Land–Water–Energy Coupling System and Low-Carbon Policy Simulation: A Case Study of Nanjing, China

Abstract

Global climate change produces large amounts of CO₂, and carbon emission reduction has become a global hot topic. As a key city in the Yangtze River Economic Belt, Nanjing plays a significant representative role in the process of achieving the “double carbon” goals. In this paper, a land–water–energy coupling system was established and urban carbon emissions were estimated. Through the SD model, the future urban carbon emissions were predicted under the adjustment of different land, water and energy consumption scenarios. We studied the relationship between urban carbon emissions and the land–water–energy coupling system, and whether Nanjing can achieve carbon neutralization in 2060 under conditions of natural development. The results show that urban carbon emissions in Nanjing have reached a peak in 2018, but low carbon measures are still needed for Nanjing to achieve its goal of carbon neutrality by 2060. Specific measures include increasing investment in technological innovation, expansion of the application scope of clean energy, reasonably planning land use structure, water conservation and wastewater utilization and the application of advanced carbon utilization technology. The results in this paper can serve as a reference for other cities and provide guidance for future urban planning and decision making.

1. Introduction

With the increasing exploitation of resources and rapid urbanization, the global warming issue has brought critical challenges for the sustainable development of human society [1]. Large quantities of fossil fuel combustion are considered to be the main cause of greenhouse gas production [2]. The Intergovernmental Panel on Climate Change (IPCC) has confirmed the quasi-linear relationship between cumulative anthropogenic carbon dioxide emissions and the global warming they cause. The International Energy Agency (IEA) released the “Global Energy Review: Carbon Emissions 2022”, which indicates that the global energy-related carbon dioxide emissions reached another record high of 36.8 billion tons. In 2020, the top six countries and regions in the world for CO₂ emissions were China (9.894 billion tons), the United States (4.432 billion tons), European Union (2.551 billion tons), India (2.298 billion tons), Russia (1.432 billion tons) and Japan (1.027 billion tons). On 12 December 2015, the Paris Climate Change Conference adopted the “Paris Agreement”, and in 2016 multiple countries signed the “Paris Agreement” to collaborate on greenhouse gas reductions. In 2020, China proposed to strive to reach carbon peaking by 2030 and achieve carbon neutrality by 2060, working towards addressing global climate change, responding to the goals of the “Paris Agreement” and promoting sustainable development in China’s society [3]. In fact, from the perspective of the current state of economic and social development and the energy structure, China’s carbon neutrality goal still faces many challenges and uncertainties [4]. In the future, it will be necessary to make efforts including resource allocation, energy innovation, ecological carbon sinks and territorial space, industrial structure and technological innovation to promote a comprehensive green and low-carbon transformation of the economy and society.

As the heartland of human production and activity, cities have become the major emission source of greenhouse gases while promoting high-quality socio-economic development [5,6]. The International Energy Agency (IEA) provides compelling evidence that cities consume 78% of the world’s total energy and that more than 60% of greenhouse gas emissions come from urban areas. The IPCC has estimated that 71–76% of global CO₂ emissions from final energy use can be attributed to cities. Some scholars argue that it is not cities themselves that are responsible for higher carbon emissions, but rather the concentration of economic activity within cities [7,8]. Some scholars believe that cities can effectively reduce the carbon intensity of development through more energy-efficient development approaches [9,10]. Although scholars have different attitudes about the role of cities in the carbon emissions system, it is widely recognized that urban development decisions have a crucial significance on global carbon reduction efforts [11].

In 2022, China’s urbanization rate was 65.22%. Dhakal believes that with the increase in China’s urbanization rate, cities will play a greater role in energy consumption and carbon emissions [12]. In China, urban carbon emissions account for about 85% of national carbon emissions [13]. However, although cities have become an important source of carbon emissions, they have also become the key to achieving carbon peaking and carbon neutrality goals [14]. Therefore, there is a need for policymakers in China to address carbon emissions from urban areas, which account for 65% of the total population. In 2010, the Chinese government initiated the low-carbon city pilot policy to enhance urban resilience to climate change. The policy designated Guangdong, Liaoning, Yunnan, Tianjin, Chongqing, Shenzhen, and five other provinces and eight cities as the first batch of low-carbon pilot areas. The National Development and Reform Commission (NDRC) subsequently identified three batches of low-carbon pilot provinces and cities in 2010, 2012, and 2017, with a total of 87 areas. The progress of these pilot cities in climate action was comprehensively evaluated, and replicable and scalable best practices were summarized from their climate action practices. Therefore, cities, with their rich resource conditions and technological means, have great potential for carbon emission reductions [2].

Water, land, and energy are essential elements in urban ecosystems, which are deeply entwined in the process of sustainable development, and all three elements contribute to carbon emissions during the process of urban economic development [1]. Exploring the coupling relationship between water, land, and energy, as well as their relationship with carbon emissions, is of great significance in seeking low-carbon and efficient urban development models, and in addressing global climate change and achieving the “double carbon” strategic goal [2].

Some scholars study carbon emissions from an energy perspective. Some studies have proved that in the long run, energy could contribute to carbon emissions in the US and the EU [15,16]. Nejat et al. believed that energy consumption and CO₂ emissions in the residential sectors in 10 countries, including China, the United States, Japan, Russia, etc., have a direct and significant impact on the world’s environment [17]. Furthermore, Dhakal’s research revealed that urban contributions will increasingly determine China’s energy consumption and carbon dioxide emissions in the future decades [12]. For example, Yang et al. investigated six energy sectors in Ningbo City and believed that the focus should be on reducing energy carbon emissions through measures such as low-carbon energy substitution and industrial transformation [18]. Gao et al. studied the carbon emission efficiency of 28 industrial sectors in China, specifically focusing on embodied carbon emissions [19]. Han et al. found that accelerating the process of China’s new urbanization can effectively control energy consumption and carbon emission intensity [5]. In a study of the Beijing, Tianjin and Hebei Urban Agglomeration, some scholars found that adjustments in energy structure and policies had a significant impact on carbon emissions [20,21].

Some scholars study carbon emissions from a land perspective. Some believe that land use change will have a significant impact on the global carbon system [22]. Reducing the intensity of soil cultivation [23] and restoring degraded soils [24] can both reduce CO₂ emissions. Griscom et al. proposed to increase carbon storage in global forests, wetlands, grasslands and agricultural lands to avoid greenhouse gas emissions [25]. Furthermore, Chen et al. highlighted the potential for substantial reductions in land input and carbon emissions in China in the coming decades [26]. At the provincial level, Zhang et al. found that total carbon emissions and carbon sequestration from land use in China both showed an annual increasing trend over the years, but with an increasing difference in their growth rates [27]. Zhao et al. discovered that the transformation of the urban form played a significant role in influencing carbon emissions from land use in Shaanxi, China [28]. Wang conducted a study on the land use structure in the Beijing-Tianjin-Hebei region and observed its relative stability. The study also revealed the adoption of land-use planning policies aimed at maintaining carbon balance [29]. Chuai et al. conducted research in Jiangsu, China, and found that urban land constraints will play a crucial role in reducing carbon emissions [30]. At the urban level, scholars found that the carbon emissions of land use in Shenzhen showed an annual increasing trend, and the emissions of different land use types were different [31,32]. Wu et al. studied the impact of land use change on multiple ecosystem services in Kunshan City in the context of rapid urbanization [33]. Zhang et al. identified that the primary driving factor of carbon emissions was due to the transition from residential to industrial land use [34].

