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China built the longest high-speed rail (HSR) network by consuming massive construction materials. However, the characterization of HSR material metabolism and associated emissions at national level remains less explored. Here, we revealed the material metabolism and life-cycle GHG emissions from 2008 to 2035 in China’s HSR network. We show that material stocks temporally amount from 0.6 gigatons in 2010 to 3.7 gigatons in 2020, dominated by aggregate and cement rather than steel. Spatially, material distributions across provinces are becoming balanced. Growing HSR speed increased the construction material demands and flows, but technological advances significantly offset these increasing consumptions, especially for developing bridge-girder spans and ballasted tracks. Our results demonstrate the life cycle emissions of HSR, with 1,008 megatons of carbon replacement value and annually 31 megatons of operational GHG emissions in 2020, and highlight the environmental benefits of HSR to inform green transportation transitions.

The huge energy consumption of metro operations has become a significant challenge faced by the urban public transportation sector to achieve low-carbon development. Using Shenzhen as an example, this study has made efforts to quantify the metro’s energy consumption and carbon emission intensity during the operation phase by using the Life Cycle Assessment approach. Furthermore, this study evaluates the actions that can be taken to reduce energy consumption and emissions. A comparative analysis between metros and other public transportation modes has also been conducted. The results show that the annual carbon emissions from the metro’s operation phase in Shenzhen city increased from 63,000 t CO2e in 2005 to 1.3 Mt CO2e in 2021, and the historically accumulated carbon emissions are 9.5 Mt CO2e. The unit operating mileage, the unit station area, and the per capita carbon emission intensity were 2.1 kg CO2e/km, 132.5 kg CO2e/m2, and 0.6 kg CO2e per capita (13th Five-Year Plan Period), respectively. By continually promoting the low-carbon operation of the subway, the cumulative carbon savings could reach 0.1 Mt CO2e (2022–2035).

Improving our comprehension of the weight and spatial distribution of urban built environment stocks is essential for informing urban resource, waste, and environmental management, but this is often hampered by inaccuracy and inconsistency of the typology and material composition data of buildings and infrastructure. Here, we have integrated big data mining and analytics techniques and compiled a local material composition database to address these gaps, for a detailed characterization of the quantity, quality, and spatial distribution (in 500 m × 500 m grids) of the urban built environment stocks in Beijing in 2018. We found that 3621 megatons (140 ton/cap) of construction materials were accumulated in Beijing's buildings and infrastructure, equaling to 1141 Mt of embodied greenhouse gas emissions. Buildings contribute the most (63% of total, roughly half in residential and half in nonresidential) to the total stock and the subsurface stocks account for almost half. Spatially, the belts between 3 and 7 km from city center (approximately 5 t/m2) and commercial grids (approximately 8 t/m2) became the densest. Correlation analyses between material stocks and socioeconomic factors at a high resolution reveal an inverse relationship between building and road stock densities and suggest that Beijing is sacrificing skylines for space in urban expansion. Our results demonstrate that harnessing emerging big data and analytics (e.g., point of interest data and web crawling) could help realize more spatially refined characterization of built environment stocks and highlight the role of such information and urban planning in urban resource, waste, and environmental strategies.

With rapid urban expansion in China, subway construction activities have been widely promoted. Accordingly, a large amount of subway-related excavated soil and rock (ESR) are generated and disposed mainly through dumping sites every year, which threatens the natural environment and poses high landslide risks. This study is therefore designed to characterize the generation, composition, flows and potential impacts of ESR in China. The results show that with the promotion of subway and increasing public awareness of recycle ESR, the generation of ESR has increased from 0.2 million m3 in 1965 to 79 million m3 in 2018 with a cumulative CO2 mitigation of 417.2 million tonnes by 2018 and will keep increase in coming decades (e.g., approximately exceed 200 million m3 by 2030). South and East China account for 60% of the total ESR which aligns well with their growing population and economic activities. Besides backfilling and landscaping, ESR were mainly sent to dumping site or unregulated landfill sites, which accounts for 87% of total generation. It was estimated and predicted that the dumping disposal of ESR has cumulatively occupied a land area of 212 ± 25 km2 from 2004 to 2030. Moreover, this study compared the management of ESR between China and developed countries, and recommendations for environmentally sound management of ESR were proposed.

Cities are responsible for the majority of global resource use and greenhouse gas emissions. Urban metabolism, which mimics metabolism of organisms as both require material and energy to function and further generate waste and emissions, is a useful framework to analyze levels of sustainability of a city through quantification of material and energy flows it uses and discards. The present study quantifies and compares the metabolic patterns of the four main Danish cities, Copenhagen, Aarhus, Odense, and Aalborg, from 2010 and 2015. We found their inflows of energy, water, and food and outflows of greenhouse gas emissions, wastewater, municipal solid waste, and construction and demolition waste have downsized over the six years. Our results affirm the impact of relevant policy and regulations in urban sustainability, with Danish cities showing more sustainable metabolic patterns than other world megacities previously reported in the literature. They also indicate that urban metabolic patterns can be highly dependent on the role and services that a city offers to its surrounding region (as a multicellular organism). Further strengthening of policies and regulations on “cities for sustainability” (i.e., focus on the sustainability of the entire multicellular organism) instead of a sole focus on “sustainable city” (i.e., each cell aiming at being sustainable independently from the others) would be important.

