Abstract:Concern about global warming, poor urban air quality and dependence on insecure sources of oil are leading many nations to examine alternative energy sources and technologies for transportation and electricity generation. This analysis shows that most of the near-term fossil fuel-based alternatives would not dramatically reduce emissions of greenhouse gases. However, in the long run, several ultra- low-emission fuels and technologies are available. The major uncertainties in the analysis are the efficiency of energy use, the warming potential of greenhouse gases other than CO₂ and the economic and political context in which energy is used.