The Energy-Environment Nexus U.S. electricity generation mix from 1950 to 2000. Fossil fuels provide more than 70 percent of our electricity--52 percent from coal, 16 percent from natural gas, and 3 percent from oil. The issue facing us is how we as a nation can move toward a more sustainable electricity future. The word "sustainable" means different things to different people. To some, it means integration of the social, economic, and environmental domains. To some it means energy that lasts forever. And to some it means energy with no environmental cost for its production and use. Energy from every source has environmental or cost consequences. The challenge for us as a society is to agree on a practical definition of sustainability and then to develop a road map to achieve it. The road map should include public policies, incentives, and research and development (R&D) agendas.