This paper computes national carbon mitigation costs using two simple principles: Incremental costs for low-carbon energy investments are calculated using the cost of coal-fired power as the benchmark.  All low-carbon energy sources are counted, because reducing carbon emissions cannot be separated from other concerns: reducing local air pollution from fossil-fuel combustion; diversifying energy sources to reduce political and economic risks; and building competitive advantage in emerging clean-energy markets. The paper estimates energy growth and incremental costs for biomass, solar, wind, geothermal, hydro, and nuclear in 174 countries from 1990 to 2008.