**1. Introduction**

To meet the global climate change mitigation target of limiting global warming to just 1.5 ◦C [1], an extensive effort toward reducing carbon emissions throughout our lifestyles is critical. Leading this effort is particularly essential for developed nations, which are more likely to emit larger amounts of carbon dioxide (CO2) and other greenhouse gases (GHG) and who have access to cleaner technologies than developing nations. Consumption-based accounting [2,3] quantifies not only direct or territorial emissions due to fuel combustion but also indirect emissions generated through the supply chains of goods and services, allowing for the consideration of broader abatement options from both the demand and supply sides [4]. This is known as a "cradle-to-gate" (i.e., from raw material extraction to final consumption) assessment. From this point of view, our lifestyle (household consumption) has been highlighted as playing a dominant role in cradle-to-gate GHG emissions, as measured through national carbon footprints (CF) [5–7]. Globally, almost 65% of CF are induced by household consumption [8]. In order to achieve a low-carbon future, drastic changes are needed not only in industrial supply structures but also in our daily lifestyle structures.

Japan, selected as the target nation for this study, is among the most industrialized GHG-emitting nations [9]. In line with Japan's Paris Agreement goals, GHG emissions need to be reduced by 26% by 2030 compared with 2013 levels. To achieve this, it is necessary to implement e ffective measures with respect to technological innovation and envisaged demographic shifts [10,11]. Japan's CF structure has been analyzed, identifying that household consumption is dominant, similar to other developed nations [12]. Further, previous research has analyzed the household CF in Japan, identifying key consumption and behavior domains for e ffective reduction [13–18]. However, this precedential research did not examine time trends of household CF or the crucial supply and demand drivers.

This study sheds light on structural changes in Japanese household CF to elucidate levers which enable the reduction of CF and enable an understanding of how supply and demand drivers underpin these CF. In addition, this research focuses on demographic trends as a demand driver of changes in CF due to an aging, shrinking population—a serious issue faced in Japan and other nations, which may affect consumption and resultant CF values [13,18,19]. For instance, Shigetomi et al. [11] evaluated the impact of demographic trends on energy-related CO2 emissions from residents during 1990–2015 using decomposition analysis, indicating that as the number of single households increased, especially older households, a commensurate increase in emissions was observed.

In order to analyze the underpinning factors of CF, this study adopts index decomposition analysis (IDA) [20] and structural decomposition analysis (SDA) [21] on a time-series data set of both direct and indirect CO2 emissions induced by Japanese households. To date, IDA has been applied mainly to examine the driving forces of energy consumption and its relation to CO2 emissions by sector [22,23]. SDA has clarified the determinants of lifecycle emissions within CF, utilizing input–output table data [24,25]. These two approaches have been utilized to analyze key determinants of home energy-related CO2 emissions [26–28] and household CF [29–31], respectively, regarding the carbon emission intensity, supply chain structure, consumption volume and composition, and population.

Studies relevant to Japan have assessed energy-related CO2 emissions at the national level [32], specific to services [33], transport [34], manufacturing [35], and residential sectors [11], identifying the key drivers for reducing emissions by sector. For instance, related to household consumption, Shigetomi et al. [11] examined the trends in residential energy-related CO2 emissions across 47 regions in Japan during 1990–2015 using the regional energy balance tables, demographic census and consumer expenditure survey, demonstrating the importance of prioritizing national and local policy interventions of the supply and demand sides in light of the di fferences in the key drivers in each region. They do not consider, however, the upstream (indirect) CO2 emissions created by the overall household consumption via the supply chain. To the best of our knowledge, this study highlights the structural changes in Japanese household CF detailing both direct and indirect energy-related CO2 emissions for the first time. This novel research uncovers insights for mitigating household CF with respect to both supply and demand factors and demographics, notably an aging, shrinking society.

The research is organized as follows: Section 2 describes the methods and data employed to calculate household CF and to estimate the contribution of drivers toward structural changes. Section 3 presents the results and discussion. Finally, Section 4 concludes with detailed policy implications and a future outlook based on the obtained results.
