1. Introduction
The quantitative assessment of regional sustainability can evaluate the sustainability of complex nature-society systems at a variety of temporal and spatial scales [
1,
2]. These assessments can help policymakers to understand the complex relationship among various components of regional sustainable development and provide effective guidance for policy-making [
3,
4]. The sustainability assessment of a region needs to not only focus on the overall sustainability state, but also to examine the differences in social, economic, and ecological development within the region [
5]. This dual focus will contribute to future regional planning and decision-making, facilitating the further exploration of potential factors that affect regional sustainable development [
6,
7]. Two perspectives existed in previous studies, strong and weak sustainability [
3,
8]. Strong sustainability assumes that socioeconomic development (or human-made capital) and environmental integrity (or natural capital) are not alternative [
9]. Replacement with human-made capital can be difficult when critical natural capital is damaged; that is, economic development cannot be achieved at the cost of environmental damage [
10]. Weak sustainability assumes that human-made capital and natural capital can be substituted, which supports the idea that economic development at the cost of environmental degradation is also sustainable [
9,
11]. However, weak sustainability is unsustainable in the long term [
2,
6].
A large number of indicators and indicator systems for sustainability assessment (
Table 1) have emerged since the first Earth Summit in 1992, which called for the development of sustainability assessment methods [
3,
12]. Some methods focus one or two dimensions of sustainability, such as the Human Development Index (HDI) developed by the United Nations Development Programme [
13], the Happy Planet Index (HPI) developed by the New Economics Foundation [
14], and the Green City Index (GCI), which was developed by the Economic Intelligence Unit and Siemens [
15]. Meanwhile, some methods include the above three dimensions, such as the Human Sustainable Development Index (HSDI) developed by Togtokh and Gaffney [
16], the City Development Index (CDI), which was developed by the United Nations Human Settlements Programme [
17], and the Sustainable Society Index (SSI) developed by the Sustainable Society Foundation [
18]. These indicators and indicator systems have been accepted as a tool to guide mankind towards the sustainable development goals, because their calculations are relatively simple [
2,
19].
Researchers have evaluated the sustainability of the Beijing-Tianjin-Hebei (BTH) urban agglomeration by using different indicators and indicator systems from the dimensions of society, economy, and the environment. In terms of social and economic dimensions, Yang et al. studied the differences in social-economic development among different provinces in China in 2003 and divided the country into four tiers that are based on the human development index (HDI) [
20]. They found that Beijing and Tianjin were in the first tier with the highest performance, while Hebei Province was in the third tier with a lower performance. Theng et al. calculated and compared the smart city index of 32 major cities in China in 2011, and found that, in the overall scores of smart city, Beijing and Tianjin ranked fourth and 12th, respectively, while Shijiazhuang, the capital of Hebei province, ranked 29th, out of 32 cities [
21]. In terms of the environment dimension, Yu et al. divided China into eight major economic zones, and then calculated and mapped the spatial distributions of the green GDP from 1990 to 2015. The results indicated that the Northern Coastal Economic Zone, which contains Beijing, Tianjin, Hebei, and Shandong, had the highest increase rates of green GDP in all eight zones and the contribution of economic development to green GDP dominated [
22]. Sun et al. established the indicator system of green development, analyzed the evolution process of green development, and divided the green development level of 30 Chinese provinces and cities into four levels. Beijing and Tianjin had the highest scores for sustaining input, and Hebei Province was at the lowest level, with a low sustaining input score [
23]. Gao et al. studied the status of regional freshwater sustainability in the BTH region by constructing the freshwater ecosystem service footprint model and found that most of the regions in the BTH urban agglomeration were in a sustainable development status, except in the regions around Beijing [
24]. Peng et al. divided Beijing into four zones according to the utilization of natural capital. They found that only the second zone did well in balancing efficiency, equity, and ecology, while the other three zones should, in future development, separately emphasize these three aspects [
25]. In summary, most of the studies have focused on only the socio-economic dimension or the environmental dimension, and have not fully covered the three dimensions of sustainable development. In addition, due to the large demand for data, few studies have evaluated sustainability at the city scale in urban agglomeration [
26].
The Human Sustainable Development Index (HSDI) provides a new way to comprehensively assess the sustainability of the BTH urban agglomeration. First, the HSDI adds the per capita CO
2 emissions on the basis of the HDI, which includes environmental factors in the sustainability assessment. This inclusion enables the HSDI to encompass the “three dimensions” of sustainable development, i.e., the environmental, economic, and social dimensions, and it facilitates a comprehensive assessment of regional sustainable development [
28,
29]. Second, in terms of calculating the HSDI, the demand for data is relatively small and the acquisition of data is relatively easy. Most of the data required to calculate the HSDI can be obtained from the statistical data released by government and international organizations, which lays a foundation for a sustainability assessment at the city scale.
