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Solar power generation and, in particular, space solar power generation seem to be one of the most promising electric power generation technologies for reducing emissions of global warming gases (denoted collectively as CO2 emissions below). Calculating the precise amount of net reduction in CO2 emissions of a solar power system over other alternative power systems requires careful life cycle considerations. For example, emissions from a space solar system must include the emissions from consuming rocket fuel during the launching the satellites, and the emissions from the energy consumed while producing the solar panels. In this paper, we calculate the CO2 emissions observed through the life cycle of a solar power satellite (SPS). This life cycle consists of the production of rocket fuel and solar panels and the construction of a Rectenna (power receiving antenna), satellite, and all other equipment listed in the Department of Energy/NASA reference system. The calculation also includes indirect CO2 emissions that occur in various stages of production of these materials. Our baseline scenario shows that the life cycle CO2 emissions for an SPS system per unit of energy generated are almost the same as the emissions for nuclear power systems and are much less than the life cycle emissions for LNG-fired and coal-fired power generation systems. Furthermore, our SPS-Breeder scenario, in which SPSs supply electricity for producing further SPS systems, shows significantly lower CO2 emissions. As electrical power generation constitutes one fourth of Japan's total CO2 emissions, reducing emissions from electric power generation is one of the most important issues on Japan's policy agenda for dealing with global warming. Our findings suggest that the SPS is the most effective alternative power generation technology.