4.4. Regional Difference in Carbon Emissions and its Decomposition
During the study period, the overall difference acquired from the Theil index and the relevant results of emissions decomposition in China regions were showed in
Figure 2. From
Figure 2, the total Theil index (overall difference) of carbon emissions in the eight major economic regions exhibited a fluctuating ascending trend of “rising-falling-rising,” ranging from 0.37 to 0.48 during 2005–2017. That is to say, the total Theil index rose slowly in 2005–2008, and then declined slightly from 2008 to 2010, finally continued to increase in 2010–2017, with a mean annual growth rate of 3.21%. This implied that the overall difference in carbon emissions at the regional level remained a strong expansion tendency. As for the reasons, it may be associated with the increasing differences of regional economic growth, industrial structure, and policy orientation, along with the imbalanced energy production and consumption during the study period.
From the perspective of the source of the overall difference, the inter-regional difference in carbon emissions varied from 0.275 in 2005 to 0.371 in 2017, which was far higher than the corresponding intra-regional differences. However, this was contrary to the results obtained by Yang and Liu [
14], who observed that the intra-regional difference of carbon emissions was far greater than the inter-regional differences. That may be attributed to the too large research unit, because Zhao’s research was based on the three regions of East, middle and West China. Moreover, the variation trend of inter-regional difference was almost similar to that of the overall difference from 2005 to 2017 (
Figure 2). This result indicated that the overall difference was mainly caused by the inter-regional difference, which could be confirmed by the contributions of inter-regional and intra-regional differences. We found that the contribution of inter-regional differences showed a slightly increasing trend, whereas that of intra-regional inequality indicated a downtrend (
Table 2). For instance, the contribution rates of inter-regional differences to the overall difference based on the Theil index were 72.55% in 2005 and 77.20% in 2017. The contribution rate of intra-regional difference to the overall difference in 2005 was 27.45%, falling to 22.80% in 2017. This showed that the contribution of inter-regional difference only had a slight increase during the research period, but future policy making and implementation should consider balancing inter-regional differences in carbon emissions.
For the inter-regional difference, it appeared that the Theil index was distinctly different, and showed the differentiation trend for various regions. As demonstrated in
Figure 3, during the entire study period (2005–2017), the ERMRYR had the largest Theil index with a mean value of 0.115, followed by the NWER, SCER, and ECER, while the ERMRYTR showed the smallest inter-regional difference based on the Theil index, with a mean of 0.004. Except for an obvious uptrend of Theil index in the ERMRYR and NWER, other economic regions only presented marginal fluctuations in Theil index. These results demonstrated that the ERMRYR was the largest contributor in total inter-regional differences increase, followed by the NWER, SCER and ECER (
Table 2). Compared with the inter-regional difference, the intra-regional difference based on the Theil index remained basically unchanged except for a slight rise (
Figure 2), but the intra-regional difference in various economic regions showed a trend of differentiation in 2005–2017. From
Figure 4, it is obvious that the NCER had the largest intra-regional difference with an average of 0.058 and a frequent fluctuation, followed by the ERMRYR, whose intra-regional difference witnessed a fluctuating upward trend, with a mean value of 0.025; while the other six economic regions only showed smaller intra-regional difference indexes, all of which were less than 0.01. This indicated that the NCER and ERMRYR were the largest two contributors in total intra-regional difference increase. However, the contribution of total intra-regional difference to the overall difference of carbon emissions was relatively small (
Table 2).