Some scholars have studied carbon emissions from a water perspective. For example, Raymond et al. studied the carbon release of water bodies [35]. Keller et al. focused on identifying the factors driving global CO₂ emissions from arid inland waters [36]. Ran et al. found that carbon dioxide emissions from inland waters in China had a substantial impact on counteracting terrestrial carbon sinks [37]. Wang et al. provided evidence suggesting the existence of a positive feedback probably occurred between the greenhouse gas emissions from urban inland waters and climate warming [38].

Some scholars have also conducted research into the coupled system of water, energy, and land. Lin et al. proposed a framework of urban land–energy–water integration to study carbon neutrality in Shenzhen city [2]. Feng et al. studied the water–energy–carbon nexus in Zhengzhou city [1]. Fang et al. used ecology, energy, carbon, and water as indicators to study the ecological footprint [39]. Liu et al. developed a new assessment framework for the water–energy nexus to study carbon emissions in the Beijing-Tianjin-Hebei urban agglomeration [40]. Dong et al. clarified the interactive relationship between urban land-use efficiency, industrial transformation and carbon emissions [41]. Yang et al. launched a study on the energy–carbon nexus and low-carbon city actions [18].

Faced with the fact that global carbon emissions have increased, countries and regions have taken different measures. The EU’s carbon emissions trading system links corporate emission reduction responsibilities with their own interests, and the effective reward and punishment mechanisms better constrain corporate behavior [42]. In China, carbon emissions’ trading platforms have gradually been established in Beijing, Tianjin, Shanghai, Wuhan, Changsha, Shenzhen, and Kunming. Environmental tax measures such as carbon taxes and energy taxes, as well as subsidies for new energy projects and carbon reduction initiatives, can stimulate innovative approaches for businesses to seek green emission reductions [43]. For the United States, the rapid development of renewable energy sources such as photovoltaic is a more important means of reducing emissions [44]. Reducing energy intensity is also an important way to reduce carbon emissions [45]. Abundant natural resources have helped in reducing Russia’s carbon emissions [46]. Currently, many countries and regions, such as the US, are expanding the scope of CCS technology and accelerating the research and development of CCUS technology [47]. In recent years, Nanjing has also reduced carbon emissions by controlling total coal consumption and developing low-carbon agriculture, green transportation and green buildings, but it is still in a weak position in respect of the construction of a carbon trading system and the development and use of carbon technology products.

The urban ecological system is a complex dynamic system with multiple loops, multiple feedback, and nonlinear characteristics. System dynamics is a powerful analytical tool for complex and variable systems with multiple feedback loops. Some scholars have used system dynamics to study complex systems. Zhang et al. used system dynamics to study carbon emissions in urban transportation systems [48]. Liu et al. constructed a system dynamics model for Changsha City to study carbon emissions [49]. Wang et al. used system dynamics to study the contribution of carbon emission reduction by sectors in China [50]. Chuai et al. studied the carbon emission intensity of land systems in the coastal region of Jiangsu, China [30].

In conclusion, scholars at home and abroad have conducted sufficient research on the subject of carbon emissions, and most of them have proposed carbon emission reduction programs from the perspective of energy and land use. However, whilst most of the existing studies have measured the time of carbon peaking, there are very few studies that have measured the time of carbon neutralization using coupled systems based at the urban level. Furthermore, although Nanjing has adopted some carbon emission policies, with the increase in economic development, population and resource consumption pressure, carbon emissions are still at a high level, and the carbon emission reduction system is still not perfect. Therefore, it is important to seek more effective carbon reduction frameworks.

Based on this, this paper selects a comprehensive system dynamics model to explore a methodological system to achieve the carbon neutral goal by establishing a coupled urban land–water–energy system, and simulate the future carbon emission path and dynamic changes under the combined effects of economic development, technological progress, and resource and energy consumption. Taking Nanjing as the research object, this paper studies the relationship between economy, population, water resources, energy, land use and carbon emission systems under the integrated framework of the urban land–water–energy system, and explores whether Nanjing can achieve carbon neutrality in 2060 at the natural development level. Then, using the Vensim PLE 9.3.5 software and system dynamics model, policy simulation scenarios are established to simulate the changes in carbon emission intensity in Nanjing City under different policy conditions, seeking strategies in terms of economy, technology, and resource consumption. This study aims to provide policy recommendations and management guidance for cities to clarify their own carbon emission status, and the research results can provide case support for carbon neutrality in global cities.

2. Materials and Methods

2.1. Theoretical Framework

The coupling system in this paper is divided into three subsystems: the land system, the water system, and the energy system. Land, water, and energy are important components of urban complex systems. They are interrelated and interact with each other [2]. In the process of urban socio-economic development, their synergistic effects result in a significant amount of carbon dioxide emissions. In urban development, the carbon emissions generated from land use, water resources, and energy consumption vary across different industries. The main connections among land, water, and energy in the coupling system are as follows: (1) in the land system, irrigation in farmland is the main way of water consumption, and the use of agricultural machinery is the source of energy consumption; (2) in the water system, water is mainly used in agricultural production, residential life, and industrial production activities; (3) in the energy system, agricultural activities on land and the use of water resources in agriculture, industry, and daily life all contribute to energy consumption.

Considering carbon neutrality from the perspective of a coupling system, exploring the coupling effects of resources such as land, water, and energy and their relationship with carbon emissions is of great scientific significance. Seeking low-carbon and efficient integrated utilization models for water, land, and energy is important for addressing carbon neutrality and global climate change. The research ideas for a coupling land–water–energy system are as follows: (1) based on the Nanjing Statistical Yearbook (2006–2022), Nanjing Water Resources Bulletin and Jiangsu Provincial Statistical Yearbook (2006–2022), variables relating to the five subsystems of population, economy, energy, water and land for the years 2005–2021 were established using IPCC carbon emission factors; (2) the logical and functional relationships between the variables are explored, and a system dynamics (SD) model is established to predict Nanjing’s ability to achieve carbon neutrality in 2060; (3) based on the goal of carbon neutrality, the measures that Nanjing should take in terms of economic development, resource consumption and land use are inversed.

