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Role of Urban Underground-Space Development in Achieving Carbon Neutrality: A National-Level Analysis in China
Jiajia Wang, Huabo Duan, Kunyang Chen, Isabelle Y.S. Chan, Fan Xue, Ning
Zhang, Xiangsheng Chen, Jian Zuo
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S2095-8099(24)00428-4
https://doi.org/10.1016/j.eng.2024.07.012
ENG 1613

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Engineering

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5 February 2024
18 June 2024
22 July 2024

Please cite this article as: J. Wang, H. Duan, K. Chen, I.Y.S. Chan, F. Xue, N. Zhang, X. Chen, J. Zuo, Role of
Urban Underground-Space Development in Achieving Carbon Neutrality: A National-Level Analysis in China,
Engineering (2024), doi: https://doi.org/10.1016/j.eng.2024.07.012

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Research
Frontier Research on Carbon Neutrality—Article

Role of Urban Underground-Space Development in Achieving Carbon Neutrality: A
National-Level Analysis in China
Jiajia Wanga, Huabo Duanb,#, Kunyang Chenc, Isabelle Y. S. Chana, Fan Xuea, Ning Zhangd, Xiangsheng
Chenc,*, Jian Zuo e,*
a Department of Real Estate and Construction, Faculty of Architecture, The University of Hong Kong, Pokfulam, Hong Kong 999077, China.
b School of Environmental Science and Engineering, Huazhong University of Science and Technology, Wuhan 430000, China
c State Key Laboratory of Intelligent Geotechnics and Tunnelling, College of Civil and Transportation Engineering, Shenzhen University, Shenzhen

518000, China
d Leibniz Institute of Ecological Urban and Regional Development, Dresden 01217, Germany
e School of Architecture and Civil Engineering, The University of Adelaide, SA 5005, Australia

# These authors contributed equally to this work and should be considered co-first authors
Corresponding authors.
E-mail addresses: xschen@szu.edu.cn (X. Chen), jian.zuo@adelaide.edu.au (J. Zuo)

Highlights
•

Urban Underground Space (UUS) presents a strategic approach for sustainable urban development.

•

A systematic framework is presented to assess UUS's carbon reduction potentials.

•

UUS released approximately 547 Mt of carbon emissions during construction phase in 2020.

•

Geothermal resources have significant potential for carbon sequestration in UUS sector.

Abstract
Surface space constraints and the associated massive carbon emissions present significant challenges to the sustainable development of megacities.
Urban underground space (UUS) construction is expected to provide a practical approach for alleviating the space constraints of surface construction.
However, in-depth examinations of the overall UUS system to reveal carbon emissions in the complex matrix are lacking. This study demonstrates the
vital role of UUS development in achieving carbon neutrality using a streamlined life-cycle assessment method. Carbon emissions and the mitigation
potential of building underground spaces, metro systems, and geothermal energy sources are analyzed. The construction of underground spaces in
buildings is the largest carbon emitter within the entire UUS system, releasing a considerable 547.2 Mt in 2020. However, geothermal carbon
sequestration, a significant element of the UUS system, provided an unexpected and impressive contribution, sequestering 170 Mt of carbon in 2020.
This study shows that UUS addresses the lack of space for urban development and is a low-carbon method of urban construction. Therefore, developing
low-carbon building technologies and improving the UUS development model is imperative to achieving better low-carbon balance. This helps to
promote more coordinated and sustainable urban development.

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Keywords: Urban underground space; Carbon emissions; Urban development; Construction sector