From the perspective of contribution rate (
Table 2), the contributions of total inter-regional difference to the overall difference in China’s carbon emissions were more than 72.50% in all years (2005–2017) and experienced an uptrend in this period, which indicating that carbon emissions between economic regions displayed significant divergence features. In China’s eight economic regions, the ones with a large contribution of inter-regional difference to the total inter-regional difference were the ERMRYR, the SCER, the ECER, and the NWER. The four regions jointly contributed to over 83.87% of the total inter-regional difference every year and showed a trend of fluctuation in the study period. Among them, the contribution of inter-regional difference of the NWER kept a continuous rising trend, which was in line with the change of inter-regional difference in the NWER described in
Figure 3. With regard to the contribution of intra-regional difference, despite not being as large as that of inter-regional difference and exhibiting a gradual downward trend (the contribution rate decreased from 27.45% in 2005 to 22.80% in 2017, with an average of 24.93%, which showed that the carbon emission within the region was converging), there were also obvious discrepancies in the contribution of intra-regional (inter-provincial) difference to the total intra-regional difference across regions. As illustrated in
Table 2, the top contributor to the total intra-regional difference appeared in the NCER, in which the contribution had a clear fluctuation, and increased sharply since 2016, with an average value of 58.0% between 2005 and 2017. It was closely followed by the ERMRYR, in which the intra-regional difference caused more than 19.83% contributions in 2005–2017, with a fluctuating upward trend, indicating that intra-regional differences between provinces in the ERMRYR were gradually broadening during the study period. Conversely, the ERMRYTR had the lowest contribution to total intra-regional difference, with an average of 0.337%, being far below all the regional average of 12.80%. Also, the intra-regional contributions of the remaining five economic regions were at medium levels and fluctuated between 0.263% and 8.255%, representing the average level of most regions.
From the above analysis, we can conclude that the inter-regional differences in carbon emissions and the internal differences in some regions such as the NCER and the ERMRYR were the main reasons for the overall difference in carbon emissions of China’s eight main economic regions during the study period, which also means that China’s carbon emission is not only different between regions, but also there is a gap that cannot be ignored between provinces within the region.
4.5. Influencing Factors of Regional Differences in China’s Carbon Emissions
The above results showed that there was an obvious spatial imbalance of carbon emissions in China’s eight major economic regions. In order to reveal the reasons for the regional differences in carbon emissions, we adopted the PLS-VIP method to explore the influences of various factors on carbon emissions in different economic regions. Considering whether it is necessary to use PLS-VIP method for regression analysis, a collinearity diagnosis on the influencing factors (i.e., explanatory variables) was first performed before using the PLS-VIP analysis. The diagnostic result confirmed that there was strong collinearity among explanatory variables because the values by a variance inflation factor (VIF) of explanatory variables (except the energy structure) were all greater than 10 (
Table 3). According to the method described in
Section 3.3, the interpretative test and the cross-validity test were used to determine the final cumulative explanatory ability of the principal components extracted from the factors affecting carbon emissions to the explanatory variables
X and the response variable
Y, as well as the cumulative cross-validity values of the principal components. As exhibited in
Table 4, in each economic region, the cumulative variance contribution rate of the principal components to the explanatory variables was more than 0.820, the cumulative variance contribution rate to the dependent variables was above 0.914, and the cumulative cross-validity was greater than 0.818. These results showed that it was reasonable to further use the PLS method to perform regression analysis based on the econometric model for carbon emissions.
Table 5 and
Table 6 were respectively the regression results and the explanation abilities of various influencing factors on carbon emissions based on PLS-VIP method in each economic region. From the perspective of average value, among all the explanatory variables, the population size had the greatest impact on carbon emission, followed by the energy intensity, economic development level (the square term of GDP per capita was included in the economic development factor), urbanization, energy structure and industrial structure, which was different from the importance ranking of each factor’s average explanatory power (i.e., VIP value) for carbon emissions in the eight regions (
Table 5 and
Table 6). In terms of population, the population size of the eight economic regions was positively correlated with carbon emissions, with the elasticity coefficients ranging from 0.257 to 0.809 and an average of 0.493. The VIP values of the population size in all regions were above 1.0. This result indicated that population size had significantly positive effects on emissions growth and a strong ability to explain carbon emissions. It also suggested that the difference in population size was the main factor causing the difference in carbon emissions between regions. From
Table 5, the regions with large population-scale effect on carbon emission increase were mainly the NCER, ECER, and SCER. Their elastic coefficients were 0.809, 0.798, and 0.607, respectively. The reasons for this may include the following two aspects: First, these regions have developed economy and superior geographical location [
57]. Second, there are three municipalities directly under the central government—Beijing, Shanghai, and Tianjin in the NCER and ECER. Due to these reasons, the above regions attracted a large number of surplus labors from other regions, which led to the continuous increase in population of these three regions. For example, based on our calculation, from 2005 to 2017, the annual average growth rate of population in the NCER, ECER, and SCER were 1.24%, 1.10%, and 1.49%, respectively. As a result, with the improvement of residents’ living standards, the impact of population-scale in these three regions on carbon emission increase was inevitably greater than that of the other regions in our study. This further confirmed Deng et al.’s conclusion that population-size effect had a great impact on carbon emissions in such regions including municipalities directly under the central government [
37]. Therefore, to control the population in the above regions and eliminate the uneconomical phenomenon of carbon emission caused by population-scale will play an important role in promoting carbon emission reduction. Furthermore, due to the influences of western development and the rise of central China, the population in the SWER, ERMRYR, and ERMRYTR obviously moved inward, which led carbon emissions to also have a certain population-scale effect. However, in the NEER and NWER, because there was no large internal migration in population [
25], the population size effect was relatively weak compared to that in the above three regions (
Table 5).
