At the 21st Session of the Conference of the Parties to the United Nations Framework Convention on Climate Change (COP21) at the end of 2015, specific greenhouse gas (GHG) emissions reduction targets for 2030 will be determined. Japan is also expected to finalize plans for reductions by the summer. We estimated the extent of possible reductions and the scale of costs and benefits from fiscal 2012 to fiscal 2050 that would result from the use of renewable energy sources, nuclear power and carbon capture and storage (CCS). A reduction target matching that of the United States is achievable and to secure a leading role in the international debate on prevention of global warming, Japan must set itself a target of a 30% reduction in emissions by fiscal 2030. In the meantime, to achieve further reductions by 2050, examination of emissions regulations and carbon pricing cannot be avoided.