1. Introduction
The construction sector has always been a pivotal driver of economic development, fueling progress, prosperity, and
modernization worldwide [1,2]. However, construction activities have been achieved at a cost; that is, the ever-increasing carbon
footprint, which poses considerable challenges in achieving carbon-neutrality goals. In China, buildings and infrastructure are
major energy consumers and significant carbon-dioxide (CO2) emitters [3]. According to the China Building Energy Consumption
Annual Report 2022 [4], emissions associated with building materials, construction, and building operations accounted for 28.2%,
1.0%, and 21.7% of China’s total energy-related CO2 emissions in 2020, respectively, equaling an accumulated emission quantity
of 5.8 billion tonnes of CO2 emissions. Despite the decrease in real estate development, infrastructure development still generates
carbon emissions. Therefore, the construction industry plays a crucial role in achieving carbon-neutrality goals. Thus, low-carbon
transformations in the construction industry are imperative [5]. In the context of national goals for achieving carbon neutrality,
mitigation measures must be taken to help steer China’s construction and building sectors towards low-carbon development
modes.
Additional to the need for emission reduction, urban areas face another pressing concern: limited surface space for expansion
[6]. As urban populations grow and urbanization rates increase, the conventional horizontal expansion of cities has become
increasingly impractical and unsustainable [7,8]. The significance of urban underground space (UUS) development lies in its
ability to provide broad opportunities to address various urban challenges and meet the growing demands of urban development.
UUS is the primary underground space for human development and utilization, and refers to the space below the surface developed
within the scope of an urban-planning area [9,10]. It provides sustainable and efficient use of underground areas for various
purposes such as transportation, commercial activities, storage, utilities, and even clean-energy production. Effective exploitation
and utilization of UUS can relieve the environmental pressure derived from traditional surface buildings and provide a new way
of thinking for densely populated areas to overcome space constraints. The use of underground-space energy sources can help
reduce carbon emissions. For example, geothermal energy is a renewable energy source that can heat and cool buildings and
reduce their reliance on fossil fuels.
In a rapidly developing country such as China, economic development is an essential goal that has led to the rapid growth of
UUSs [10,11]. As of the end of 2020, China has been actively promoting and making significant strides in the development of
UUSs [12–15]. The cumulative construction of UUSs in the Mainland of China has reached a remarkable 2.4 × 109 m2 [16]. In
2020, the country witnessed the addition of approximately 2.59 × 108 m2 of new UUS, exhibiting an annual growth rate of 0.78%
[16]. Newly added underground spaces accounted for approximately 22% of the total completed construction area in urban regions
during the same period [16]. As China continues to face rapid urbanization and population growth, promoting UUS development
is expected to remain a key priority for the future [17–19]. Globally, cities use underground space as an essential solution to meet
the challenges of urbanization and optimize land use. For example, Helsinki, Finland, has more than 1.0 × 107 m3 of underground
space for parking, sports facilities, storage, and metro systems [20,21]. Helsinki uses UUS for transportation purposes in
conjunction with the existing transportation infrastructure. For example, an underground metro system can be seamlessly
connected to underground pedestrian tunnels, making moving around the city easy and efficient. Singapore is committed to
doubling its metro network from 180 km by 2030, a global model for promoting underground-transport systems [22–25]. In Paris,
France, where the population density is as high as 927 people per square kilometer owing to good ground conditions, a large
amount of underground infrastructure is used for transportation networks, public utilities, and multiple functions [26]. These
quantifiable examples highlight the fact that UUS forms an integral part of modern urban planning, enabling cities to maximize
land use.
Considering the Sustainable Development Goals of the United Nations, the question of whether the intensive development of
UUS is a low-carbon activity must be addressed, particularly in many megacities in China. Notably, the development of UUS
involves a large amount of resources and energy inputs, and is likely to incur indirect carbon emissions [27–30]. This apparent
negative effect deserves further investigation. For example, the energy-saving performance of metros during operation has
replaced part of the ground automotive transportation as a low-carbon mode of transportation [31–35]. However, the considerable
carbon emissions during the construction of underground metro tunnels contradicts the carbon-emission reduction potential that
occurs during metro operation. With the common goal of carbon neutrality, the consideration of UUS as a low-carbon
development approach at the national level must be properly justified. However, existing studies have mainly focused on
quantifying carbon emissions from specific UUS projects [36–38]. Insufficient analysis can be attributed to various factors,
including complex system boundaries, limited data availability, and the evolving characteristics of UUS development.
Therefore, this study conducts a comprehensive assessment of China’s UUS development and embodied carbon impact over
the past two decades using a top-down approach at the national level. Unlike previous studies that focused on specific UUS types,
such as underground rail transit, this study takes a holistic approach to evaluate the overall carbon impact of multiple UUS types
at a national scale. By examining the cumulative effects of UUS development and operation over nearly two decades, this study
aims to elucidate the role of sustainable construction in reducing carbon emissions. Overall, this study aims to ① quantitatively
analyze the performance of UUS in low-carbon sustainable development through statistical data and calculations of typical UUS