For the impact of economic development on carbon emissions, the elastic coefficients of GDP per capital in the eight economic regions were all positive, with a range of 0.226 to 0.762, and the average ranked the second among all factors, indicating that economic growth was the major factor to promote regional emissions increase during the study period. Specifically, the positive effect of GDP per capital on carbon emissions was the largest in the NCER and ECER, followed by that in the SWER, NEER, SCER, ERMRYTR, and NWER, while the per capital GDP in the ERMRYR showed the smallest positive effect in emissions increase (
Table 5). This meant that economic development levels resulted from different development conditions (e.g., location difference, resources endowment and policy support, etc.) were distinctly various in different regions, and thus had different impacts on regional carbon emissions. From the perspective of the VIP value, the explanatory ability of GDP per capital in the eight major regions varied from 0.984 to 1.253, with an average of 1.091, ranking second in all the factors (
Table 6). It showed that economic development had a strong ability to explain carbon emissions in all regions. Among them, in the NWER, the economic development had the strongest ability to explain carbon emission, indicating that it played a decisive role in the increase of carbon emissions in this region. Nevertheless, the weakest interpretability occurred in the ECER, in which the VIP value was 0.984. The reason may be that the ECER belongs to developed regions. With the enhancement of emission reduction awareness and the promotion of emission reduction technologies, the ability of economic development to explain carbon emissions was gradually weakened. Comprehensive considering the elasticity coefficient and explanatory capacity of per capital GDP to carbon emission, we can conclude that economic growth was the second driver for the differences in regional carbon emission, which was similar to the results reported by some previous researchers [
14,
19]. They found that per capital GDP was an important factor affecting the difference of carbon emission levels in East, Central, and West China. It is interesting to note that the square term of GDP per capital had a relatively weak effect on regional carbon emission (excluding the SWER), but most of its elastic coefficients were more than 0.25 (
Table 5), and the VIP value was mostly close to or more than 1.0 (
Table 6). On the one hand, it illustrated that the square term of GDP per capital could not be ignored in the interpretation of carbon emissions [
58]. On the other hand, it also showed that no inverted U-shaped relationship appeared between economic development and carbon emissions in this period (2005–2017). Therefore, with the rapid economic development, the peak (or inflection point) of regional carbon emissions has not come. It is worth mentioning that, although the coefficients of the square term of per capital GDP were positive in all regions, the NCER coefficient almost approached to 0 (
Table 5). This means that when the economic development reaches a certain level, it is possible to restrain emissions growth by improving emission reduction technology, optimizing the industrial structure and enhancing environmental awareness of the government and residents. Simultaneously, it also shows that the peak of Kuznets curve between carbon emission and GDP per capita will appear in the NCER in advance.
