A need to make substantial future reductions in greenhouse gas emissions would require major changes in national energy systems, Nine industrialized countries have explored the technical boundaries of CO2 emission restrictions during the next 40 to 50 years using comparable scenario assumptions and a standard model, MARKAL, Quantitative results for the countries are shown side by side in a set of energy maps that compare the least-cost evolution of the national energy systems by the main factors that contribute to CO2 emissions, The ability to restrict future CO2 emissions and the most cost-effective measures for doing so differ among the countries; an international agreement that would mandate substantial emission restrictions among countries by an equal percentage reduction is clearly impossible, The results are a first step toward a basis for allocating such international reductions, and the multinational process by which they were produced provides an example for further international greenhouse gas abatement costing studies.