sum _{i=1}^{4}MCF·f_i·DO{C}_i·DO{C}_F·(e^{-(T-1)·k_i}-e^{-T·k_i})\hfill \\ & & ·F·16/12·(1-R)·(1-OX)\hfill \end{array}$$ (1)

where CH₄ represents the CH₄ emissions during T years of operation, M is the mass of MSW landfilled when the site starts to operate, MCF is the correction factor, f_i is the proportion of waste component i (i includes food waste, paper, fabric, wood, etc.), DOC_i is the proportion of degradable organic carbon in waste component i, DOC_F is the proportion of the decomposed fraction in degradable organic carbon, T is the time length from the start-up operation to the studied year, k_i is the reaction constant, F is the volume ratio of CH₄ to the total landfill gas, R is the CH₄ recovery rate, and OX is the oxidation factor.

For China, the parameters of the FOD model were obtained from field investigations and laboratory analysis as described in our previous studies [11,[25], [26], [27]] (section S3). We developed an emission factor matrix from three dimensions: the geographic region, disposal time, and site scale (Tables S1–S3). For the US, the default emission factors recommended by the Intergovernmental Panel on Climate Change were mainly used during the calculation process [6,28,29]. The results were verified using data published by the Greenhouse Gas Reporting Program through the Facility Level Information on GHGs Tool. The schematic diagram of the calculation and analysis process is presented in Fig. S1.

2.2. Data sources

The baseline year was set at 2012, allowing for the inflection time of the Environmental Kuznets Curve (EKC) for landfill CH₄ emissions in China and accessibility of related data (Fig. S2). In total, 1248 landfills sites in all states and territories of the US, including Puerto Rico, and 1955 sites in all provinces of Chinese mainland were investigated in this study. For China, information on landfill sites was obtained from an investigation conducted by the Ministry of Ecology and Environment of China and local environmental bureaus. For the US, site-specific data were gained from the Landfill Methane Outreach Program published on the website of the US Environmental Protection Agency (https://www.epa.gov/lmop). The detailed site information contained the geographical coordinates (longitude and latitude), physical address, annual and historical amounts of landfilled MSW, and management levels.

The data on socio-economic factors in the US and China, including the population (POP), urbanization rates (URB), GDP, and per-capita income (INC), as well as the ratio of the primary industry (PIR), secondary industry (SIR) and tertiary industry (TIR) to the GDP of an individual state or province, were obtained from the US Census Bureau (https://www.census.gov), US Bureau of Economic Analysis (https://www.bea.gov/), and National Bureau of Statistics of China (http://www.stats.gov.cn/english/). Detailed definitions and information are presented in Table S4.

2.3. Data analysis

The spatial distribution of CH₄ emissions was obtained through a kernel density estimation using ArcGIS 10.3 (Esri, US). The kernel density was classified into nine levels using the natural breaks (Jenks) method [17,20,26].

A stepwise multiple linear regression was utilized to develop the double logarithmic regression model for CH₄ emissions and socio-economic factors as follows:

$$InC{H}_4=a_0+a_1\ln x_1+a_2\ln x_2+\dots +a_i\ln x_i+\epsilon$$ (2)

where CH₄ represents landfill CH₄ emissions, x_i is the value of socio-economic factors (i includes GDP, URB, INC, etc.), a₀ is a constant, a_i is the corresponding coefficient of x_i, and ε is the random error. Three independent variables were selected based on the F-test, P-value, and variance inflation factor to check for significance and collinearity.

The Pearson regression model at 95% confidence level was used to analyze the correlation between CH₄ emissions and socio-economic factors. The Pearson correlation analysis and stepwise multiple linear regression were conducted using SPSS 24.0 (IBM, US).

3. Results

3.1. CH₄ emission patterns from landfills in the US and China

3.1.1. Comparative analysis across national and regional levels

The historical CH₄ emission patterns from landfills in the US and China showed opposite trends from 1990 to 2015 (Fig. S2). China witnessed two different CH₄ emission growth periods: a rapidly increasing stage (1990–2000) and a steady growth stage (2001–2015), with annual growth rates of 10.4%–33.0% and 3.1%–6.9%, respectively. In contrast, the US maintained a stable decrease during the same period, with an annual decreasing rate of 0.4%–6.0%. The landfilled MSW and the corresponding CH₄ emissions were much higher in the US than in China. The CH₄ emissions from landfills in the US and China reached 3.73 and 1.48 Mt in 2012, with the amounts of landfilled MSW reaching 138.1 and 124.0 Mt, respectively.