As far as the industrial structure was concerned, although the average of the elastic coefficients of the ratio of the second industry to GDP was the smallest, it was all positive in each economic region, which was consistent with the expectation that the increase of the proportion of the second industry will promote the growth of carbon emissions. However, from
Table 5, the effect of industrial structure on energy-related carbon emission was quite different in various economic regions, and the elastic coefficients changed from 0.014 to 0.751, indicating that the industrial structure was also the contributor to regional carbon emission difference. The regions with great influence of industrial structure on carbon emission in order were the NCER, ERMRYTR, and ECER, and the VIP values of industrial structure in the NCER and ERMRYTR were close to 1.0, indicating that the industrial structure still had a strong ability to explain carbon emission in these two regions. It also means to further optimize the industrial structure and reduce the proportion of the secondary industry in the above regions, which is conducive to carbon emission mitigation. The regions with less impact of industrial structure on carbon emissions were the NWER and NEER, with the elastic coefficients of 0.014 and 0.022, respectively, and the VIP value of the NEER was less than 0.8 (
Table 6), showing that the positive effect of industrial structure on carbon emissions was weak in these two regions. The results above also further indicated that the carbon emissions in the NCER and ERMRYTR were more easily affected by the industrial structure than those in the NWER and NEER, which may be related to the dependence of regional economic growth on the pull of the secondary industry [
27]. For the developed NCER and ERMRYTR, although the industrial allocation has improved and the energy consumption of GDP has constantly reduced, the industrial structure is still relatively traditional. In addition, economic growth relies mainly on the energy-consuming secondary industry with high input-output efficiency. As the carbon emission of the secondary industry is far greater than that of the first and third industries, the regional carbon emission was related closely with the proportion of the secondary industry. This can be verified by the changing trend of the output proportion of the secondary industry to the GDP in the ERMRYTR (
Figure 5). As for the share of secondary industry’s output in the GDP of the NCER, there was a significant downward trend. That may be caused by the decline of the proportion of the secondary industry in the Beijing-Tianjin area due to industrial structure optimization and upgrading and “capital function relief” (since 2014, the coordinated development of Beijing-Tianjin-Hebei region has become a national strategy, the industries with energy-intensive consumption and high pollution were shut down or transferred to the surrounding areas, and the proportion of secondary industry in Beijing decreased to 19.0% in 2017), which partially counteracted the influence of the large proportion of the secondary industry in Shandong and Hebei provinces and caused a significantly decline in the share of the output value of the secondary industry in the whole NCER. However, this does not mean that the carbon emission in the NCER was not significantly affected by the share of the secondary industry. The reason is that in the NCER, Shandong and Hebei are resource-intensive provinces. The economic development impelled the pillar industries such as chemical, metallurgy, building materials, machinery and automobile to consume large amounts of high-carbon fossil energy and produce a lot of carbon emissions, causing the elastic coefficient of the share of the secondary industry affecting carbon emissions reached 0.751 in this region (
Table 5). This indicated that the development of the secondary industry in the NCER has reached a stage of serious diseconomy. Taking Shandong Province in the NCER as an example, its GDP increased from 1836.7 billion yuan in 2005 to 6300.2 billion yuan in 2017 (ranking the third in China), with an annual average growth of 24.3%, but its economic development was dominated by the secondary industry at the expense of energy resources. According to our calculation, the output value of the secondary industry accounted for over 52.0% of the provincial GDP during 2005–2017, which resulted in a large number of high carbon energy consumption, and thus carbon emissions. For example, the carbon emissions caused by coal consumption increased from 580 million tons in 2005 to 910 million tons in 2017, with an average annual growth of 5.2%. The NEER and NWER belong to underdeveloped areas. Although the proportion of resource-based cities and industries is relatively high, the industrial structure adjustment is vulnerable to the impact of macroeconomic policies. As a result, the proportion of the secondary industry output in total GDP changed in an “M” pattern (
Figure 5). Moreover, the NWER had undertaken a large number of industries from the developed eastern region, which ultimately restrained the carbon emissions increase to a certain degree, resulting in a weak positive impact of the secondary industry’s share on the regional carbon emissions (
Table 5,
Table 6). This shows that the moderate transfer of the secondary industry to these regions may reduce national total carbon emissions.