This paper focuses on the carbon emissions related to land, water, and energy systems. The land system includes seven types of land (cultivated land, garden land, forest land, grassland, water area, land for settlements, industrial, and mining, and transportation land). The water system includes produced water, domestic water and sewage water, while the energy system includes coal, petroleum, natural gas and electricity. There are close interconnections among these three systems. Additionally, the population subsystem and the economic subsystem directly or indirectly influence the functioning of the other systems, ultimately impacting carbon emissions in urban areas. The research framework of this paper are presented in Figure 1.

2.2. Study Area

Nanjing (Figure 2) is the capital city of Jiangsu Province, located in the eastern region of China. It covers a total area of 6587.02 square kilometers and serves as an important city at the intersection of the eastern coastal economic belt and the Yangtze River economic belt. Nanjing boasts a strong economy, ranking among the top ten in terms of GDP nationwide. From 2005 to 2021, the permanent population of Nanjing City increased from 6.898 million to 9.423 million, with an average annual growth of 158,000 people. The urbanization rate also rose from 76.24% to 86.9%. However, the rapid development of the city has also brought about a series of issues, such as excessive energy consumption, land use changes, and water resource depletion. The emission of greenhouse gases has significantly increased, making carbon reduction actions urgent.

Nanjing is an energy-consuming city, which has the characteristics of a scarcity of non-fossil energy resources and a heavy reliance on industrial and energy structures. As the capital of Jiangsu Province and a key city in the Yangtze River Delta region, Nanjing’s decisions in dual carbon action have been a topic of high concern for other cities. Therefore, Nanjing’s practices in achieving carbon peak and carbon neutrality can serve as a reference for other cities with similar resource structures and development status.

On 22 September 2020, during the 75th United Nations General Assembly, President Xi Jinping proposed that “China strives to achieve carbon peak before 2030 and carbon neutrality before 2060”. Following President Xi Jinping’s proposal of those dual carbon goals, Nanjing has actively promoted the formulation of carbon peak plans in various fields. The focus is on energy revolution and industrial transformation, with basic requirements of being project-based, list-based, and indicator-based approaches. The city plans to establish a “1 + 3 + 12 + N” low-carbon development policy system, coordinating efforts to achieve the carbon peak and carbon neutrality. In “1 + 3 + 12 + N”, 1 refers to the overall implementation plan for carbon neutrality in 2060, 3 refers to the implementation plan for carbon peaking in 2030, the 14th Five-Year Plan for low-carbon Development, and the implementation plan for green and low-carbon development, and 12 refers to the 12 districts in Nanjing, including Gaochun, Lishui, etc. Each district has formulated its own implementation plan for carbon peaking. N refers to the realization of carbon peaks in various sectors, including agriculture, enterprises, transportation, and buildings. In order to effectively implement the concept of low-carbon and promote green development, it is imperative to simulate and measure the urban carbon emission system of Nanjing through scenario modeling. These efforts will provide valuable insights for decision-makers in controlling carbon emissions.

2.3. Data Sources

The data sources include area of population, economy, water, energy consumption, land use types and other data, and the time span is 2005–2021. The details of data sources are as follows: (1) data on population, economy and energy consumption are from the Nanjing Statistical Yearbook (2006–2022); (2) data on water are from the Nanjing Statistical Yearbook (2006–2022) and Nanjing Water Resources Statistical Bulletin; (3) data on land use are from the Nanjing Statistical Yearbook (2006–2022), the Jiangsu Provincial Statistical Yearbook (2006–2022) and the Jiangsu Province Land Use Change Survey 2005–2021, including cultivated land area, sown area of crops, irrigated area, sown area, fertilizer application, use of agricultural plastic film, and total mechanical power. In this paper, the land use system was divided into seven land use types, which are cultivated land, garden land, forest land, grassland, land for settlements, industrial and mining, transportation land, and water area. Types and areas of land use in Nanjing are shown in

Table 1. Types and areas of land use in Nanjing from 2005 to 2021 (hm²).

Year	Cultivated Land	Garden Land	Forest Land	Grassland	Water Area	Land for Settlements, Industrial and Mining	Transportation Land
2005	245,600	141,062	1,108,919	761	18,117	9889	711
2006	243,690	142,452	1,000,000	720	18,117	9584	737
2007	242,810	146,629	1,106,570	705	18,090	9280	764
2008	242,090	145,984	73,483	703	18,063	8976	790
2009	242,070	13,867	74,400	8600	142,933	8782	1614
2010	239,910	12,467	73,267	8200	142,133	9049	1685
2011	238,860	11,933	72,933	8067	141,200	9187	1756
2012	238,570	11,667	72,333	7933	140,000	9338	1832
2013	238,410	11,333	72,133	7800	139,333	9396	1899
2014	237,190	11,067	71,800	7667	138,667	9485	1939
2015	237,170	10,933	71,533	7533	137,800	9556	1961
2016	236,650	10,733	71,267	7400	137,333	9676	1983
2017	235,580	11,978	99,198	8756	143,418	9759	2007
2018	235,290	13,222	121,129	10,111	148,624	9843	2030
2019	234,670	14,467	155,060	11,467	150,813	9926	2054
2020	234,670	14,235	151,302	11,107	149,854	10,388	2124
2021	142,090	14,353	152,571	11,365	150,701	10,323	2202

. In addition, the carbon emission coefficients used in this paper are from the carbon emission estimation method proposed by the IPCC, which is currently the most widely used and applied method.

2.4. System Dynamics Model

System dynamics is a system simulation method originally proposed by Professor J. W. Forrester for analyzing business problems such as production management and inventory management [51], which was first applied in industry [48] and then used in a wide range of social sciences [50]. System dynamics uses computer simulation techniques to analyze systems, model them and find various countermeasures to solve problems. System dynamics models are better suited to the science of analyzing social, economic, and ecological problems of these highly nonlinear, multivariate systems, and it provides some reference for policy makers by analyzing the feasibility of policies.

The spatial boundary of this research model is set as Nanjing City, with a time boundary from 2005 to 2060. The target year is set as 2060, and the simulated prediction period is from 2022 to 2060, with a simulation time step of 1 year to reduce errors caused by temporal variations in the prediction. Mathematical methods are employed in this study to determine the model parameters, and the Vensim software is utilized to simulate the main trends in land use carbon emissions in Nanjing City from 2022 to 2060. Additionally, scenario simulations are conducted by adjusting controllable parameters to analyze the degree of change in land use carbon emissions under different simulated scenarios.

This paper primarily analyzes the relationship between land use systems, energy systems, water systems and carbon emissions from a systemic perspective. The urban carbon emissions system is divided into five subsystems: the land subsystem, energy subsystem, water subsystem, economic system and population subsystem. The structure of the system is determined by the interactions between these five subsystems. The composition of the urban carbon emissions system and its main feedback mechanisms are shown in Figure 3 and Figure 4.