From a socio-economic perspective, the US (Rocky Mountains, New England, Plains, Southeast, Great Lakes, Mideast, far west, and Southwest) (Table S5) and China (Northwestern, Northeastern, Southwestern, Middle Yellow River, Middle Yangtze River, southern coastal region, northern coastal region, and eastern coastal region) (Table S6) were separated into eight regions [10,25,30], with CH₄ emissions fluctuating from 10° to 10⁵ tonnes and 10⁻⁵–10⁵ tonnes, respectively (Fig. 1). The median value of CH₄ emissions from individual regions increased with the rise of GDP in both countries, and the US had a relative higher level. The high-density CH₄ emission regions spatially overlapped with the developed urban agglomerations (Fig. 2), which were characterized by high industrial outputs, urbanization rates, and populations [23,31,32]. CH₄ emissions of the US were mainly concentrated in four regions (kernel density > 13,300) (Fig. 1): the Southeast, Great Lakes, far west, and Southwest, which corresponded to the urban agglomerations of Atlanta-Georgia, Chicago-Illinois & Columbus-Ohio, San Francisco & Los Angeles-California, and Houston-Texas. In China, the three regions with the highest CH₄ emissions were the southern, northern, and eastern coastal areas (kernel density > 12,600), corresponding to the urban agglomerations of the Pearl River Delta, Beijing-Tianjin-Hebei, and Yangtze River Delta.

Fig. 1.

Frequency distributions of landfill CH₄emissions for eight economic regions in the US and China. The left and right side of violin plots represent the emission distributions of the US and China, respectively; The box plots inside violins show the first, second (median value), and third quartiles of the emissions; The numbers of landfill sites in each region are shown in parentheses; the regions are listed in a GDP ascending order from left to right; RM: Rocky Mountain, NE: New England, PL: Plains, SE: Southeast, GL: Great Lakes, ME: Mideast, FW: Far West, SW: Southwest; NW: Northwestern, NEC: Northeastern, SWC: Southwestern, MY: Middle Yellow River, MYR: Middle Yangtze River, SC: Southern coastal, NC: Northern coastal, EC: Eastern coastal.

Fig. 2.

Geographic spatial patterns of landfill CH₄ emissions in the US (a) and China (b) based on kernel density estimation.

3.1.2. Disparities in unit CH₄ emissions

The CH₄ emissions generated per tonne of landfilled MSW in the US and China were approximately 26.93 and 11.94 kg, respectively (Fig. 3). The urban per-capita CH₄ emissions in the US and China were 14.55 and 2.07 kg/capita, respectively. The urban per-capita CH₄ emissions in all regions of the US (from 7.88 kg/capita in Mideast to 26.40 kg/capita in Southeast) were much larger than those in China (from 1.71 kg/capita in southern coastal region to 3.59 kg/capita in Northwestern). The CH₄ emissions generated per unit of the GDP (i.e., 1000 US dollar (USD)) in the US and China were 0.25 and 0.16 kg/(1000 USD), respectively. In China, the per-GDP CH₄ emissions were generally higher in regions with lower GDP [24,25]. The highest per-GDP CH₄ emissions of the US were 0.42 kg/(1000 USD) in the Southeast, while those of China were 0.31 kg/(1000 USD) in the Northwestern.

Fig. 3.

The total emissions (tonnes) (a), per-GDP emissions (kg/1000 USD) (b), and urban per-capita emissions (kg/capita) (c) of landfill CH₄ for eight economic regions in US and China. SW: Southwest, FW: Far West, ME: Mideast, GL: Great Lakes, SE: Southeast, PL: Plains, NE: New England, RM: Rocky Mountain; EC: Eastern coastal, NC: Northern coastal, SC: Southern coastal, MYR: Middle Yangtze River, MY: Middle Yellow River, SWC: Southwestern, NEC: Northeastern, NW: Northwestern.

Approximately 40.6% of landfill CH₄ emissions in China were attributed to urban agglomerations, especially in Yangtze River Delta (18.4%). Urban agglomerations in the US accounted for 32.0% of the total landfill CH₄ emissions, of which 18.0% were from Chi-Pitts. Urban agglomerations, as the economic growth poles, accounted for 36.6% and 30.3% of the GDP in the US and China, respectively [33]. The industrial development levels of urban agglomerations in the US differed remarkably from those in China (Fig. S5). The tertiary industry was dominant in all urban agglomerations in the US with the TIR value more than 76.6%, while the secondary industry was the main contributor to the GDP in China, except for in Beijing (22.7%) and Shanghai (38.9%), with the SIR ranging from 48.5% to 51.7%. The tertiary industry in the US urban agglomerations contributed more to CH₄ emissions, and that for China was the secondary industry.

