This book has addressed the energy flows through the climate system. It has made the point that the total energy generation by humans is relatively small compared with natural flows, and the main way humans cause climate change is by interfering with the natural energy flows. Ironically, that interference comes mainly from our use of energy and the associated emissions of greenhouse gases and aerosols. In this case, “energy” encapsulates electricity, heat, transport, industrial, and agricultural activities. The latter include changes in land use. The primary source of the errant emissions is the burning of fossil fuels, generating carbon dioxide. A key issue then is how to decarbonize energy systems.