2

applications in China from 2001 to 2020; ② examine the factors related to the performance of various typical urban underground
spaces in terms of carbon emissions; ③ draw conclusions regarding the positive outcomes of China’s UUS development in
effectively reducing carbon emissions; and ④ suggest a low-carbon path for UUS development in China and beyond.
2. Methods
2.1 Research boundary
The primary assessment method used in this study was life-cycle assessment (LCA), which is a method for assessing the
environmental impact of a product over its entire life cycle. It is suitable for sustainability assessments at regional, national, and
global scales (details in Note S1 in Appendix A). In terms of an intensive literature review, the LCA method is more appropriate
for calculating UUS carbon emissions at the national level.
UUS comprises a variety of categories, including buildings, tunnels, basements, underground parking lots, and other
subterranean infrastructure [39,40]. The major categories of underground spaces related to carbon emissions, which are the focus
of this study, can be summarized into three typical types: construction of commercial and residential underground spaces,
development of underground metro systems, and utilization of geothermal energy [41–43]. These UUS types involve the intensive
consumption of building materials, energy, or renewable-energy resources. Further details on the relationships between these
three categories and their specific definitions are presented in Note S2 in Appendix A. In this study, we focus on a quantitative
analysis of the carbon-emission mitigation associated with UUS development in these three specific aspects (Fig. 1). UUS
contributes significantly to mitigating carbon emissions in the context of urbanization; however, considering the complexity of
each aspect, a comprehensive examination of their effects presents considerable challenges. Therefore, to streamline this research
and gain a deeper understanding of the impact of USS on carbon reduction, we focused on these three key areas.

Fig. 1 Quantification boundary of the carbon mitigation of UUS.

2.1.1 Underground building
The term “underground building” explicitly pertains to the subterranean levels of both commercial and residential buildings. A
comprehensive top-down methodology was employed to determine the cumulative area of the constructed underground spaces in
China between 2001 and 2020. The annual aggregated floor areas of completed buildings from 2001 to 2020 were sourced from
the Statistical Yearbooks of China 2020 (covering the years 2002–2019) [44]. According to “The 13th Five-Year Plan for the
Development and Utilization of Urban Underground Space” [45] and The Blue Book on the Development of Urban Underground
Space in China 2021 [16], the annual proportion of underground area in completed building floor areas increased from 10% to
19% during the past two decades. This proportion involves the annual completed metro area. Hence, the annual completed metro
areas were calculated using Eq. (1) and then deducted using Eq. (2). Annual statistics on the metro operating mileage and in-use
stations from 2001 to 2020 were obtained from the China Urban Construction Statistical Yearbook (2002–2020) [46] as well as
the China Urban Rail Transit Association (2020).
𝑆sub(𝑖) = 𝐿tunnel(𝑖) × 𝑤tunnel + ∑𝑛(𝑖)
𝑡=1 ℎ𝑡 × 𝑤𝑡, (1)

