**4. Conclusions**

This study identified the driving forces of direct and indirect household CO2 emissions (i.e., household CF) both overall and at the sectoral level using IDA and SDA. In the analysis, emissions from di fferent age groups were also considered. The main findings from our analysis are as follows:


Based on these findings, the following conclusions are o ffered to contribute toward achieving Japan's future carbon reduction target. It is necessary to comprehensively consider the household CF, and, considering the impact of an aging society on Japan's overall CO2 emissions, more practical measures should be taken to reduce CO2 emissions by sector. Specifically, the following policy implications are proposed based on our findings.

First, carbon-reduction policies in an aging society should consider demographic influences. Under an aging society with an increase in the proportion of elderly people, the proportion of young people will gradually decrease. Therefore, household-related sectors need to pay more attention to the CO2 emissions from consumption by elderly people. In addition, from the perspective of factors affecting CO2 emissions within sectors, the increase in small-sized households greatly increase CO2 emissions. Thus, for further CO2 emission reductions, enterprises in household-related sectors should adjust their business orientation to meet the needs of small-scale households.

Second, consumers should be encouraged to consume responsibly; i.e., by choosing environmentally friendly products or restricting the irresponsible consumption of goods and services (e.g., less food waste). As a first step toward doing so, considering the fact that the indirect CO2 emissions of residents' lives are higher than their direct CO2 emissions, it is imperative that householders are made aware of this fact. This educational aspect will be essential in order to reduce indirect CO2 emissions as well as promoting low-carbon production for industry. The governmen<sup>t</sup> should expand financial support for enterprises to develop low-carbon technologies to alleviate financial pressures and stimulate enterprise to invest in related research and development. From the consumer side, the key is to develop green-consumption awareness and to encourage the purchase of environmental-friendly commodities. For example, for household products, which account for a large proportion of households' indirect CO2 emissions, financial support should be provided when purchasing environmentally friendly products. Moreover, a healthy lifestyle is of grea<sup>t</sup> significance for residents to reduce potential CO2 emissions; for instance, healthy living habits can keep the body in good condition, which is likely to reduce certain medical expenses and decrease the indirect CO2 emissions from the medical sector.

In addition to policy implications from the domestic perspective, there is also an important implication from the international perspective. Although the aging society is a serious issue in Japan, this issue is also emerging in developed countries. Even for developing countries, China is also beginning to experience an aging society. Therefore, achieving the goal of CO2 mitigation under the constraints of an aging society has important practical significance for many countries. In order to reduce per household emissions, policy encouraging multi-generational households may be appropriate. A complementary policy for per capita emission amelioration may be to encourage the sharing economy, whereby individual consumption is o ffset by shared facilities and conveniences (i.e., car sharing, etc.). As illustrated above, such a demographic shift would gradually change toward the positive and increase indirect CO2 emissions to a level which is higher than direct emissions, which is associated with an increase in the number of households rather than population. Thus, it should be noted that e fforts for recovering the population and labor force through policy measures will eventually lead to an increase in overall CO2 emissions. On the basis of clarifying influential factors toward the CF, particularly those for sectors which presented an increasing trend such as food, public transport, private transport, and the medical sector, the implementation of carbon reduction schemes are crucial. Furthermore, the governmen<sup>t</sup> needs to formulate fit-for-purpose carbon reduction policies based on the characteristics of CF for di fferent age groups. Finally, because the aging society issue occurs over a very long time-span, CO2 emission reduction policies should be continuously adjusted according to the evolution of demographic changes.

**Author Contributions:** Conceptualization, Y.S. and K.M.; methodology, Y.S.; software, Y.S.; validation, K.M; formal analysis, Y.S.; investigation, Y.H.; resources, Y.S.; data curation, Y.S.; writing—original draft preparation, Y.H. and Y.S.; writing—review and editing, A.C. and K.M.; visualization, Y.H. and Y.S.; supervision, K.M.; project administration, Y.S.; funding acquisition, Y.S. and K.M.

**Funding:** This research was supported by JSPS KAKENHI Grant Numbers JP18K11754, JP18K11800, and JP18K18231; and the Integrated Research Program for Advancing Climate Models (TOUGOU program) of MEXT. These organizations did not have any involvement or influence in the implementation of this study.

**Conflicts of Interest:** The authors declare no conflict of interest. The funding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.
