We analyse long-term energy prospects from the point of view of future carbon dioxide emissions and constraints imposed by possible changes in the global climate. The approach is based on scenario simulations of technical, economic, social, and cultural changes which determine future energy use and their environmental impacts to the middle of the next century. For this purpose, two scenarios have been formulated: one with changes in society, the economic system, and the energy sector that follow the dynamic-as-usual pattern and the other with enhanced energy-efficiency improvements and conservation efforts. For each scenario, three different cases have been investigated reflecting possible situations on the energy supply-side. The consequences of various policy options in energy supply and demand have been evaluated to understand the efficacy of different measures in approaching a global greenhouse-gas-abatement energy policy.