3

where 𝑆sub(𝑖) refers to the area newly occupied by metro in the year 𝑖 (m2); 𝐿tunnel(𝑖) denotes the newly added metro operating
mileage in the year 𝑖 (m); 𝑤tunnel is the width of the metro tunnel, which is equivalent to the diameter of shield segments, that is,
6.98 m; 𝑛 refers to the number of newly opened metro stations in the year 𝑖; ℎ𝑡 and 𝑤𝑡 are the length and width of the 𝑡th newly
opened metro station, and for this study, these values are 240 and 24 m, respectively [47].
The CO2 emissions of an underground building during the construction phase can be calculated as follows:
𝐸c―ug(𝑖) = 𝑆com(𝑖) × 𝑑com(𝑖) ― 𝑆sub(𝑖) × 𝑒c―ug, (2)
where 𝐸c―ug(𝑖) is the CO2 emission of building underground space in the construction phase in the year 𝑖 (tonnes of CO2, tCO2);
𝑆com(𝑖) refers to the newly completed floor area of buildings in the year 𝑖 (×104 m2), and the full set of data are provided in Note
S2; 𝑑com(𝑖) is the ratio of the underground area to the total annual completed building floor area in the year 𝑖; 𝑒c―ug refers to the
carbon-emissions factor of underground building in the construction phase, that is, 0.72 tCO2·m−2 [48].
Greenery and vegetation also contribute to carbon sequestration. Assuming that the areal equivalent of the building underground
space had all been developed into greenery and vegetation on the ground, the quantity of the resulting fixed CO2 in a certain year
can be estimated as follows:
𝐸cs―ug(𝑖) = (𝑆com(𝑖) × 𝑑com(𝑖) ― 𝑆sub(𝑖)) × 𝑒g × 3.67, (3)
where 𝐸cs―ug(𝑖) is the amount of fixed CO2 in the year 𝑖 (tCO2), and 𝑒g refers to the average carbon-sequestration factor of urban
greenery, that is, 2.16 tonnes of carbon per hectare per year (tC·hm−2·a−1) [49].
2.1.2 Metro
CO2 emissions associated with the construction phase are non-negligible when the low-carbon effect of the metro is considered.
Metro construction can be divided into two distinct categories: tunnels and stations. In the construction phase, the total CO2
emissions are the sum of the emissions from both parts, as reflected in Eq. (4):
𝐸c―sub(𝑖) = 𝐿tunnel(𝑖) × 𝑒tunnel + ∑𝑛(𝑖)
𝑡=1 ℎ𝑡 × 𝑤𝑡 × 𝑒station, (4)
where 𝐸c―sub(𝑖) denotes the CO2 emissions associated with the construction phase of metro in the year 𝑖 (tCO2); 𝑒tunnel and 𝑒station
refer to the carbon-emissions factors associated with the construction of metro tunnels and stations, that is, 10.192 tCO2·m−1 and
3.924 tCO2·m−2, respectively [50].
In addition to the metro, public buses and taxis are the major types of urban public-transport vehicles in China. This is reflected
by the annual total ridership data for each type of vehicle from 2001 to 2020, which can be found in the China Urban Construction
Statistical Yearbook (2002–2020) [46] and the Statistical Bulletin on the Development of the Transportation Industry (2009–
2020) [51]. The three types of vehicles were compared on a macro scale in terms of operation-induced CO2 emissions, which
were acquired using Eq. (5):
𝐸𝑡o―trans(𝑖) = 𝑁𝑡𝑖 × 𝑀𝑡𝑖, (5)
where 𝑡 refers to the type of vehicles, namely metro, public bus, or taxi; 𝐸𝑡o―trans(𝑖) denotes the CO2 emissions of the type 𝑡 vehicle
in the year 𝑖 (kgCO2); 𝑁𝑡𝑖 is the total ridership of the type 𝑡 vehicle in the year 𝑖 (passenger-ride); 𝑀𝑡𝑖 represents the CO2 emissions
per passenger-ride of the type 𝑡 vehicle in the year 𝑖, that is, 0.7 kgCO2 per passenger per ride for metro [52], 0.68 kgCO2 per
passenger per ride for public bus [53], and 1.72 kgCO2 per passenger per ride for taxi [53]. The emission factors of public buses
and taxis consider the combined emission indicators of both petrol-powered and electric vehicles.
2.1.3 Geothermal energy
The utilization of hot, dry rock geothermal resources remains limited worldwide. Thus, for simplicity, this section is concerned
only with shallow and hydro-geothermal energy. The carbon-mitigation effect of shallow geothermal energy can be quantitatively
determined using heat-pump data [54], as follows:
𝐸geoth―s(𝑖) = 𝑁pump(𝑖) × 𝐽ave × 𝑒tr, (6)