3.2. Co-relationship between CH₄ emission patterns and local socio-economic conditions

Landfill CH₄ emissions were significantly correlated with GDP (R² = 0.814, 0.811; p < 0.01), POP (R² = 0.882, 0.687; p < 0.01), and URB (R² = 0.304, 0.380; p < 0.05) in the US and China, respectively (Fig. 4). It might suggest that the increase in landfill CH₄ emissions was mainly caused by an increase in the local GDP and population [25,30]. There were also significant correlations amongst the GDP, POP, and URB. Moreover, URB was positively correlated with the TIR (R² = 0.441, 0.532; p < 0.01) and INC (R² = 0.332, 0.747; p < 0.01) in the US and China, respectively.

Fig. 4.

Correlation relationship between landfill CH₄ emissions and socio-economic factors in US (n = 51) and China (n = 31).

Based on the stepwise multiple linear regression, three independent variables, namely the GDP, TIR, and INC, were selected to develop a double logarithmic regression model with landfill CH₄ emissions for the US (R² = 0.834) and China (R² = 0.829) (Fig. 5). The coefficients of each variable in the US were larger than those in China, suggesting greater elasticity of regional CH₄ emissions and greater sensitivity to changes in socio-economic factors in the US [19,26,30]. The standardized coefficients indicated that an increase in the GDP would enhance landfill CH₄ emissions and exert a greater influence than the INC and TIR. The INC for the US and China had opposite effects on landfill CH₄ emissions, the higher INC resulted in the more emissions in China but fewer ones in the US [34,35]. An increase in the TIR would reduce landfill CH₄ emissions in both countries. Tertiary industry accounted for 67.8% and 44.6% of the GDP in the US and China in 2012, respectively (Fig. S5). It has been reported that the service industry in China, particularly the emerging service industry, will enter a booming stage and the TIR could reach 60.4% in 2030 [23,36,37]. The tertiary industry in the US was projected to grow at an annual rate of 2.4% from 2020 to 2030, and the TIR would reach 74.8% [38]. Owing to an improvement in the tertiary industry, especially in the emerging service sector, the US and China could reduce CH₄ emissions from MSW landfills by 18.6% and 16.9% in 2030, respectively.

Fig. 5.

The practical (CH₄-pra) and predicted (CH₄-pre) CH₄ emissions based on double logarithmic regression model in US and China.

4. Discussion

As the two largest economies undergoing different development stages, the US and China are good references for identifying the triangular relationship between waste generation, landfill CH₄ emissions, and local socio-economic conditions. The GDP per capita and landfill CH₄ emissions in the US and China had inverted U-shape EKCs (section S2). The US had entered the downward stage for a long time, that an increase in the GDP would not induce the rise of CH₄ emissions [5,30], while China was in the upward trend of EKC and the turning point appeared around 2013. Successive CH₄ emission control and reductions in the future may be useful for waste management in China [7].

Large disparities were observed in CH₄ emissions from landfills between the US and China. Landfills in the US emitted 2.5 times more CH₄ than those in China and 2.3 times more CH₄ per tonne of landfilled waste in 2012, owing to the differences in waste composition, landfill operation processes, and CH₄ control measures [13,21,26]. The higher waste generation rate in the US (2.02 kg/(capita·day)) resulted in the quantity of their landfilled MSW being larger than that in China (0.66 kg/(capita·day)) [27,39], despite the US population (313.83 million) being smaller than China (1354.04 million) [8,10]. The MSW in China presented with a higher content of organic fractions than that in the US, while the corresponding collection and disposal rate resulted in the lack of a regular management system [11,15,29]. For instance, parts of food scraps are privately collected and are served as feedstock by some local poultry breeders [15,26,27]. Source separation campaigns were preliminarily implemented in some developed cities in China, such as in Shanghai, Beijing, Shenzhen, and Hangzhou [40]. Approximately 67.4% of landfill sites in the US have an annual capacity greater than 100,000 tonnes, while 62.1% of those in China are medium scale, with an annual capacity ranging from 10,000 to 100,000 tonnes (Fig. S3). According to our previous field investigations [11,[26], [27], [28]], large-scale landfills are more likely to implement efficient measures (e.g., gas collection and power generation, gas purification for further utilization, and refinement landfill processes) to reduce CH₄ emissions and have a relatively high recovery rate, while those with medium scales generally adopted cost-saving but low-efficiency methods, e.g., functional soil cover and gas collection and flaring. However, the reaction constants (k) for landfills with leachate recirculation in some US states were elevated due to favourable humidity levels, and more CH₄ emissions were generated during the initial landfilling stage [28,41,42]. CH₄ emissions based on the FOD model were estimated according to the length of time that the landfill had been in operation at the time of the study [11]. The historical amounts of landfilled MSW in the US was approximately twice of China during 1990–2015 [10,39].