2.5. Measurement of Carbon Emissions

2.5.1. Carbon Emissions from Land Use

In this part, the main calculation method is to calculate the carbon source and carbon sink of each type of land use in Nanjing, and the net carbon emission of land use in Nanjing is obtained by subtracting the carbon sink from the calculated carbon source.

In this study, the land use types in Nanjing City were divided into cultivated land, garden land, forest land, grassland, water area, land for settlements, industrial and mining, and transportation land. Among them, garden land, forest land, grassland, and water area are carbon sinks; land for settlements, industrial and mining, and transportation land are carbon sources; while cultivated land is both a carbon sink and a carbon source.

The carbon emissions from cultivated land mainly come from the carbon released during agricultural production processes, including fertilizer application, plastic film application, agricultural mechanization, and farmland irrigation. Based on the calculation methods of the IPCC and Wu [52], carbon emissions from cultivated land were estimated by Equation (1):

$$E_{CL}=E_f+E_a+E_m+E_i,$$

(1)

where $E_{CL}$ is carbon emissions from cultivated land, $E_f$ is carbon emissions from fertilizer application, $E_a$ is carbon emissions from agricultural film application, $E_m$ is carbon emissions from agricultural mechanization, $E_i$ is carbon emissions from agricultural irrigation.

$$E_f=G_f\times A,$$

(2)

where $G_f$ is the discounted amount of fertilizer application, A = 0.85754 tC/mg.

$$E_a=F_a\times B,$$

(3)

where $F_a$ is the amount of agricultural film used, B = 0.00384 tC/mg.

$$E_m=A_m\times C+W_m\times D,A_m=S_m\times E$$

(4)

where $A_m$ is the arable sown area, $S_m$ is the cultivated area, $D$ is the replanting index, $W_m$ is the total power of agricultural machinery, C = 0.01647 tC/hm², D = 0.00018 tC/kw.

$$E_i=A_i\times F,$$

(5)

where $A_i$ is the irrigated area, F = 0.26648 tC/hm².

The carbon absorption of arable land relies on the photosynthesis of crops during their growth and development stage. The amount of carbon absorbed by arable land can be calculated based on the quantity of crops planted, the economic coefficient, and the carbon absorption rate. The specific formula is as follows:

$$C_{CL}={\sum }_i{C}_{fi}{Y}_i/H_i,$$

(6)

where $C_{CL}$ represents the carbon sequestration in cultivated land, $C_{fi}$ represents the carbon absorption required for the synthesis of organic matter per unit of crop $i$, $Y_i$ represents the economic production of crop $i$, $H_i$ represents the economic coefficient of crop, which is shown in

Table 2. Economy coefficient and carbon absorption rate of main crop.

Crop	Paddy	Wheat	Corn	Soybean	Potato	Peanut	Rapeseed	Cotton	Sugar Cane	Other Crops
C	0.41	0.48	0.47	0.45	0.42	0.45	0.45	0.45	0.45	0.45
H	0.45	0.40	0.40	0.34	0.70	0.43	0.25	1.00	0.50	0.40

.

Carbon sink land also includes garden land, forest land, grassland and water area, which show little change in carbon emission intensity over a long time range. Carbon absorption from other land is estimated by Equation (7):

$$C_i=S_i\times V_i,$$

(7)

where $C_i$ represents the carbon absorption of land type $i$, $S_i$ is the area of land type $i$, and $V_i$ is the carbon emission coefficient of land type $i$ (

Table 3. Carbon emission coefficient of various land-use types.

Land Use Type	Coefficient
Garden land	−0.21
Forest land	−0.644
Grassland	−0.021
Water area	−0.218

).

Carbon source land also includes land for settlements, industrial and mining, and transportation land. Carbon emissions from these two types of land can be calculated by Equations (8) and (9):

$$E_S=S_1\times V_1,$$

(8)

$$E_T=S_2\times V_2,$$

(9)

where $E_S$ represents the carbon emissions of land for settlements, industrial and mining, $S_1$ is the area of land for settlements, industrial and mining, and $V_1$ is the carbon emission coefficient of land for settlements, industrial and mining. $E_T$ represents the carbon emissions of transportation land, $S_2$ is the area of transportation land, and $V_2$ is the carbon emission coefficient of transportation land. $V_1$ = 314.15 t/ha, $V_2$ = 117.96 t/ha.

The total carbon emissions from the land use system can be expressed by the following equation, Equation (10):

$$E_{LU}=E_{CL}+E_S+E_T-C_{CL}-C_i,$$

(10)

2.5.2. Carbon Emissions from Energy Consumption

Carbon emissions from energy consumption mainly come from various types of production and constructive activities carried out by human beings. The calculation of carbon emissions from energy consumption uses the indirect carbon emission coefficient method, which converts the consumption of different types of fuels into standard coal and multiplies it with the corresponding carbon emission coefficient to obtain the carbon emissions generated from construction land. The fuel consumption data are obtained from the Nanjing Statistical Yearbook (2006–2022), and the carbon emission coefficients refer to IPCC (2006). Carbon emissions from construction land were estimated by Equation (11):

$$E_{EC}={\sum }_k{Q}_k{C}_{fk},$$

(11)

where $E_{EC}$ is the carbon emissions from construction land, $Q_k$ is the consumption of the energy type $k$, $C_{fk}$ is the carbon emission coefficient of the energy type $k$ (

Table 4. Carbon emissions of coefficients of various energy (tC/t).

Energy	Specific Categories	Coefficient
Coal	Raw Coal	0.7559
	Finely washed coal	0.7559
	Other washed coal	0.2155
	Coke	0.855
	Other Coking Products	0.6449
	Coke Oven Gas	0.3548
	Blast Furnace Gas	0.3548
	Other Gas	0.3548
Natural Gas	Natural Gas	0.4483
Petroleum	Crude Oil	0.5857
	Gasoline	0.5538
	Kerosene	0.5714
	Diesel oil	0.5921
	Fuel oil	0.6185
	Liquefied Petroleum Gas	0.5042
	Refinery dry gas	0.4602
	Other Petroleum Products	0.5857
Electricity	Electricity	2.5255

).

2.5.3. Carbon Emissions from Water Consumption

Carbon emissions from water resources are divided into carbon emissions from domestic water use, production water use and wastewater treatment. Water for production is divided into water for agricultural production and water for industrial production. Among them, carbon emissions from agricultural water use mainly come from energy consumption in the irrigation process, and carbon emissions from industrial water use are mainly concentrated in industrial water supply, cooling system water, boiler heating water and other links [53]. Different energy sources consume different amounts of water, and the main stages that generate consumption are different. For example, the water consumption in coal production is mainly related to mining and processing stages. Similarly, oil and gas fields require water during extraction. Thermal power plants consume water mainly in the cooling system.