4

where 𝐸geoth―s(𝑖) refers to the amount of CO2 reduced by utilizing shallow geothermal energy in the year 𝑖 (tCO2); 𝑁pump(𝑖) is
the number of heat pumps in the year 𝑖 (pumps); 𝐽ave is the average amount of energy of each pump for building heating and airconditioning (J·pump−1); 𝑒tr is the carbon-emissions factor of electricity (tCO2·J−1).
As the embodied energy of one tonne of standard coal is well known, amounts of geothermal energy are typically expressed as
a particular quantity of standard coal. Thus, the carbon-mitigation effect of geothermal energy can be calculated as follows:
𝐸geoth(𝑖) = 𝑁coal(𝑖) × 𝑒coal, (7)
where 𝐸geoth(𝑖) refers to the amount of CO2 reduced by utilizing geothermal energy in the year 𝑖 (tCO2); 𝑁𝑐𝑜𝑎𝑙(𝑖) is the equivalent
quantity of standard coal in the year 𝑖 (t); 𝑒coal is the carbon-emissions factor of standard coal (tCO2·t−1).
2.2 Data collection
Conducting a systematic and long-term analysis at the national level presents significant challenges for data acquisition. The
primary data sources for this study were government documents, such as the China Statistical Yearbook, and industry reports,
such as the 13th Five-Year Plan for the Development of Urban Underground Space in China and the Blue Book for the
Development of Urban Underground Space in China 2021 [16]. Government and industry data are relatively reliable and
extensive, covering 20 years (2001–2020). For example, the China Statistical Yearbook provides the annual gross floor area of
completed buildings. Other professional reports and yearbooks also provide valuable data on metro operations and other types of
public transportation such as buses and taxis. Some parameters or data were assumed if they were difficult to gather; however,
uncertainties were captured. The error values of the results were obtained using a Monte Carlo simulation.
3. Results and discussion
3.1 Total carbon emission and mitigation of UUS
The comprehensive analysis presented in this study sheds light on the pivotal role of UUS as a significant contributor to
sustainable urban development and carbon-emissions reduction. Urban and national development is intrinsically tied to
construction activities, which inevitably lead to carbon emissions [55]. Therefore, the pursuit of low-carbon development
approaches has become critical. In this context, the development of underground spaces has emerged as an effective means of
mitigating the constraints of the limited availability of surface land while minimising carbon emissions.
The results of this study show that the construction process is the main source of carbon emissions during the entire life cycle
of the UUS. Production and transportation of construction materials are the primary sources of carbon emissions [56]. In addition,
emissions showed a continuous upward trend over time. Although emissions are generated during the construction phase, carbonsink offsets during operation make UUS a sustainable construction model that complements urban development. From a life-cycle
perspective, UUS does not lead to higher carbon-emission outcomes. Simultaneously, UUS can be used for transportation and
commercial facilities through proper planning, thereby reducing the pressure on surface space development and promoting
sustainable urban development. The space saved on the surface provides opportunities for green plants, and plant carbon
sequestration is useful for reducing carbon emissions.
Geothermal energy plays a vital role in carbon reduction in UUS. The use of geothermal resources provides UUSs with a
considerable capacity for carbon sequestration. China has a large land area and abundant natural resources, and the rational
utilization of geothermal resources provides powerful assistance for the low-carbon development of UUS. Economic growth must
be connected to infrastructure, and UUS offers new ideas for developing countries such as China. Moreover, the combination of
geothermal-energy utilization and underground-space development offers an innovative and sustainable model. This synergy
provides a new direction for low-carbon urban developments. The results of this study also confirm that UUS can be used as a
low-carbon construction method, which has been validated in other developing countries. In addition to analyzing historical data
on UUS emissions, this study projected future trends under different scenarios. The results illustrate the potential paths for carbon
emissions and sequestration from 2021 to 2035 under various scenarios. These scenarios are critical for understanding the impact
of current measures.
Overall, the carbon-reduction potential of UUS is clear. Although underground buildings still produce emissions during
construction, UUS is a low-carbon system based on the overall evaluation of the system. Urban-planning authorities can use UUSs
to enhance planning and design to further reduce emissions. Rationally planning the distribution of UUS and seizing low-carbon
construction opportunities derived from it are essential. These findings demonstrate the urgent need to implement low-carbon
strategies and take proactive measures in carbon strategies, particularly when considering the overall construction of cities. Future
UUS development should explore advanced construction materials, improved energy-efficient systems, and innovative carboncapture and -storage techniques. Carbon negativity can be achieved more efficiently through synergy with geothermal-energy
utilization. Moreover, sustainable design principles, such as maximizing green spaces and employing smart ventilation systems,
can further contribute to carbon negativity. These technologies and approaches are pertinent to the UUSs in China and have
implications for urban development.

5

In addition, the assumptions made during the carbon-emission calculation process are limited. For example, the average carbonsequestration factor may not accurately represent sequestration rates in different regions or ecosystems. Additionally, our data
may not account for the potential impact of technological advances on emission factors, which may introduce uncertainties into
the results. However, we captured these uncertainties in the modelling results. Most importantly, these assumptions and limitations
did not significantly alter the overall trend of carbon calculations in the macro-level analysis. In fact, in some extreme hypothetical
scenarios, the use of assumed parameters can provide a more intuitive understanding of the analytical results. Future research
should aim to refine these assumptions and incorporate more precise data to further enhance the accuracy of carbon calculations.
3.1.1 Underground building
Fig. 2 shows the carbon emissions associated with various stages of underground space for building construction, juxtaposed
with the carbon-sequestration potential attributed to urban greenery and vegetation in equivalent surface areas. CO2 emissions
associated with the construction of underground buildings have increased continuously from 70.2 Mt in 2001 to 547.2 Mt in 2020.
The average annual growth rate was calculated as 11.4% over the entire two-decade period. As shown in Fig. 2, the annual
increments in emissions appeared relatively stable and modest during the first decade. However, the annual emissions growth
became more significant thereafter. Between 2011 and 2013, emissions increased noticeably, the momentum of which prompted
annual emissions to exceed 400 Mt in 2015. During the 13th Five-Year Plan period, annual emissions increased to over 500 Mt
for the first time in 2018, and peaked at 547.2 Mt in 2020. Notably, the increase from 2019 to 2020 was small and even negligible,
which can be attributed to the coronavirus disease 2019 (COVID-19) pandemic in 2020.
Notably, the figure reveals a steady increase in CO2 emissions during the construction phase of underground buildings,
primarily driven by the stages of building-material production and transportation, particularly cement production. The production
of cement involves numerous chemical reactions. Therefore, the carbon emission from cement production during the construction
process is a topic that requires attention. Transportation consumes a considerable amount of energy, and the estimated results
verify that energy savings in transportation require special attention. Notably, while this carbon-emissions segment is linked to
underground-space construction, equivalent surface building construction similarly necessitates building-material production and
transportation, including cement production. Consequently, this fraction of carbon emissions is not exclusively associated with
underground-space development, but constitutes an inherent aspect of the holistic building process. This awareness helps to
mitigate the misattribution of increased carbon emissions solely to the construction of underground spaces.
The results showed that plant carbon sequestration in cities did not result in considerable carbon reduction. As assumed in this
study, ① the development of underground space for buildings has released the same size as the surface space, and ② all this
surface space is used for carbon sequestration in the form of green plants and vegetation. The purpose of these two assumptions
is to estimate the role of plant carbon sinks in urban areas. Using these assumptions, we calculated the trends and significance of
plant carbon sinks in cities. Simultaneously, we assumed that the carbon-sequestration capacity of urban greening remains
constant, that is, 2.16 tC·hm−2·a−1. Even under extreme conditions, plant carbon sinks in cities do not result in an order-ofmagnitude reduction of carbon. However, UUS solves part of the surface-land problem and provides opportunities for the
development of urban plant carbon sinks. This study evaluated the development trends of urban carbon sinks using the limiting
condition hypothesis. Fig. 2 illustrates the amount of CO2 compensated by these means during a 20-year period. The amount of
fixed CO2 increased continuously and rapidly over the two-decade period, reaching a total of 6 Mt in 2020, and annual growth
appeared to accelerate. The gradual increase in carbon sequestration observed over the years suggests that urban greening plays
an auxiliary role in the sustainable development of UUS.