Differences in the GDP, urbanization rate, and industry structure were probable reasons for the gap in unit CH₄ emissions between the US and China [22,23,30]. The urban per-capita and per-GDP CH₄ emissions in the US were 7.0 and 1.6 times higher than those in China, respectively. To certain degree, the amount of landfilled MSW could reflect the regional economic situations and consumption levels [27,43]. Urbanization in China was relatively backward, with most provinces (77.4%) being a low ratio of urban population (< 60.2%), while population urbanization in 79.6% of the US states exceeded 60.4% [31,39]. The urban population in China was approximately 2.8 times larger than that in the US in 2012, and their per-capita income was 5.9 times lower than that of the US, resulting in less consumption and waste generation [44], [45], [46]. Higher levels of the GDP, associated with economic production and consumption activities, generally would produce more waste [19,21,30]. Meanwhile, there would be more investment in urban environmental infrastructure attributed to a higher GDP, which might lead to an improved waste collection efficiency, recycling rate, and landfill management [13,15,22]. In 2012, the GDP of the US was 1.7 times higher than that of China, with 1.5 times more MSW generation in the US (Fig. S5). The waste recycling rates in the US and China were found to be 26.0% and 15.6%, respectively [8,15]. Moreover, the industrial structure would greatly influence CH₄ emissions due to the different waste properties. China was about to transition from the secondary to tertiary industries, while the US had prevailed in the service industry for decades [23,37]. The ratio of the tertiary industry to the GDP in the US was much larger than that in China, but both countries were characterized by a high proportion of emerging service industries [35,36,38]. The finance, insurance and real estate (29.8%), professional and business services (18.2%), and educational services and health care (12.6%) were the major contributors to the tertiary industry in the US, while the finance and real estate (23.1%), wholesale and retail trade (23.0%), and transportation and warehousing (11.6%) were the main contributor to the service industry in China [10,38]. Waste generation from these emerging service industries was relatively small and most of the waste (e.g., packaging and office paper) was recyclable, resulting in fewer organic fractions and landfilled MSW [42,46]. Enhancing the development of the tertiary industry, especially the emerging service sectors, might be an effective strategy for controlling and reducing CH₄ emissions [23,37,47,48]. In addition, MSW management strategies and landfill policy innovation schemes at the city, regional, and national scales need to be reviewed by incorporating new and efficient technologies that help reduce CH₄ emissions in the short- and long-term.

5. Conclusion

Landfills are one of the main contributors to CH₄ emissions in both the US and China, and landfills in the US emitted 2.5 times more CH₄ than those in China. Moreover, owing to differences in waste composition, landfill operation processes, and the CH₄ control measures, 2.3 times more CH₄ per tonne of landfilled waste were generated in the US in 2012 compared to in China. High-density centers of CH₄ emissions in the US and China were distributed in economically developed districts, such as urban agglomerations and metropolitan areas. The GDP, per-capita income, and proportion of the tertiary industry correlated with landfill CH₄ emissions. The results of double logarithmic regression models predicted that, by promoting the tertiary industry, especially the emerging service sector, the US and China could reduce CH₄ emissions from MSW landfills by 18.6% and 16.9% in 2030, respectively.

Declaration of competing interest

The authors declare that they have no conflicts of interest in this work.

Biographies

Yijun Liu is a master graduated from School of Environmental Science and Engineering, Shanghai Jiao Tong University. Her research focuses on greenhouse gas emission accounting and reduction from waste sectors.

Ziyang Lou(BRID: 09661.00.20328) is currently a professor of School of Environmental Science and Engineering, Shanghai Jiao Tong University; Young Yangtze River winner of the Ministry of Education, professor of China Institute for Urban Governance. He focuses on carbon emissions accounting and reduction from waste sectors, waste management and secondly pollution control system development. He served as the deputy director of Shanghai Engineering Research center of Solid Waste Treatment and Recycling; Council member of Chinese Society of Environmental Sciences and solid waste branch of China Silicate Society. He has published more than 100 papers.