According to Zhao [53], this paper proposes the following formula for calculating carbon emissions from water consumption:

$$E_{AW}=Q_{AW}\times W_{AW}\times K,$$

(12)

where $E_{AW}$ is the carbon emissions from agricultural water (kg), $Q_{AW}$ is the amount of water used in agriculture (m³), $W_{AW}$ is the energy intensity of agricultural water, taking the value of 0.336 kWh/m³. $K$ is the carbon emission coefficient of electric power, taking the value of 0.801 kg/kWh:

$$E_{IW}=Q_{IW}\times W_{IW}\times L,$$

(13)

where $E_{IW}$ is the carbon emissions of industrial water (kg), $Q_{IW}$ is the industrial water consumption (m³), $W_{IW}$ is the energy intensity of industrial water, taking the value of 2088.4 kWh/m³. $L$ is the carbon emission coefficient of industrial water consumption in Nanjing City, taking the value of 0.642 kg/kWh:

$$E_{RW}=Q_{RW}\times W_{RW}\times M,$$

(14)

where $E_{RW}$ is the carbon emissions of residential water (kg), $Q_{RW}$ is the water consumption of residential water (m³), $W_{RW}$ is the energy intensity of residential water, taking the value of 1696.21 kWh/m³. $M$ is the carbon emission coefficient of the comprehensive energy consumption of residential water in Nanjing, taking the value of 0.601 kg/kWh:

$$E_{ST}=Q_{ST}\times W_{ST}\times K,$$

(15)

where $E_{ST}$ is the carbon emissions of sewage treatment system (kg), $Q_{ST}$ is the volume of sewage treatment (m³), $W_{ST}$ is the energy intensity of wastewater treatment, taking the value of 0.281 kWh/m³.

The total carbon emissions from the water consumption system can be expressed by the following equation, Equation (16):

$$E_{WC}=E_{AW}+E_{IW}+E_{RW}+E_{ST},$$

(16)

In conclusion, the total carbon emissions from Nanjing City can be expressed by the following equation, Equation (17):

$$E=E_{LU}+E_{EC}+E_{WC}$$

(17)

3. Results

According to the previous description of the calculation formula and model, we first analyzed carbon emissions from land use, energy consumption and water consumption in Nanjing City from 2005 to 2021. Then, carbon emissions from 2022 to 2060 were predicted by using a system dynamics model. Through drawing the causal relationship of the carbon emission system and the stock and flow diagram of the carbon emission system, we conducted a scenario simulation of carbon emissions in Nanjing and finally introduced measures to be taken to achieve carbon neutrality.

3.1. Carbon Emissions from Land Use

Table 5. The amount of carbon emissions from land use in Nanjing from 2005 to 2021 (t).

Year	Carbon Emissions from Cultivated Land	Carbon Emissions from Land for Settlements, Industrial and Mining	Carbon Emissions from Transportation Land	Carbon Absorption from Cultivated Land	Carbon Absorption from Other Land	Carbon Emissions from Land Use
2005	127,502.7909	3,104,950.897	83,798.2377	5,225,915.628	747,732.402	−2,657,396.104
2006	115,373.9406	3,009,457.460	86,915.6061	4,416,954.618	677,879.647	−1,883,087.258
2007	106,100.9512	2,913,964.023	90,032.6599	4,447,327.581	747,381.937	−2,084,611.884
2008	84,939.3571	2,818,470.796	93,150.3429	4,739,577.851	81,932.036	−1,824,949.392
2009	80,944.1203	2,757,539.097	190,308.8095	4,643,719.857	82,165.667	−1,697,093.497
2010	77,348.6459	2,841,312.434	198,697.1028	4,625,903.120	80,959.002	−1,589,503.94
2011	72,519.8865	2,884,595.186	207,085.3175	4,577,644.905	80,426.064	−1,493,870.579
2012	70,402.9453	2,932,066.813	215,997.9037	4,661,309.859	79,719.265	−1,522,561.461
2013	69,097.9155	2,950,217.773	223,861.9041	4,784,145.817	79,372.330	−1,620,340.554
2014	66,379.8216	2,978,142.149	228,580.3043	4,800,799.948	78,953.535	−1,606,651.208
2015	63,954.6234	3,000,481.775	231,201.6116	4,741,630.114	78,562.064	−1,524,554.167
2016	63,498.7223	3,038,179.777	233,822.9188	4,285,669.388	78,243.801	−1,028,411.770
2017	60,676.1112	3,064,382.192	236,601.5061	4,007,412.395	97,847.765	−743,600.350
2018	57,645.0290	3,090,584.607	239,380.0933	4,315,147.468	113,396.137	−1,040,933.875
2019	48,891.0549	3,116,787.023	242,158.7592	4,160,731.555	136,014.747	−888,909.465
2020	46,315.5118	3,261,843.279	250,405.8937	4,160,750.839	133,329.232	−735,515.387
2021	45,659.2299	3,241,446.566	259,565.1736	4,178,712.331	134,361.346	−766,402.707

shows the carbon emissions from land systems in Nanjing spanning from 2005 to 2021. The carbon emissions originating from cultivated land have exhibited a consistent decline over the years. This decline can be attributed to the reduction in cultivated land within Nanjing, as well as the government’s efforts in promoting low-carbon agricultural practices. Conversely, carbon emissions stemming from settlements, industrial and mining land, and transportation land have shown a steady increase. This increase reflects the constant rapid urbanization in the region, characterized by an improved urban transportation infrastructure and accelerated urban industrial development. Additionally, the carbon absorption capacity of arable land showed a fluctuating pattern until 2014, followed by a downward trend. This trend may be linked to the reduction in the arable land area and land intensification. The carbon sequestration of other carbon sink types of land shows an upward trend, which indicates that the measures taken by Nanjing to protect forests and waters have achieved good results.

3.2. Carbon Emissions from Energy Consumption

Table 6. The amount of carbon emissions from energy consumption in Nanjing from 2005 to 2021 (t).