6

Fig. 2 Carbon emissions and sequestration in construction of underground building.

3.1.2 Metro
Fig. 3 summarizes the CO2 emissions attributed to the different stages of metro development over the past two decades (2001–
2020). The figure visually represents these emissions, highlighting distinct metro construction and operation trends. Carbon
emissions linked to metro construction, including stations and tunnels, exhibited a significant shift in 2010, increasing sharply
from 2.8 Mt in 2009 to 12.5 Mt. This indicates a notable division between the pre-2010 period and subsequent years. This is
because of increased government investment in urban-infrastructure development, with metro construction being an important
aspect. Policy support facilitated the development of metro-construction projects. With the acceleration of urbanization, the urban
population is proliferating and the pressure on urban transportation is increasing. As an efficient and fast mode of transportation,
the metro can effectively alleviate the problem of urban traffic congestion. Therefore, metro construction has grown rapidly during
the process of urbanization. Regarding metro operations, emissions exhibited steady growth from 2001 to 2009. Subsequently, a
noticeable acceleration in emission was observed until 2019, peaking at 14.3 Mt. Notably, in 2020, the COVID-19 pandemic led
to a considerable decline in the total ridership, which, in turn, had a discernible impact on the operational carbon emissions of the
metro.

Fig. 3 Trends in carbon emissions at different stages of metro construction and operation.

7

Fig. 4(a) shows the operational CO2 emissions associated with the three primary categories of public-transport vehicles from
2001 to 2020. A closer examination showed that public buses and taxis generated more CO2 emissions in the latter ten-year period
than in the first decade, a trend similar to that observed for the metro. Notably, the collective emissions of public transport,
including the metro, public buses, and taxis, consistently exceeded 120 Mt between 2012 and 2019, peaking at 130 Mt in 2014.
As the economy develops, people’s willingness to travel increases and public transportation grows. Simultaneously, with the
promotion of environmental protection, people are choosing a higher proportion of public-transportation trips. Fig. 4(a) shows
the distinction in operational carbon emissions between the metro and other public-transportation options. Metro emissions were
consistently lower than those of public buses and taxis. Quantitatively, the annual emission proportions attributed to the metro
within the broader public-transport context ranged from 1.05% to 11.77%, with an average of 4.53% over the twenty-year period.
Transportation modes are vulnerable to external factors, and the impact of the COVID-19 pandemic led to a significant reduction
in carbon emissions from transportation after 2019. The development of the carbon-emission trend in the graph reveals a positive
correlation with transportation development.
A scenario analysis was conducted under extreme conditions to delve deeper into the roles of major public-transport types in
CO2 emissions. This analysis involved allocating the genuine annual aggregate ridership to the three major vehicle categories. As
shown in Fig. 4(b), the scenario analysis results align with those shown in Fig. 4(a). The scenario analysis highlighted that taxis
provided significantly higher CO2 emissions than in the actual situation. In contrast, metro and public buses exhibited emissions
that were considerably lower than the actual data. Additionally, the metro and public buses appear to have reasonably comparable
presumptive carbon emissions, with the metro maintaining only a modest lead over public buses. The metro continues to perform
well in terms of low-carbon performance while significantly easing road traffic and effectively improving transportation
efficiency.

