To identify a reliable path to zero greenhouse gas emissions after the middle of this century, bottom-up scenarios were examined focusing on anthropogenic CO2 emission reduction that would support a shift to a sustainable decarbonizing energy supply and demand structure economically, avoiding technological development risks. The scenarios were developed as combinations of measures and activity level: three cases for measures, 0) none, 1) good technical practice (GTP) dissemination and 2) GTP+expansion in electric furnace steelmaking; and two cases for activity level, i.e., A) mass production and B) medium activity, where activity was decreased in proportion to the population from the 2015. Primary energy supply, final energy consumption and CO2 emissions in 2030 and 2050 were calculated and compared. Technical measures were limited to those already existing in 2019, except for the transport sector, where commercialization of electric vehicle technology was assumed. Fossil fuel utilization was assumed to continue for high temperature industrial heating and transport by sea and air. Nuclear power, hydrogen, CCS, CCU, climate engineering, carbon credit purchasing from overseas were not assumed. The result showed clearly that energy-derived CO2 emissions in fiscal 2050 can be reduced by 90% or more compared to 1990 by the diffusion of existing technical excellence, which showed much larger effects than the level of activity. In addition, the relationship between the cumulative CO2 emissions and the carbon budget were examined, and it was clarified that the temperature rise of 2 degrees C and that of 1.5 degrees C can be satisfied if measures to 2030 are implemented two to five years, respectively, ahead of schedule.