Year	Coal	Petroleum	Natural Gas	Electricity	Carbon Emissions from Energy Consumption
2005	11,111,620.13	16,246,760.38	219,372.7247	3,657,755.353	31,235,508.58
2006	12,354,794.18	16,641,998.45	295,755.4124	4,066,542.654	33,359,090.7
2007	12,750,879.79	17,745,855.24	496,668.5776	4,271,281.995	35,264,685.61
2008	12,104,793.69	17,386,453.31	618,752.9846	4,459,688.933	34,569,688.92
2009	12,504,312.64	19,102,931.61	693,436.3912	4,726,512.823	37027,193.46
2010	15,468,995.59	19,230,748.90	728,839.5724	5,298,547.339	40,727,131.41
2011	18,243,389.91	18,978,427.68	1,009,703.916	5,647,699.021	43,879,220.53
2012	19,059,435.48	18,814,969.18	1,057,631.962	6,117,333.092	45,049,369.72
2013	19,092,073.31	21,606,245.96	1,020,688.829	6,288,262.821	48,007,270.92
2014	19,216,227.35	22,212,481.81	1,024,233.396	6,847,685.747	49,300,628.30
2015	19,057,357.83	24,271,536.52	1,096,327.008	6,933,359.003	51,358,580.36
2016	19,225,559.54	25,512,446.25	1,057,757.798	7,277,498.759	53,073,262.35
2017	19,150,696.11	25,348,094.89	1,169,239.585	7,445,569.341	53,113,599.92
2018	18,855,443.13	27,967,244.62	1,190,452.934	7,813,529.079	55,826,669.76
2019	18,725,858.72	26,894,833.64	1,199,240.841	7,929,819.516	54,749,752.71
2020	19,438,659.56	25,976,108.58	1,190,002.118	7,999,371.903	54,604,142.17
2021	19,703,598.43	25,762,645.34	1,131,522.698	8,342,239.893	54,940,006.36

shows the carbon emissions from the energy system in Nanjing from 2005 to 2021. Among them, carbon emissions from coal and electricity consumption show a consistent upward trend, while carbon emissions from oil and natural gas consumption exhibit a declining trend after 2018 and 2019. This decline follows a gradual increase and may be attributed to the government’s efforts in promoting energy conservation and emission reductions within the industrial sector. In general, energy-related carbon emissions in Nanjing City have shown an increasing trend over the years. However, starting from 2018, there has been a gradual stabilization and then a slow decline.

3.3. Carbon Emissions from Water Consumption

Table 7. The amount of carbon emissions from water consumption in Nanjing from 2005 to 2021 (t).

Year	Produced Water	Domestic Water	Sewage Treatment	Carbon Emissions from Water Consumption
2005	903,312.7272	72,073.1502	242,938.9265	1,218,324.804
2006	905,261.7737	85,937.2923	402,699.1695	1,393,898.236
2007	891,463.8451	86,243.1189	319,092.8321	1,296,799.796
2008	857,340.0572	91,743.9212	447,339.4843	1,396,423.463
2009	823,216.2694	97,244.7234	402,449.3296	1,322,910.322
2010	789,092.4815	102,745.5257	413,512.0608	1,305,350.068
2011	754,975.4269	108,246.3279	408,443.2367	1,271,664.992
2012	665,371.8424	93,262.8603	445,955.2361	1,204,589.939
2013	651,000.8299	88,098.4673	600,473.3426	1,339,572.640
2014	610,797.1444	93,767.4743	409,050.9554	1,113,615.574
2015	607,059.9622	87,223.8031	424,117.8775	1,118,401.643
2016	638,759.5152	92,724.6053	444,643.0139	1,176,127.134
2017	643,984.1598	98,225.4076	475,497.1174	1,217,706.685
2018	675,683.7128	103,726.2099	479,654.3634	1,259,064.286
2019	680,908.3574	109,227.0121	480,671.7296	1,270,807.099
2020	712,607.9104	102,675.1856	440,834.6434	1,256,117.739
2021	717,832.5550	96,123.3590	431,288.9582	1,245,244.872

presents the carbon emissions from the water system in Nanjing spanning from 2005 to 2021. Among them, carbon emissions associated with production water and domestic water exhibit a pattern of initial decline followed by a gradual increase starting in 2018. This trend could be attributed to the rapid urbanization witnessed in recent years, which has substantially augmented the water requirements of both the industrial and residential sectors. Conversely, carbon emissions from sewage treatment display a fluctuating pattern. Overall, the carbon emissions from the water system demonstrate a gradual decline with intermittent fluctuations.

3.4. Total Carbon Emissions in Nanjing City

Table 8. The amount of carbon emissions in Nanjing from 2005 to 2021 (t).

Year	Carbon Emissions from Land Use	Carbon Emissions from Energy Consumption	Carbon Emissions from Water Consumption	Total Net Carbon Emissions
2005	−2,657,396.104	31,235,508.58	1,218,324.804	29,796,437.28
2006	−1,883,087.258	33,359,090.70	1,393,898.236	32,869,901.68
2007	−2,084,611.884	35,264,685.61	1,296,799.796	34,476,873.52
2008	−1,824,949.392	34,569,688.92	1,396,423.463	34,141,163.00
2009	−1,697,093.497	37,027,193.46	1,322,910.322	36,653,010.28
2010	−1,589,503.940	40,727,131.41	1,305,350.068	40,442,977.53
2011	−1,493,870.579	43,879,220.53	1,271,664.992	43,657,014.94
2012	−1,522,561.461	45,049,369.72	1,204,589.939	44,731,398.19
2013	−1,620,340.554	48,007,270.92	1,339,572.64	47,726,503.01
2014	−1,606,651.208	49,300,628.30	1,113,615.574	48,807,592.67
2015	−1,524,554.167	51,358,580.36	1,118,401.643	50,952,427.84
2016	−1,028,411.770	53,073,262.35	1,176,127.134	53,220,977.71
2017	−743,600.350	53,113,599.92	1,217,706.685	53,587,706.26
2018	−1,040,933.875	55,826,669.76	1,259,064.286	56,044,800.17
2019	−888,909.465	54,749,752.71	1,270,807.099	55,131,650.35
2020	−735,515.387	54,604,142.17	1,256,117.739	55,124,744.52
2021	−766,402.707	54,940,006.36	1,245,244.872	55,418,848.52

shows the carbon emissions data for Nanjing spanning from 2005 to 2021. Carbon emissions from land use exhibit negative values, indicating that the land in Nanjing absorbs more carbon than it emits. This phenomenon can be attributed to the city’s active promotion of green and low-carbon agricultural policies, as well as the implementation of energy-saving and emission-reducing agricultural technologies in recent years. However, carbon emissions from energy consumption have been steadily increasing over time, with the highest growth rate observed in 2012. It is worth mentioning that the rate of increase has gradually slowed down thereafter. This trend suggests that Nanjing has been actively undertaking measures to reduce carbon emissions in industrial production. Nevertheless, due to rapid urbanization, the city continues to generate a substantial amount of carbon dioxide annually. In contrast, carbon emissions resulting from water consumption have exhibited a declining trend year after year. This trend can be attributed to China’s persistent policy of water conservation, given the country’s vast land area and limited water resources. The decline may also be linked to the adoption of energy-saving and emission-reduction technologies promoted by Nanjing. In summary, the overall carbon emissions in Nanjing City have shown a year-on-year increasing trend, reaching a peak of 56.05 million tons in 2018, followed by a gradual decline.

