3.2.1. The Energy Sources Level

In energy sources (Figure 5a), results show that the most significant structural changes were from coal. During 2005–2015, although the coal was still the largest contributor to energy-related carbon emissions with largest increment (total amount increased by 3076Mt), its proportion conversely decreased a lot (occupation ratio decreased by 1.2%). Compared with China's policy objectives [18], it can be seen that the effect of coal reduction work in China during this period was quite successful. The proportion of coal in China's energy structure decreased by 6.4% in this period. For one reason, it was related to the national efforts to increase the proportion of natural gas and non-fossil energy consumption [15]. For another reason, it was also closely related to the industrial upgrading and technological progress of the coal industry itself [28].

In contrast, natural gas has become an important energy source for replacing coal in the transition of China's energy structure leading to a rapid growth rate of carbon emissions by 205%, which was closely related to China's strong investment in natural gas infrastructure construction and deep international cooperation. Facts show that in 2009, China cooperated with Russia and Central Asian countries to build the first natural gas pipeline for the introduction of long-distance natural gas delivery from Central Asia; in 2013, China-Myanmar natural gas pipeline was established in cooperation with Myanmar, which became the second onshore natural gas pipeline; in the meantime, the liquefied natural gas (LNG) business was also booming, and the countries China imported LNG from had expanded from Australia, Indonesia and Malaysia to Qatar and Brunei [29]. Renewable energy represented by wind and solar power also developed rapidly in China during this period. For example, the wind power became the energy source with fastest relative growth rate (by 55.7 times) in the decade and contributed a lot of increase of the proportion in energy structure (increased by 1.1 %). The rise of natural gas and renewable energy had slowed the growth of energy-related carbon emissions to some degree.

## 3.2.2. The Vehicle Passive System Level

In the vehicle passive system (Figure 5b), results show that the largest driving force was the passenger car, which not only had the largest increase in total amount change (231Mt CO2) but also remained a high relative growth rate (158%), making its occupation ratio increased by 9.5% in vehicle passive system and exceed trucks as the largest emission source. While trucks and ships represented a significant decline in the occupation ratio of the emissions structure (trucks reduced by 8.1%, ships reduced by 5.2%). Compared with other statistics, the results kept in line with the fact that China's highway infrastructure had been gradually improved in the past decade (in 2015, the total length of China's highways reached 4.57 Million km, the total length of freeways reached 123,500 km, the proportion of towns with roads reached 99.99% [30]), and the number of civilian vehicles increased rapidly (reached 163 million in 2015, which was ten times more than that in 2005 [26]). In this period, new energy vehicles such as electric vehicles had not been promoted, so the increment of private cars were mainly gasoline-powered cars, which significantly increased the carbon emissions of passenger cars. The transition of the economic structure was another proof. In 2015, the proportion of increased GDP in the tertiary industry exceeded 50% for the first time [31]. As in this period, the tertiary industry had less demand for physical goods compared with the primary and secondary industry, the overall freight demand and the growth rate of the road freight industry slowed down in 2015. Thus the energy consumption growth of trucks and ships slowed down, and carbon emissions did not increase significantly.

## 3.2.3. The Building Passive System Level

In the building passive system (Figure 5c), in terms of the energy consumption, the most significant increase came from hot water systems (relative energy consumption growth rate by 157%, energy consumption proportion increased by 4.9%), but as for carbon emissions, household appliance was

noteworthy. Although appliances energy consumption growth rate was only 19%, the relative growth rate of carbon emissions was as high as 158%, and its occupation ratio in the emissions structure increased the most by 4.1%. This result shows the role of R index to reveal potential trends. To explain this, we further calculated the carbon emissions per unit energy consumption (CPE) of these two sectors and found the difference. In this period, the CPE of the hot water system decreased by 31.4%, but that of the household appliance increased by 116.8%. This finding was consistent with another statistics [26], which shows that in the process of replacing traditional home appliances, a large part of the original biofuel appliances were replaced (such as electric cookers, gas stoves replaced firewood stoves, biofuel direct use decreased by 29.7% from 2010 to 2015 [26]), which made carbon emissions of appliances increase significantly although the total energy consumption did not change so much, for biomass was assumed to be carbon neutral. As a policy implication, it could be solved by adjusting the power source structure and introducing more non-fossil energy or biomass power.

The heated/cooled space system contributed the largest increase in total amount change (242 Mt CO2), but considering its large emission base, the relative growth rate is just 42%, which is the smallest in the building system, its occupation ratio also decreased by 4.7%. This was mainly due to the gradual improvement of the infrastructure of municipal heat pipe networks. Centralized heat-supply and gas heat-supply replaced traditional heating methods and improved heating efficiency. The changes were also related to the improvement of energy efficiency of household devices such as heaters. Wang et al. [32] also agreed that China's domestic heating reformation could play a crucial role in achieving energy saving and emission reduction goals. At the spiritual level, this trend reflected a significant improvement of the residents' living standards, the constantly increasing demand for life quality.