In terms of urbanization, the average value of the urbanization elasticity coefficient of the eight regions was 0.363, ranking the fourth in all factors. This factor exerted positive effects on carbon emissions in most regions except the ECER where the negative effect appeared (
Table 5). This result was not consistent with those of previous studies. Previous research showed that the urbanization had a positive role in promoting carbon emissions in the eastern region, no significant impact on carbon emissions in the central region, while had a negative impact in the western region [
59]. In addition to the impact of time span (the sample data spanned the period from 1980 to 2014 in previous research), another reason for this phenomenon might be related to the large difference in study area division. In our study, the most significant effect of urbanization on carbon emissions occurred in the NEER, in which for every 1.0% increase in urbanization level, carbon emissions increased by about 0.583%. With regard to the VIP values of urbanization at the regional level, the ones of all economic regions except NCER were above 1.0 (
Table 6), which showed that urbanization had an important explanatory capacity for carbon emissions of all regions in China [
60]. Especially in the ERMRYR, the ability of the urbanization level to explain carbon emissions ranked the first among all the factors in the region, indicating that the urbanization played a decisive role in carbon emissions increase in the region. The reason may be that some provinces in the ERMRYR are in the early stage of industrialization and urbanization. Especially with the implementation of the strategies of western development and central rise, the focus of national investment and urbanization growth began to shift from the eastern coast to the central and western hinterland, which accelerated the extensive urbanization process in the ERMRYR, and increased the proportion of the urban population, so as to stimulate the consumptions of high-carbon energy and materials such as coal, cement and steel while promoting the infrastructure construction, consumer demand, and quality of life. Thus, the above reason ultimately brought more carbon emissions in the region. As regards the negative effect of the urbanization on carbon emission in the ECER, it may be related to three reasons. First, the ECER has entered the stage of new urbanization and high-quality development. The infrastructure construction has been basically completed, and the urban form has gradually changed to a compact type. Generally, the compact cities are conducive to reducing the demand for high-carbon fossil energy [
20]. Second, the ECER promotes urbanization by the market allocation of resources and economic development. Therefore, with the rise of urbanization rate, the degree of industrial capital concentration will continue to improve, and then through scientific and technological innovation to improve the efficiency of energy utilization, leading to the gradual weakening of the positive effect of urbanization on carbon emissions, thereby cutting down carbon emissions. Third, the focus of national economic development and urbanization began to shift from the coastal to the inland provinces, and the speed of urbanization in the ECER slowed down. While exploring the new urbanization path, more attention was paid to economic transformation and upgrading, so as to promote the expansion of high energy consumption industries in the region to the central, western and the northeast regions, and thus inhibiting carbon emissions. Therefore, in the course of urbanization, the only way to reduce carbon emissions in the eight economic regions is to carry out new low-carbon urbanization.
Energy intensity played an active role in carbon emissions increase in most economic regions, but its elasticity coefficient and VIP value fluctuated greatly among the regions. The elasticity coefficient ranged from –0.120 to 1.317, with an average value of 0.401 (
Table 5), and the VIP values varied from 0.539 to 1.143, with the average of 0.936 (
Table 6), which indicated that the impact of energy intensity on the differences in regional carbon emission could not be ignored. To be specific, energy intensity played the most promoting role in carbon emissions increase in the NEER and NCER. For every 1% increase in energy intensity, the carbon emissions of the two regions increased by 1.317% and 0.791%, correspondingly, and their VIP values were as high as 1.065 and 1.143, respectively, indicating that energy intensity had a strong ability to explain carbon emissions. The reason was that in the research period, the industrial structure of the NEER and NCER (excluding the Beijing-Tianjin region) was relatively traditional, and there were more extensive resource-dependent enterprises. Additionally, in 2005–2006, with the improvement of the domestic investment environment and the acceleration of investment growth, the rise of absolute output inevitably brought more energy input (i.e., the increase of energy consumption intensity), causing carbon emissions growth. Hence, improving the efficiency of energy utilization is an important way to carbon emissions reduction in these regions. Contrarily, the positive effects of energy intensity in the ECER and SCER were relatively weak, and their VIP values were less than 1.0, indicating that energy intensity had no significant effect on carbon emissions. The main reason was that the industrial restructuring in such regions has been basically completed, and the overall level of energy efficiency has been improved. Besides, the demand for energy-consuming products due to economic development was close to saturation, and the public awareness of emission reduction was enhanced. The two reasons led to the decrease in carbon emissions, thus a weak positive effect of energy intensity. What was interesting was the energy intensity had the least positive effects on carbon emission in the NWER and SWER, while the negative effects were found in the ERMRYR and ERMRYTR (
Table 5). Except that the VIP value of energy intensity in the NWER was less than 0.8, those in the other three regions were all in the range of 0.8–1.0 (
Table 6). It was apparent that the energy intensity had a certain ability to explain carbon emission in the SWER, ERMRYR, and ERMRYTR. As for the above four regions, the reason why the energy intensity showed the smallest promoting or negative effects on carbon emissions, maybe that these economic regions are located in the central and western regions, and resource-dependent and high-energy consumption enterprises accounted for a large proportion in secondary industry, which was vulnerable to the impact of national macroeconomic policies (Since 2005, China has implemented energy-saving and emission-reduction policies for key industries, resulting in a significant reduction of energy consumption per unit product). Therefore, the adjustment of industrial structure or technological progress has great elasticity to restrain carbon emissions. Besides, these regions undertook some high-tech industries from the eastern developed areas, and eliminated some local backward production enterprises, causing the enhancement of energy efficiency and the corresponding reduction of carbon emissions [
61].
