The United States and other developed countries currently and historically have transferred considerable resources overseas to further their foreign policy objectives and to purchase oil and natural gas. These transfers are comparable in magnitude to estimates of the scale of the economic effort that would be required to create a world-wide energy system with zero carbon emissions by the middle of this century. Solar energy, the most abundant of the alternative energy supply sources, is currently the most expensive of the alternatives to fossil fuels but a substantial body of research and practical experience suggests that solar costs could fall to competitive levels with sufficient technological progress and increases in solar energy production and capacity. (C) 2010 Elsevier Ltd. All rights reserved.