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In this paper we draw up future roadmaps for technologies, policies, and optimal investment timing toward the achievement of a low-carbon society (LCS) in Japan by 2050. Future pathways for Japan to follow are calculated using an analytical model based on a backcasting methodology. Early actions can lead to pathways for minimizing the costs toward a LCS in Japan. However, to take early actions, large investments will be needed at the initial stages. The investment requirements for the residential and commercial sector and transportation sector will be especially costly: The former reaching 2.5 trillion JPY annually from 2010 to 2025 and the latter reaching 2.5 trillion JPY annually from 2010 to 2015. There are four main advantages to early actions. (1) Technologies have learning-by-doing effects: The additional cost of reducing CO2 emissions will fall as the technologies spread. (2) If actions are delayed, learning-by-doing effects may fail to work sufficiently, resulting in higher total investment requirements for achieving a LCS. (3) No infrastructure can be built immediately; hence it would be difficult to switch suddenly to a LCS in the years just before 2050. (4) Future technological development has several uncertainties. If the development of one of the currently dominant technologies falls behind schedule, it will fail to spread as expected and CO2 emission targets will not be met. Early actions will open up new opportunities for the spread of alternative actions toward the LCS should a dominant technology fail in some way. [ABSTRACT FROM AUTHOR]

The aim of this paper is to define low carbon society (LCS) based on the phase diagram with the variables from the IPAT [Human Impact (I) on the environment equals the product of population (P) affluence (A) and technology (T)], starting from the problem definitions: (1) Economy is not everything for LCS and (2) it is not clear if speed on the constraint is more important than liberation from it. To tackle these problems, two research questions are set: (1) What are the basic indicators, objects, and constraints to shape the argument of LCSs and (2) what are the historical paths of several countries and what can be said for their future paths toward carbon societies? Time series data from 1900 or before are used, while the phase diagram with the variables from the IPAT equation is used as the core methodology. In addition to the IPAT variables, the importance of land per capita shall be considered as another basic indicator for LCS related to the carrying capacity. The three different kinds of objects [i.e., total gross domestic product (GDP), GDP per capita, and social indicator] and the two constraints (i.e., total emission and emission per capita) are considered, and it is shown that the combination of these objects and constraints strongly affects the argument of the carrying capacity of climate and land, development of economic system and economic man, and other human developments. Among the four cases analyzed in the phase diagram, the case where GDP per capita is object and total emission is constraint is given the most attention and requires the further research in the future since it does not contradict with a definition of LCS. It is also suggested that if changing the direction of the path with low carbon technology development takes time and cost, institutional arrangement would be additionally necessary in addition to market mechanism where optimal solutions are found on constraint. [ABSTRACT FROM AUTHOR]