This paper aims to evaluate the regional sustainability of the BTH urban agglomeration from multiple scales for the last 15 years. First, the HSDI values of BTH urban agglomeration were calculated at the provincial and city scales in 2000 and 2015. Subsequently, the dynamics of the HSDI in the BTH urban agglomeration was examined in the past 15 years using the changes in the HSDI and cluster analysis. Finally, the paper discusses the main causes of sustainability changes in the BTH urban agglomeration and provides suggestions for regional sustainable development in the future.
4. Results
4.1. Regional Sustainable Development Status of the BTH Urban Agglomeration in 2015
The regional sustainability of the BTH urban agglomeration was above the average level. The
HSDI of the BTH urban agglomeration was 0.85 in 2015, a value that was higher than the average
HSDI of China in 2010, which was 0.83 [
28]. Within the urban agglomeration, there was a large disparity in the HSDI among the Beijing, Tianjin, and Hebei provinces, and smaller differences were found among prefecture-level cities in Hebei Province. Beijing had the highest
HSDI of 0.89 (
Figure 2), which was 5.06% higher than that of Tianjin and 7.86% higher than that of Hebei Province. The
HSDI of all prefecture-level cities in Hebei Province was greater than 0.80, and the discrepancies between prefecture-level cities were relatively small. The highest
HSDI of 0.84 was in Qinhuangdao, which was only 3.58% higher than that in Hengshui.
Cluster analysis showed that the 13 cities in the BTH urban agglomeration in 2015 could be classified into three categories (
Figure 3). The Kruskal–Wallis H test supported that the clustering results were reliable (
Table 3). In terms of
LEI,
GDPI,
EMI, and
HSDI, the three categories of cities were significantly different. The differences in
LEI and
GDPI were significant at the 0.01 level, and the
p values of EMI and
HSDI were less than 0.05. In terms of
EI, the
p value was 0.077, which suggested that it passed the significance test at the 0.1 level.
Among the three categories of cities, the first category of cities, which included Beijing and Tianjin, had the highest level of overall sustainability, economic sustainability, and healthy sustainability. The average HSDI, GDPI and LEI values for the cities in the first category were 0.87, 0.92 and 0.91, respectively. In terms of environmental sustainability, cities in the first category were at an intermediate level, and the EMI of these cities was 0.839, which was slightly higher than that of cities in the second category (i.e., 0.81) and smaller than that of cities in the third category (i.e., 0.91). For educational sustainability, the cities in the first category showed the lowest EI, which was 0.82.
The second category (i.e., Shijiazhuang, Langfang, Cangzhou and Tangshan) included the cities with an intermediate overall sustainability level and a low environmental sustainability level. The average HSDI of these cities was 0.83. The environmental sustainability level of this category was the lowest among the three categories, and the EMI was only 0.81, which was 12.50% lower than that of the third category. In terms of the educational sustainability level, the second category of cities had the highest EI, which was 0.86.
The third category consisted of the cities with a high level of environmental sustainability but low levels of overall sustainability and economic sustainability. Those cites included Qinhuangdao, Chengde, Handan, Hengshui, Xingtai, Zhangjiakou, and Baoding, in which the highest EMI was 0.91, the lowest HSDI was 0.82. Among these cities, the GDPI was significantly different from that of the other two categories. The GDPI in the third category was 22.17% and 11.74% lower than those in the first and second categories, respectively.
4.2. Regional Sustainable Development Dynamics of the BTH Urban Agglomeration from 2000 to 2015
From 2000 to 2015, the regional sustainability of the BTH urban agglomeration showed an overall increasing trend (
Figure 4). The
HSDI increased from 0.772 to 0.849, with an increase of 0.078 and a growth rate of 10.1%. From the perspective of the three dimensions of sustainability, the levels of economic and social sustainability of BTH urban agglomeration increased, and the level of environmental sustainability decreased from 2000 to 2015. The
GDPI, which represented economic sustainability, increased from 0.584 to 0.830, with an increase of 0.246 and a growth rate of 42.0%. The
LEI and
EI, representing social sustainability, increased from 0.804 and 0.807 to 0.859 and 0.844, with growth rates of 6.8% and 4.6%, respectively. The
EMI, which represents environmental sustainability, decreased from 0.935 to 0.864, with a decrease of 0.071 and a change rate of −7.6%. Among the changes in the three dimensions, the change rate of economic sustainability was the largest. The variation in
GDPI was 5.00, 6.25, and 3.57 times the variation in
LEI,
EI, and
EMI, respectively.
The 13 cities in the BTH urban agglomeration can be divided into two categories, according to the changes of EMI and GDPI (
Table 4,
Figure 5). The Mann–Whitney U test results showed that the clustering results were also reliable (
Table 5). In terms of the changes of GDPI and EMI, the two categories of cities are significantly different, and they passed the significance test at the 0.01 level.