3.5. System Dynamics Simulation in Nanjing

Figure 5 demonstrates the change in total carbon emissions in Nanjing from 2005 to 2060. The data reveal that urban carbon emissions in Nanjing reached their peak in 2018, with a total of 56.05 million tons. Subsequently, carbon emissions exhibited a gradual decline. However, under natural development conditions, the total carbon emissions of Nanjing are about 31 million tons in 2060, failing to achieve carbon neutrality.

Policy simulation serves as a valuable tool for assessing the impact of specific policies on the overall system. Through policy simulation, we can explore the path for reducing land carbon emissions in Nanjing where carbon emissions are related to land use changes, the economic structure, and energy efficiency. Therefore, this study mainly conducted scenario simulations on five policies (

Table 9. Policy simulation program.

Scene	Feature	Specific Planning Objectives
Original Scene	Natural development	Current policy measures and technological level
Scene 1	Increase investment in science and technology innovation	Rate of change in investment in science and technology innovation will increase by 20%
Scene 2	Water-saving policy adjustment	Water saving policies will increase by 20%
Scene 3	Economic development	GDP economic development will decrease by 20%
Scene 4	Energy consumption for water treatment adjustment	Energy consumption for water treatment will decrease by 20%
Scene 5	Carbon sink level adjustment	Increase carbon sink level by 20%

): adjustment in investment in science and technology innovation, adjustment in water saving policy, adjustment in GDP, adjustment in energy consumption for water treatment, adjustment in carbon sink levels. Figure 6 illustrates the changing trends in total carbon emissions in Nanjing City under five simulated scenarios.

It can be seen that the strengthening of science and technology innovation (STI) inputs, water conservation policies and carbon sink levels can reduce carbon emissions, as a result of which, the total carbon emissions of Nanjing in 2060 will be about 29.5 million tons under the scenario of a 20% increase in STI inputs, 27 million tons under the scenario of a 20% strengthening of water conservation policies, and 20 million tons under the scenario of a 20% strengthening of carbon sink levels, which shows that the positive influence of water conservation policies on carbon emission reduction is stronger than that of STI inputs. Furthermore, when considering the scenarios of a 20% decrease in both GDP and water treatment energy consumption, the total carbon emissions of Nanjing in 2060 will drop to 24 million tons and 22 million tons, respectively. However, in the process of urban development in most cities, the growth in urban GDP and energy consumption is inevitable, thus leading to an increase in carbon emissions.

3.6. Model History Check

The historical data from 2005 to 2021 are substituted into the model for simulation and verification. In this paper, population, output value of primary industry, output value of secondary industry and output value of tertiary industry are selected as test variables from the system, and the historical data are compared with the simulated value calculated by the model. The relative error rate is all within 6%, indicating that the error is within the allowable range. The simulation results of system dynamics are reliable (

Table 10. Relative errors of simulation (unit: %).

Year	Population	Output Value of Primary Industry	Output Value of Tertiary Industry
2005	0.000	0.000	0.000
2006	−0.131	0.000	0.006
2007	−0.275	0.000	0.009
2008	−0.430	0.000	0.016
2009	−0.584	0.000	0.023
2010	−0.744	−0.007	0.029
2011	−1.483	0.000	0.028
2012	−1.626	0.000	0.027
2013	−1.807	4.788	0.027
2014	−2.360	0.000	0.029
2015	−2.537	−0.004	0.031
2016	−2.650	0.000	0.036
2017	−2.813	0.000	0.039
2018	−3.455	−1.057	0.041
2019	−3.552	0.695	0.039
2020	−3.695	0.000	0.038
2021	−3.793	0.003	0.043

).

4. Discussion

4.1. Analysis of Carbon Emission Characteristics in Nanjing

Based on the findings in the third section, this study reveals that the total carbon emissions in Nanjing exhibited an upward trend until 2018, reaching a peak of 56.05 million tons in 2018. Subsequently, the total carbon emissions gradually decreased over the years. It is projected that the total urban carbon emissions will reach about 31 million tons by 2060. Notably, the carbon absorption capacity of the land system in Nanjing displayed a declining trend year by year. It is anticipated that the carbon absorption capacity will reach one million tons by 2060, which may be attributed to the vigorous implementation of carbon emission reduction policies in Nanjing. However, carbon emissions from the energy system in Nanjing exhibited a fluctuating upward trend; however, it is expected that carbon emissions from the energy system will gradually stabilize over time, which can be attributed to energy-saving and emission reduction policies in the industrial sector in Nanjing. Furthermore, carbon emissions from Nanjing’s water system reached a peak of 1.27 million tons in 2019 and have since gradually declined.

The proportion of carbon emissions from the land system in the total carbon emissions of the urban composite system gradually decreased from 7.5% and eventually stabilized at 1%. Conversely, the proportion of carbon emissions from the energy system in the total carbon emissions of the urban composite system increased from 89% in 2005 to 96% in 2021, and is expected to stabilize at around 96% in the future. The proportion of carbon emissions from the water system exhibited a slow decline from approximately 3% in 2005 to 2.1%, eventually stabilizing at around 2.2%. These trends may reflect the rapid urbanization occurring in Nanjing. As cities expand rapidly and populations increase, agricultural land is converted to non-agricultural land, and per capita water resources gradually diminish while industry and services experience rapid growth. Therefore, these findings suggest that carbon emissions from the energy system have a significant impact on the urban carbon system, and future policy development should prioritize energy conservation and emission reduction in the energy sector.

Under the natural development scenario, Nanjing will be able to accomplish the goal of carbon peaking in 2030, but not the goal of carbon neutrality in 2060. Therefore, additional external control measures are necessary to facilitate the realization of the dual-carbon goal.

4.2. Analysis of the Carbon-Neutral Scenario in Nanjing

In urban ecosystems, various factors are interrelated and have a complex relationship with each other. Therefore, examining carbon emissions’ intensity solely through the lens of a single element would impose limitations on research. To overcome this constraint, the implementation of a land–water–energy coupled system becomes crucial as it allows for a comprehensive analysis of the interactions among various elements. This coupled system enables a thorough investigation of the synergistic effects between land, water, and energy, as well as their connections with natural resources and socio-economic systems.

In urban ecosystems, population, economy, land, water, and energy subsystems play vital roles in the functioning of urban operations. Changes in land use structure and area can have direct or indirect impacts on urban carbon emissions by affecting production activities on the land, thereby changing the extent of carbon sources and sinks. The interconnection between water resources and energy is significant, as they are linked through material flows. This study found that when water-saving policies are implemented more rigorously, the energy consumption resulting from water resource consumption decreases. Simultaneously, the water consumption related to energy also decreases, indicating the existence of complex feedback relationships between the water and energy systems. Changes in land use types also influence water and energy consumption. For instance, an increase in the area of carbon sink land leads to a rise in agricultural water consumption, while an expansion in construction land area results in increased energy-related carbon emissions. Therefore, it is crucial to pay close attention to the feedback relationships among the urban system’s internal elements when studying urban systems.