Fig. 4 Comparison of carbon-emission trends in (a) the operation of different public-transportation options for actual passenger flow and (b) after
average passenger distribution (blue belt represents the range of the uncertainty analysis).

As urban populations grow and cities become more congested, the expansion of surface-level transportation infrastructure
encounters challenges owing to limited physical space. Thus, the significance of the metro as a pivotal player in the transportation

8

landscape has become even more pronounced. The metro, an underground transport mode, operates independently of surface
constraints and effectively utilizes the underutilized space beneath the city. Beyond its capacity to alleviate urban congestion, the
metro’s inherent attributes align with low-carbon urban-development goals. The results show that the metro system generates
emissions that are no higher than those of other public-transportation options. Thus, the metro system can address the twin
challenges of urban congestion and carbon emissions. Its ability to provide a reliable, efficient, and low-carbon transportation
option, while bypassing the limitations posed by surface-level transport, underscores its importance in achieving sustainable urban
mobility.
3.1.3 Geothermal energy
Table 1 presents a comprehensive overview of the present state and potential carbon-reduction impact achievable by utilizing
geothermal resources. This data-driven analysis underscores the pivotal role of geothermal energy in UUS development. A notable
trend is the steady expansion of geothermal-energy applications, particularly for shallow heating/cooling and hydro-geothermal
heating. This not only aids in mitigating reliance on conventional fossil fuels, but also empowers cities with sustainable
alternatives for heating, cooling, and electricity generation. The use of underground space for the development of UUS may
destroy the structure of the underground strata and impact the utilization of geothermal resources, such as the conduction and
extraction of geothermal energy. However, these effects are typically local and controllable and can be reduced with appropriate
design and management. Therefore, the temperature loss of geothermal resources was not considered in this study. Geothermal
energy, similar to solar, wind, and other clean-energy sources, is an essential approach for carbon reduction that cannot be ignored.
China’s abundance of natural geothermal-energy resources holds promise for low-carbon development or even the carbon
neutrality of the UUS system. This expansion signifies the growing recognition of the versatility of geothermal energy across
diverse sectors, which considerably supports urban carbon-reduction aspirations.
Furthermore, the data highlighted the substantial potential of geothermal resources to replace fossil fuels. As geothermal-energy
applications continue to expand, the corresponding displacement of fossil fuels increases annually. Projections suggest that
geothermal resources may replace an energy equivalent of 280 Mt of standard coal by 2035. This projection promises a significant
reduction in fossil-fuel consumption, which is vital for catalyzing efforts to reduce urban carbon emissions. From a research
perspective, the significance of the 2025 and 2035 data points lies in their foundation for the existing national planning. These
projections are not arbitrary. Instead, they are rooted in well-established strategies and policies. Incorporating this contextual
information enriches the analysis, highlighting the harmonization of these projections with the country’s long-term sustainability
objectives. Geothermal energy plays an irreplaceable and vital role as a clean energy source in UUS development.
Table 1 Current Status and potential for carbon reduction through the application of geothermal resources.

Year
2015
2020
2025
2035

Shallow geothermal
heating/cooling area
(× 106 m2)

Hydro-geothermal heating area
(× 106 m2)

Installed power-generation
capacity (MW)

Substitution of fossil
energy
(million tce)

392.0

102.1

27.28

20

1118.5

502.1

527.28

70

2237.0

1004.2

1054.60

140

4474.0

2008.4

2109.20

280

Reduction in CO2
emissions (Mt)
48.6
170.0
340.0
680.0

Data were cited from the "13th Five-Year Plan" for developing and utilizing geothermal energy [42], and the future-development target data were
derived from the National Energy Administration. tce: tonnes of coal equivalent.

