To stabilize greenhouse gas concentrations at an equivalent doubling, CO2 emissions must be limited to 5 PgC y–1 in 2050, compared to 8 PgC y–1 today. This will require the decarbonization of world energy supply, in which fossil fuels, which today account for 85% of energy supply, are replaced by carbon-free sources. Only five sources are capable of supplying a substantial fraction of the required carbon-free supply: biomass, fission, solar, wind, and decarbonized fossil fuels. Other sources are either too limited, too expensive, or too unproven to make a substantial contribution by 2050. Each of the major alternatives has significant economic, technical, or environmental handicaps. Biomass can supply affordable portable fuels, but would require vast areas of land, in competition with agriculture and natural ecosystems. Fission is a mature technology, but suffers from public-acceptance problems related to the risks of accidents, waste disposal, and proliferation. Solar is environmentally benign but expensive and would require massive storage or transmission. Wind is economically competitive at windy sites, but attractive sites are limited. Fossil fuels are cheap and abundant, but the cost of CO2 capture and disposal may be high and the environmental impacts unknown.