Abstract
Global greenhouse gas emission, major factor driving climate change, has been increasing since nineteenth century. STIRPAT and CEVSA models were performed to estimate the carbon emission peaks and terrestrial ecosystem carbon sinks at the provincial level in China, respectively. Utilizing the growth characteristics and the peak time criteria for the period 1997–2019, the patterns of energy consumption and CO2 emissions from 30 Chinese provinces are categorized into four groups: (i) one-stage increase (5 provinces), (ii) two-stage increase (10 provinces), (iii) maximum around 2013 (13 provinces), and (iv) maximum around 2017 (2 provinces). According to the STIRPAT model, the anticipated time of peak CO2 emissions for Bei**g from the third group is ~ 2025 in both business-as-usual and high-speed scenarios. For **njiang Uygur autonomous region from the first group and Zhejiang province from the second group, the expected peak time is 2025 to 2030. Shaanxi province from the fourth group is likely to reach carbon emission peak before 2030. The inventory-based estimate of China’s terrestrial carbon sink is ~ 266.2 Tg C/a during the period 1982–2015, offsetting 18.3% of contemporary CO2 emissions. The province-level CO2 emissions, peak emissions and terrestrial carbon sinks estimates presented here are significant for those concerned with carbon neutrality.
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Introduction
Global greenhouse gas emissions have been increasing since the nineteenth century, recognized as a major factor driving global warming and climate change. More than 127 countries, especially parties to the Paris Agreement, have made individual climate pledges to cut down greenhouse gas emissions since 20151,2. China pledges to achieve the peak of carbon dioxide (CO2) emissions before 2030, and carbon neutrality before 2060. This forces China to take the lead in reducing CO2 emissions to net zero in the long run3. Understanding the balance between Chinese carbon emission and sink is crucial to achieve net zero carbon emissions.
Energy consumption is the predominant source of greenhouse gas emissions. In China, coal accounts for ~ 56.8% of total energy consumption in 20204. The high-emitting and low-efficiency utilization of coal, together with construction and cement production, positions China currently being the largest emitter of CO2 in the world5. Despite comprehensive summaries of China’s energy consumption and CO2 emissions growth patterns in the past decades6,7,8,9,10,11,12,13,14, discrepancies arise in gigatonnes gap of CO2 emissions due to different energy datasets. Notably, province-level growth patterns are limited reported. With respect to the future trajectories of China’s CO2 emissions, several approaches (e.g., environmental Kuznets Curves, grey model, Markov model, and STIRPAT model) were performed to predicate the optimal year to reach carbon emissions peak and cumulative emissions15,16,17. Significant differences of the optimal year and peak emissions are presented in these studies due to the different scenarios’ setup, calculation models and regression methods.
Terrestrial ecosystem (e.g., forests, agriculture, grasslands, and wetlands) is closely coupled to the climate system. This ecosystem is suggested to be the most cost-effective and readily available player to absorb CO2 and other greenhouse gases18,19. Numerous estimates and models have been employed to calculate China’s terrestrial carbon sinks, yet consensus remains elusive due to the uncertainty of parameters and observations20,21,22,23,24,25. These studies reveal that China’s terrestrial carbon sinks have a wide range from 70 to 1910 Tg C/a, and mostly fall into the range from 100 to 450 Tg C/a. Regional estimates of terrestrial carbon sinks have also been conducted, exposing significant disparities in different studies, such as in Qinghai-Tibet Plateau (43.16 Tg C/a26 vs. 182 Tg C/a27) and Guangdong province (53.2–54.5 Tg C/a28 vs. 69 Tg C/a29).
To estimate province-level carbon emissions peak and terrestrial carbon sinks, STIRPAT (Stochastic Impacts by Regression on Population, Affluence, and Technology) and CEVSA (Carbon Exchange between Vegetation, Soil, and the Atmosphere) models are performed in this study. China and provinces-level energy consumption for the period 1990–2020 and CO2 emissions values for the period 1997–2019 are also presented. While China comprises 34 provinces (autonomous regions and municipalities), this study focused on 30 provinces for calculating their CO2 emissions and 33 provinces for assessing terrestrial ecosystems carbon sinks, based on the currently available data. This study aims to: (i) present the growth patterns of energy consumption and CO2 emissions in 30 provinces; (ii) estimate carbon emissions peak and corresponding time for 4 typical provinces; and (iii) calculate 33 provinces’ terrestrial ecosystems carbon sinks and also discuss their implications for achieving carbon neutrality.