As one of the largest energy consumers, the transport sector generated direct and indirect emissions which significantly affect the environment, accounting for approximately one-third of the total carbon emissions. While the major impacts are from transport energy use and emissions, very few studies attempted to examine the impacts from transport infrastructure, especially at a city or country level. This paper, taking Shenzhen in China (a fast developing megacity) as the case study, is specially designed to quantify the carbon footprint of the urban roads by using streamlined life cycle assessment method. For given years (ranged from 2004 to 2013), various activities of urban roads (e.g. newly planned road construction, maintenance of road in use, and road renovation and demolition) have been examined in this study. The results show that the total carbon footprint from urban roads in Shenzhen was 260 (±20) thousand tons CO2e in 2013. The major contributor was the materials use (embodied impact) from newly constructed roads, which accounts for 52.3% of the total carbon footprint, followed by the maintenance stage (24.3%). The eco-design process of road construction plays a vital role in achieving the effective carbon footprint reduction. These findings help to better understand the carbon footprint from urban roads in megacities, and provide useful inputs for policy making process in terms of identifying carbon reduction opportunities for the transport sector. In addition, the methodologies are useful for the quantification of carbon footprint in other cities of China and beyond.

The rapid urbanization in China since the 1970s has led to an exponential growth of metal stocks (MS) in use in cities. A retrospect on the quantity, quality, and patterns of these MS is a prerequisite for projecting future metal demand, identifying urban mining potentials of metals, and informing sustainable urbanization strategies. Here, we deployed a bottom up stock accounting method to estimate stocks of iron, copper, and aluminum embodied in 51 categories of products and infrastructure across 10 Chinese megacities from 1980 to 2016. We found that the MS in Chinese megacities had reached a level of 2.6-6.3 t/cap (on average 3.7 t/cap for iron, 58 kg/cap for copper, and 151 kg/cap for aluminum) in 2016, which still remained behind the level of western cities or potential saturation level on the country level (e.g., approximately 13 t/cap for iron). Economic development was identified as the most powerful driver for MS growth based on an IPAT decomposition analysis, indicating further increase in MS as China's urbanization and economic growth continues in the next decades. The latecomer cities should therefore explore a wide range of strategies, from urban planning to economy structure to regulations, for a transition toward more "metal-efficient" urbanization pathways.

With the continuing growth of urbanization and the boom of the construction sector in China, a substantial quantity of construction and demolition waste has been generated. However, there has been little research to characterize the waste from buildings’ interior renovation (repair and maintenance) or decoration activities. The primary purpose of this study was to provide a systematic analysis for the generation and flows of decoration and renovation waste based on field investigation. Taking Shenzhen city - a fast-growing megacity (over 20 M population) in South China - as an example, the results show that approximately 1 Mt of decoration and renovation waste was generated in Shenzhen city in 2017, with an annual growth rate of 10% over the past decade. Concrete waste and brick (block) account for 72% of the total decoration and renovation waste. The landfilling rate of decoration and renovation waste has reached 83%, of which a majority was disposed of by open dumping. The renovation of older residential buildings produced more than half of the total decoration and renovation waste. Although the amount of hazardous materials such as gypsum and painting materials only account for a very low weight fraction of 2%, they could still pose significant environmental impacts, compared to other types of construction and demolition waste. Accelerating the construction of sorting facilities to separate toxic and valuable materials is crucial, to realize decoration and renovation waste minimization and lower environmental risks. Overall, the findings of this study could be applied to formulate environmentally sound management measures from a policy standpoint, for Shenzhen city. The methods could also be useful for decoration and renovation waste estimation and projection in other cities in China and beyond.

Massive construction and demolition waste is generated in China's rapid development. Within this kind of waste, glass material produced form the widely applied curtain wall system contributes a lot. Currently, the majority of waste glass curtain wall is simply disposed in landfills or dumping, which not only significantly threats the scarcity of land resources, but also raises risks for surrounding environment and human health. However, there is relatively little work in China which focuses on the management of glass curtain wall waste produced from demolition and reconstruction sites. Therefore, this study systematically reviews current application and further development of glass curtain wall system in Shenzhen city (a mega city of China), and then analyzes the compositions and characteristics of the generation of waste glass curtain wall. The results show that the total gross surface area of buildings that use glass curtain wall system has approximately reached to 3 million m 2 in the region (Nanshan district in Shenzhen). It also indicates that there is around 2 kg of waste which could be produced from the construction of glass curtain wall per m 2 and will generate more than 6000 tons of glass curtain wall waste, including the whole life cycle from manufactory and construction. In additional, several recommendations to sound management of the waste glass curtain wall are proposed based on the further review of the management strategies and process methods employed in developed counties. Overall, the findings in this study could be helpful to contribute valuable knowledge and methods to the environmental management and waste recycling in China.

The urgent need to develop low carbon urban transport systems particularly in Asian megacities is facing the significant challenge of growing motorization following population increase and economic development. Sustainable urban public transport (UPT) plays a crucial role to fulfil the ambitious targets on carbon emission reduction. In this study, life cycle assessment was employed to quantify the environmental impacts (measured by carbon emissions) of UPT systems (including bus and subway) in Shenzhen, a leading megacity in South China, and then to examine corresponding carbon intensity reduction potentials. Results showed that the total carbon emissions from UPT in Shenzhen have increased rapidly from 0.70 Mt in 2005 to 1.74 Mt in 2015 due to the fast growth of the volume of transport turnover. However, current low-carbon UPT mode has only reduced 0.21 Mt CO2 e (cumulative value, from 2005 to 2015), and thus could not contribute proportionally to the city's overall emission reduction target. Three advanced scenarios (from conservative to optimistic) were further simulated to estimate carbon emissions and their intensity reduction potentials over the next 15 years (2016–2030). Compared to the business-as-usual scenario, all these three low-carbon transition scenarios could significantly mitigate the rapid growth of carbon emissions and consequently help achieve Shenzhen's carbon intensity reduction goal by 2030 (60%, compared to 2005 level). These findings could not only inform evidence-based policy making to facilitate the low-carbon transition of the urban transport sector in Shenzhen, but also shed light on sustainable urban transition in other megacities.