4.3. Discussion on Low-Carbon Policy

4.3.1. Increase Investment in Technological Innovation to Improve Energy Efficiency

Nanjing City should improve energy efficiency and reduce carbon emissions through technological innovation. The government should increase the investment ratio in technological innovation and promote technological innovation in various industries, which is of great significance for achieving low-carbon development in Nanjing City. At the same time, the Nanjing City government needs to provide support and assistance to companies that develop environmentally friendly, low-carbon, and green energy, encouraging them to accelerate research and development and introduce these environmentally friendly energy sources into the market as soon as possible, thereby changing the energy structure of Nanjing City. Nanjing can learn from the EU’s carbon trading system and gradually set up its own carbon emissions trading system, so as to promote enterprises to independently innovate carbon emission reduction technologies.

4.3.2. Expand the Application Scope of Clean Energy

Nanjing should explore external markets for new energy and expand the use of clean energy. The reality facing the Nanjing region is that the conditions for wind power are not ideal, and the production of energy resources is limited. The proportion of energy sources such as hydropower, wind power, and solar power that can be used on a large scale is extremely small. Even in a year with strong winds, wind power generation accounts for less than 0.2% of the annual electricity consumption. Therefore, Nanjing should focus on developing external markets for clean energy and further penetrate the green electricity trading market. It should also increase the use of clean energy from other cities in Nanjing. This requires cooperation at the national and even global level. In the past five years, the proportion of electricity from other regions has increased from 0.5% to about 35%, with clean energy accounting for 39% of the electricity from other region. Promoting the continuous introduction of green electricity in the Nanjing region and promoting the application of bioenergy, solar energy, wind energy, and other clean energy sources in the city are important pathways to achieve the dual carbon goals.

The renewable characteristics are not only in clean energy, but also in buildings. Nanjing, as a highly urbanized city, can continue to promote green building policies, and renewable energy buildings can reduce 560,000 tons of CO₂ per year on average. Green building policies can also be extended to other highly urbanized areas.

4.3.3. Reasonably Plan Land Use Structure

Nanjing should rationalize the use of land resources, strengthen the protection of arable land and develop low-carbon and efficient modern agriculture. It should promote low-carbon agriculture, improve agricultural production efficiency, and improve the ecological environment of agriculture, thereby reducing carbon emissions from agricultural production. According to the simulation results, the carbon sink level per unit area increases by 1%, resulting in a reduction of approximately 80 thousand tons of carbon emissions. Therefore, Nanjing should increase the protection of grassland, forest land, garden land and water area, and fully utilize their functions as carbon sinks. In addition, ecological protection and restoration should be strengthened, so as to improve the carbon sequestration capacity of natural ecosystems. There should be strict control of the scale of construction land and intensive utilization should be achieved through reasonable planning and layout, continuously tapping into the internal potential of construction land.

4.3.4. Water Conservation and Wastewater Utilization

Based on the results of the simulation, the implementation of water-saving policies contributes to the reduction in carbon emissions, so Nanjing should implement a strict water-saving management system and strengthen the management of water-saving policies by setting standards for water use in various industries. In agriculture, it promotes efficient water-saving irrigation technologies such as sprinklers, micro-irrigation and low-pressure pipe irrigation to bring its effective farmland water-use rate closer to the developed countries’ rate of about 0.8. In industry, it promotes the adoption of water-saving processes in factories and the integrated treatment of discharged waste. Nanjing will achieve 25% recycled water utilization rate by 2030.

At the same time, gray water for toilet flushing at the multi-residential scale and black water for landscaping and irrigation are policies that Nanjing can adopt. In addition, Nanjing’s contractual water-saving policy (water-saving service enterprises and water users provide water-saving renovation and management services on a contractual basis, with the benefits of water-saving being shared by both parties) has also been well received and deserves to be further promoted.

4.3.5. Apply Advanced Carbon Utilization Technology

Nanjing will not be able to achieve carbon neutrality by 2060 in its natural state of development, and even with more stringent policy adjustments, it will be difficult for carbon emissions to be rapidly reduced to 0 over time. Therefore, more effective carbon technologies can be considered. For example, Carbon Capture, Storage and Utilization (CCUS) technology can capture, store and reuse carbon emissions from the production process, which is based on Carbon Capture and Storage (CCS) with the addition of Utilization, and has already gained international recognition. CCS technology has been widely recognized internationally. Cormos claimed that CCS technology can significantly reduce specific CO₂ emissions in the power sector and will play an important role in reducing greenhouse gas emissions in the coming decades [54]. Currently, China’s CCUS technology projects are located in 19 provinces, and the types of utilization show a diversified distribution. This technology has great potential to be promoted in the future.

5. Conclusions

This paper analyzes the trends and logical relationships of urban carbon emissions in Nanjing, constructs a system dynamic-coupled system of urban land–water–energy, predicts whether Nanjing can achieve carbon neutrality in 2060, and sets different scenarios of future policy changes to simulate the changes of carbon emissions in Nanjing under different policies. The conclusions are as follows:

Under natural development conditions, Nanjing can achieve the national goal of carbon peaking in 2030. However, there are still 31 million tons of remaining CO₂ that cannot be neutralized. Therefore, it is necessary for Nanjing City to constrain resource consumption and implement more carbon reduction measures to achieve the “net zero carbon emissions” target as soon as possible. The urban system is a complex multi-causal and multi-feedback loop system, where changes in each subsystem can have interconnected impacts on carbon emissions activities in other systems. Therefore, when studying urban development strategies, policymakers need to understand the coupling relationships among multiple subsystems such as energy, land, water, the population, and economy.

Finally, five key points of carbon emission reduction were obtained in the reverse push process: first, increase investment in technological innovation to improve energy efficiency; second, expand the application scope of clean energy; third, reasonably plan land use structure (including increasing land for carbon sinks); fourth, focus on water conservation and wastewater utilization; fifth, apply advanced carbon utilization technology.

In general, this study fills the gap in the literature in studying carbon neutrality from the perspective of urban complex systems, providing a reference for achieving dual carbon goals in other cities in the Yangtze River Economic Belt and cities with similar development status in other countries and regions. At the same time, this study may inspire future research in the following aspects: (1) The operational modes of coupled systems are more complex in real-world scenarios, and this study has not delved deeper into this aspect. It is recommended that more attention is paid to this aspect in future research; (2) Regions with different levels of economic development have different focuses in carbon emission reduction tasks. The research area selected in this study is economically developed cities, and it is suggested to further deepen the research on underdeveloped regions in future studies.