Methods and data
Energy-related carbon emissions accounting
Following previous methods7,8, CO2 emissions generated by energy consumption were calculated as follows:
where CS refers to the fossil fuels consumption by the fuel type x; NCV represents net caloric value generated by per unit of consumption; CC refers to CO2 emission per net caloric value by fuel; and OE is the oxygenation efficiency, respectively. Moreover, the increment value and rate of provincial CO2 emissions were defined as follows:
where EMISSION represents CO2 emissions of a province in one year; the subscript a rerefers to provinces; and b as well as c is a specified year.
STIRPAT model and scenarios
Compared to other methods (e.g., logarithmic mean Divisia index (LMDI)), STIRPAT model could examine more impact factors towards environment30. This model has become an increasingly dominant method in examining the impact factors for CO2 emissions17. This is a stochastic regression model related to population, affluence and technology parameters, defined as follows:
where I is a given environmental indicator; P is population; A is affluence (i.e., GDP per capita); T is technology; a is a constant term; b, c and d are the exponential parameters estimated; and e is the random error, respectively. This formula (4) could be re-written as follows:
where I is replaced by C (carbon emissions) in this study. Following previous studies17,31,32 and integrating additional factors, STIRPAT model is extended as follows:
where ET is carbon emission intensity; EC is energy consumption intensity (tonne of standard coal equivalent/ten thousand RMB); ES is energy structure (coal consumption/total energy consumption); IS is industrial structure (secondary industry/GDP); UR is urbanization rate; f, g, h and i are the exponential parameters estimated, respectively.
Due to the multicollinearity of above independent variables, the ordinary least square (OLS) regression generally fails to bring forward the reliable relationship between carbon dioxide emissions and those factors. Thus, ridge regression33 is used here to improve the accuracy and reliability of parameter estimation.
To better estimate provincial carbon emissions trajectories, three types of scenarios/ sensitivity analysis, including high-speed, business-as-usual (BAU) and low-speed, are performed here. In the BAU model, the annual increment rates of factors are set in accordance with the 14th Five-Year Plan related to energy transition, urbanisation, and investment34,35 and 2035 domestic goals. Compared to BAU scenario, increment rates are set somewhat lower or higher than those in low-speed and high-speed scenarios, respectively. The detailed increment rates of these factors of four provinces (i.e., **njiang, Zhejiang, Bei**g, and Shaanxi provinces) are listed in supplementary Table S1.
CEVSA model
Carbon fluxes in terrestrial ecosystem is controlled by eco-physiological and environmental factors, such as vegetation pattern and structure, photosynthesis, temperature, water, and nutrients. To estimate the controls of these factors on carbon fluxes, CEVSA model is used in this study. This model is predominantly composed of a biophysical sector calculating the transfers of heat and water; a plant growth sector related to photosynthesis, autotrophic respiration, carbon and nitrogen allocation and accumulation among plant organs, leaf area index and litter production; and a soil sector simulating decomposition of organic carbon and the inputs and outputs of nitrogen. Detailed descriptions, explanations, parametrization, and calculations for this model have been documented in previous work18,36. Using observation-based data of climate, vegetation, soil, and atmospheric CO2 for the period 1982 to 2015, the CEVSA model was run to analyse the terrestrial carbon sinks of 33 provinces.
Data
Chinese energy consumption and CO2 emission data are derived from earlier studies37,38,39. Additionally, 30 provinces-level energy consumption and CO2 emission data are primarily obtained from previous studies4,7,8,40. No data in Tibet, Hong Kong, Macao, and Taiwan. Due to data availability, the research period for energy consumption and CO2 emission data are from 1990 to 2020 and from 1997 to 2019, respectively.
Population, GDP per capita, carbon emission intensity, energy consumption intensity, energy structure, industrial structure, and urbanization rate data of four provinces used in STIRPAT scenarios are from the 14th Five-Year Plan and 2035 domestic goals, available on provincial government websites (e.g., https://www.bei**g.gov.cn/gongkai/shuju/) and previous studies4,7,8. Primary data performed in CEVSA model are collected from National Bureau of Statistics of China. The research period used in CEVSA model is from 1982 to 2015 due to data availability.