As shown in
Table 5, the energy structure represented positive and negative effects in the eight economic regions. On the whole, the average value of elasticity coefficients of energy structure was small and ranked behind in all factors, but the elasticity coefficients fluctuated greatly among the regions, with the range of −0.437 to 0.938, indicating that the energy structure was also an important factor causing the difference in regional carbon emissions. Among the eight economic regions, the absolute value of the energy structure effect in the SWER and NCER was larger than that in other economic regions (
Table 5). Specifically, the energy structure effect of the NCER was significantly negative, and the VIP value was close to 1.0 (
Table 6). On the one hand, it showed that the energy structure was important to the change of carbon emissions in this region; on the other hand, with the increase in the proportion of new energy use, the energy diversification effect in the region was increasingly obvious, and the energy structure is becoming more reasonable. For example, the Beijing-Tianjin area, located in the NCER, is rich in clean energy resources. In recent years, with the advantages in economy, science-technology, and education, as well as good external environment, Beijing and Tianjin have vigorously developed a new energy industry and formed a green energy industry cluster with the wind, solar, biomass, lithium batteries, and geothermal energy as the main body. In 2017, the ratio of coal, oil, natural gas and other energy sources was adjusted to 27.6:22.1:1.7:48.6 in the Beijing-Tianjin area. Although the coal consumption of Shandong Province, located in the NCER, has always been dominated (In the past 13 years, the average coal consumption ratio of this province was 72%, which was greater than the national average of 70%), the proportion of raw coal in total energy consumption of the whole region decreased from 65% in 2005 to 49% in 2017. As coal (including its products) is the energy with the largest carbon emission coefficient, the higher the share of coal in primary energy consumption, the more carbon emissions (Every 1% increase in coal proportion may increase carbon emissions by about 0.053%). Thus, the improvement of energy structure can effectively inhibit the growth of carbon emissions and shows a negative driving effect on carbon emissions. As regards the significantly positive effect of energy structure in the SWER, the reasons may include the following three aspects. First, it was related to the economic underdevelopment and the inelastic demand of energy (especially coal) for economic growth of the SWER. Second, the region is China’s main energy-produced base, and the energy consumption structure dominated by coal has not been changed for a long time. Third, the promotion and application of carbon emission reduction technology could not keep up with the increasing speed of energy consumption, especially coal. The above reasons resulted in a strong positive impact of energy structure on carbon emissions. In addition, the VIP value of energy structure in the SWER reached 0.993 (
Table 6), indicating that the energy structure dominated by coal is one of the important factors to promote carbon emissions increase in the region. In the developed ECER, SCER, and ERMRYTR, economic development needs to consume a large amount of energy as the driving force. Although there are large-scale energy inputs from outside the regions (such as West–East electricity transmission, North–South coal transportation, and West–East gas transmission), the impact of energy consumption structure on carbon emissions is greatly affected by energy policies and macroeconomic situation. Especially in 2007–2010, owing to the influence of the financial crisis, alternative energy sources in the above-mentioned regions were used again, coupled with the slowdown in new energy investment, leading to a weak positive effect of energy structure on carbon emissions (
Table 5). However, the VIP values of the energy structure in the ECER and ERMRYTR were all close to 1.0 (
Table 6), indicating that the energy structure had a good explanation ability for carbon emissions of these two regions. Additionally,
Table 5 showed that the energy structure effects in the NEER, ERMRYR, and NWER were negative, and the VIP values were all below 0.8 (
Table 6). This demonstrated that the impact of the energy structure in these three regions was not important. As for the negative correlation between energy structure and carbon emissions, it may because the NEER, ERMRYR, and NWER are important coal-oil production bases in China, and fossil energy accounts for over 90% of the total energy resources. Although the industries of these regions are relatively backward, the coal-fired power industry is relatively developed. When the inhibition effect of coal treatment technology is greater than the promoting effect of coal proportion increase on carbon emission, the carbon emissions will gradually reduce (namely negative effect).