3.2 Scenarios of future carbon emissions in UUS
The carbon-emission results show that UUS is currently in line with the concept of low-carbon sustainability; however, more
strategies are still needed to improve low-carbon efficiency. Scenario projections can provide decision support and a better
understanding of possible future scenarios. Based on the results of this study, these three types of emissions are summarized as
historical data to predict future emission trends. Therefore, this study provides nine different scenarios for a comprehensive
projection of the potential trajectory of carbon emissions and sequestration in the UUS from 2021 to 2035, based on economic
development goals and policy development (shown in Fig. 5). This predictive illustration is instrumental in comprehending the
ramifications of contemporary actions on the future environmental tableau. Central to the figure is the depiction of three principal
trajectories concerning carbon emissions and sequestration: planned, increased, and decreased, each symbolizing either a
continuation of present trends, an activity escalation, or a decline, respectively. These pathways are juxtaposed to formulate nine
amalgamated scenarios, each pinpointing a unique confluence of emission and sequestration prospects (the scenario set can be
found in Note S3 and Table S1 in Appendix A).

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Notably, the most optimistic scenario (S1) involves strong environmental stewardship, leading to reduced emissions and
enhanced sequestration. Similarly, the most pessimistic trajectory, S6, is worth observing and is characterized by rampant
industrial activities paired with insufficient carbon-capture initiatives. These extremes encapsulate a spectrum of possibilities,
starkly admonishing the divergent outcomes that have emerged from our current choices. This scenario shows that even a UUS
currently assessed as low-carbon cannot be developed and built indiscriminately or without restraint. In baseline Scenario S1,
geothermal-energy development alone is insufficient to offset the carbon emissions generated by UUS construction under the
state’s current planning. Therefore, additional measures and innovative techniques for carbon sequestration must be implemented
to reduce the carbon footprint. Scenario S7 is predicated on escalated UUS construction aligned with urban-development demands
coupled with enhanced policy-driven geothermal carbon sequestration, and it presents a future in which carbon sequestration
potentially neutralizes emission impacts, manifesting a steady decline in overall carbon discharge.
Specifically, Fig. 5 underscores the need for assertive interventions in carbon strategies and the pivotal role of foresight in
sculpting sustainable environmental outcomes. By orchestrating a suite of scenarios, the illustration provides a nuanced
perspective on conceivable futures, enlightening policymakers, scholars, and environmental proponents on critical junctures in
decision-making.

Fig. 5 Projections of carbon emissions and sequestration of UUS by scenario over time.

4. Conclusions
The construction sector has long been a driver of economic growth. However, the surge in carbon emissions has called for
urgent action to reduce and mitigate carbon emissions. The development of UUS presents an innovative solution that relieves
pressure on surface space and presents a low-carbon construction model. This study comprehensively analyzed the interactions
between various aspects of UUS, such as urban development, energy consumption, and carbon reduction. This study focused on
the vital role of the operational phase in UUS systems, particularly in metro networks. Metros provide low-carbon transportation
options while reducing roadway congestion.
Moreover, this study quantified the capacity for geothermal energy to displace considerable fossil energy, thereby aligning with
China’s sustainability objectives and contributes globally to climate change mitigation. Similarly, future trends and possibilities
of the carbon impact of UUS were analyzed. The potential trajectories of carbon emissions and sequestration from 2021 to 2035
were presented through a scenario analysis. In essence, UUS development has become a strategic approach to achieving
sustainable urban growth while minimizing carbon footprints. For developing countries, the development of UUS presents a
positive solution for land-resource constraints. UUS offers a new solution for the conflict between economic growth and
environmental protection. From the perspective of environmental benefits, the carbon-emission reduction advantages of the UUS
construction phase may not be significant in the short-term. However, both the operation of the metro system and utilization of
geothermal resources can offer significant carbon-sequestration potential in the long-term, which can help meet the overall carbonemission reduction goal. Long-term thinking is essential for regional and national development. Therefore, policymakers should
focus on new technologies and materials for the development of UUS to further expand low-carbon construction.
This study has certain limitations, such as limited attention given to geothermal energy. The primary focus of this study was
on shallow and hydraulic geothermal energy. By contrast, other forms of geothermal resources, such as hot, dry rock geothermal

10

resources, were not considered. Because of the macroscale calculations involved, this study made assumptions about the setting
of some coefficients, such as the average carbon-sequestration coefficient. Technological advances in emission factors were not
included in the data. As mentioned above, UUS can be developed for purposes other than building underground spaces, metros,
and geothermal energy. Future research can extend the scope of this study to cover other types of UUS developments.
Additionally, private motorized vehicles can be incorporated into ridership allocation.
Acknowledgements
This work was financially supported by the Shenzhen Science and Technology Plan (JCYJ20190808123013260).
Compliance with ethics guidelines
Jiajia Wang, Huabo Duan, Kunyang Chen, Isabelle Y. S. Chan, Fan Xue, Ning Zhang, Xiangsheng Chen, and Jian Zuo declare
that they have no conflicts of interest or financial conflicts to disclose.

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Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that
could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential
competing interests:

15