Result
Energy consumption and CO 2 emissions
National and provincial level for the past 23 years
Energy consumption in China shows a slow increase from 1990 to 2002, a significant increase from 2003 to 2013, and a slight increase since 2013 (Fig. 1). Coal is the largest share in the total consumption, and its share yields a significant decrease from 72.5% in 2007 to a value of 56.8% in 2020. Meanwhile, the percentage of oil shows a stable increase from 16.4% in 2009 to 18.9% in 2020. Gas and other energy (hydroelectricity, renewables, and nuclear) have share values of 8.4% and 15.9%, respectively, in 2020.
Likewise, CO2 emissions in China is characterized by a slow growth from 1990 to 2002, a significant increase from 2002 to 2013, and a slight increase since 2013 (Fig. 1). Compared to 1997, 28 provinces yield similar CO2 emissions in 2002 (Fig. 2A,B). From 2002 to 2013, these provinces’ CO2 emissions show a robust increase with an average increment rate of 210% (Fig. 2C). In detail, Shanxi and Inner Mongolia yield the highest (620%) and second highest (490%) rate, respectively. 30 provinces (adding Ningxia and Hainan provinces) yield similar CO2 emissions in 2019 to their counterpart in 2013 (Fig. 2D).
Based on the growth variation and the time of reaching the maximum for the period 1997–2019, the growth patterns of energy consumption and CO2 emissions from 30 China provinces are divided into four groups (Figs. 3, 4): (i) one-stage increase; (ii) two-stage increase; (iii) maximum around 2013, and (iv) maximum around 2017. Five provinces (** Tools package (version 5.4.1) based on the standard map of China No. GS (2023) 2767.
The third group consists of 13 provinces, including Qinghai, Sichuan, Chongqing, Henan, Anhui, Heilongjiang, Bei**g, Shanghai, Jiangsu, Hubei, Jilin, Hebei and Tian**, dominantly located along Yangtze River or in the northern China. These provinces reach the peak of both historic energy consumption and CO2 emissions in 2012 or 2013. Bei**g city yields stable energy consumption after reaching the maximum. The fourth group is composed of Shaanxi and Guizhou provinces, whose historic CO2 emissions reach the maximum in 2016 or 2017. Compared to Guizhou province, Shaanxi province is characterized by significantly high energy consumption and carbon emissions.
Peak carbon emissions and times of 4 provinces
STIRPAT model shows that **njiang, an example of the “one-stage increase” group, might reach peak emission around 2030 in the context of three scenarios (Fig. 6A). Zhejiang province from the “two-stage increase” group is estimated to achieve peak emissions around 2025–2030 in three conditions (Fig. 6B). Bei**g, the case study for the “maximum around 2013” group, likely reaches peak emission around 2020 in low-speed setting, and yields peak values around 2025 in the context of BAU and high-speed scenarios (Fig. 6C). The optimal year of peak emissions for Shaanxi province from the fourth group is around 2025 in high-speed scenario (Fig. 6D).
Prior to peak emissions, the cumulative CO2 emissions in high-speed scenario of these four provinces are higher than those in low-speed and BAU scenarios. This tendency is further shown in **njiang Uygur autonomous region and Zhejiang province after reaching the peak of emissions. However, Bei**g and Shaanxi provinces have cumulative CO2 emissions of high-speed scenario lower than those in the other conditions after emissions peak. In the context of the three scenarios, the lowest peak CO2 emissions for **njiang, Zhejiang, Bei**g, and Shaanxi are 636.06, 386.94, 58.37 and 564.90 Mt, respectively.
Terrestrial ecosystems CO 2 sinks
Based on CEVSA model, China’s terrestrial ecosystem yields carbon sinks of ~ 266.2 Tg C/a (or CO2 uptakes of ~ 976.2 Mt/a) for the period 1982–2015. Overall, the northern and western China yield higher values than the rest of areas (Fig. 7A, Table 1). Among 33 provinces, Heilongjiang province has the largest CO2 uptakes with value of 94.7 Mt/a, while Hong Kong yields the smallest value of 0.1 Mt/a.
Annual carbon sequestration by China’s terrestrial ecosystems offsets 18.3% of its CO2 emissions for the studied period. The northern and western China have higher ratios of CO2 uptakes compared to CO2 emissions (mainly ranging from 20 to 40%) than the other provinces (< 20%, Fig. 7B). Notably, Qinghai is the only province with CO2 uptakes higher than coeval CO2 emissions.