The first category included cities with rapid growth in economic sustainability and a salient decline in environmental sustainability; these cities included Tianjin, Shijiazhuang, Tangshan, Chengde, Cangzhou, and Langfang. The average change in the GDPI (which represents economic sustainability) of such cities was 0.255, with an average increase of 45.1%. The EMI, which represents environmental sustainability, decreased by 0.111, which was a 12.0% decline. Among them, Tangshan is an example of a city that had relatively slow economic development as environmental sustainability declined. From 2000 to 2015, the economic sustainability growth of Tangshan only ranked seventh in the BTH urban agglomeration, but its environmental sustainability exhibited the greatest decline. During 2000–2015, the GDPI in Tangshan increased by 41.1%, which was smaller than the overall change in GDPI in Hebei Province (41.8%) and the overall growth of the GDPI in the BTH urban agglomeration (42.0%). Meanwhile, the reduction rate of the EMI in Tangshan was the largest among the 13 cities in the BTH urban agglomeration, accounting for 17.5%, which was 2.32 times the value of the EMI reduction rate (7.6%) in the BTH urban agglomeration.
The second category of cities has a low degree of environmental degradation with economic development. The cities included Beijing, Qinhuangdao, Hengshui, Baoding, Xingtai, Handan, and Zhangjiakou. These cities had a small decline in the EMI as the GDPI increased. Beijing and Zhangjiakou are examples of cities with rapid economic growth and a low degree of environmental degradation. Among them, Beijing had the smallest degree of environmental deterioration with economic growth. The EMI in Beijing decreased by only 0.8% between 2000 and 2015, which was much lower than the overall EMI declining rate of 7.6% in the BTH urban agglomeration; the value for Beijing also represented the best value in the BTH urban agglomeration. From 2000 to 2015, the growth rate of GDPI in Zhangjiakou was 52.2%, ranking third among the 13 cities in the BTH urban agglomeration, and the EMI reduction rate was 3.2%, being second only to Beijing.
4.3. The Correlation between Evnrionmental Sustainability and HSDI
The decline of environmental sustainability in the BTH urban agglomeration has played an important role in inhibiting regional sustainable development (
Figure 6). The changes in the regional
EMI were positively correlated with the changes in the
HSDI, with a Pearson correlation coefficient of 0.50 (
p < 0.1). This result implied that the cities with a higher degree of decline in environmental sustainability in this urban agglomeration were accompanied by a smaller increase in overall sustainability. Additionally, the Pearson correlation coefficients between the increase of the
HSDI and the increase of the
LEI,
EI and
GDPI were 0.10 (
p = 0.739), 0.42 (
p = 0.155), and 0.45 (
p = 0.126), respectively. The result suggested that an increase in social or economic sustainability in the region did not necessarily lead to an increase in overall sustainability.
Among the 13 cities in the BTH urban agglomeration, Tangshan was the representative city of the regional sustainable development restricted by the decline in environmental sustainability. Tangshan had the smallest increase in overall sustainability and the largest decrease in environmental sustainability. From 2000 to 2015, the HSDI of Tangshan increased by 0.056, which was the lowest rate of increase among all cities. The EMI reduction in Tangshan was the greatest, with a decrease of 0.159. In contrast, Beijing and Zhangjiakou were representative cities in which the decline of environmental sustainability had less impact on regional sustainable development. The overall sustainability of these cities significantly increased, while the environmental sustainability decreased slightly. From 2000 to 2015, the increase of the HSDI in Beijing was relatively large, with an increase of 0.085, ranking the third in the BTH urban agglomeration. Meanwhile, its EMI reduction was the smallest among the 13 cities in the BTH urban agglomeration, decreasing by only 0.007. From 2000 to 2015, the HSDI in Zhangjiakou increased by 0.103, ranking the second in the BTH urban agglomeration. The reduction of environmental sustainability was small and the EMI decreased by 0.030, a value that was only higher than that of Beijing.
6. Conclusions
From 2000 to 2015, the overall sustainability of the BTH urban agglomeration showed an increasing trend, with an HSDI growth rate of 10%. Among the three dimensions of sustainability, economic sustainability improved the most, while environmental sustainability declined. The change rates of the GDPI and EMI were 42% and −8%, respectively. The environmental sustainability of six cities, including Tianjin, Shijiazhuang, and Tangshan, decreased by 0.111, which was 56.3% higher than the regional average.
The decline in environmental sustainability became an important factor limiting regional sustainable development. A significant positive correlation was found between changes in the regional EMI and HSDI, and the Pearson correlation coefficient reached 0.50 (p < 0.1). In contrast, the HSDI was not significantly correlated with the LEI, EI, or GDPI. Urban energy structure optimization played an important role in affecting environmental sustainability. Among the six cities with regional average declines in environmental sustainability, the proportion of the CO2 emissions that were generated by coal use in the total CO2 emissions decreased by only 1.8%, which was far smaller than the 12.0% decline that was observed in the other seven cities in the region. In the future, the development of the BTH urban agglomeration should focus on improving environmental sustainability while achieving economic development. Optimizing the energy structure in the region will be a powerful path for building a resource-saving and environmentally friendly society.