The Kaya Identity tells us exactly what families of tools we have in the policy toolbox to reduce carbon dioxide emissions to some desired level. Specifically, carbon dioxide accumulating in the atmosphere can be reduced only by influencing the following four levers.  1. We could reduce the population.  2. We could reduce per capita GDP. 3. We could become more efficient.  4. We could switch to less carbon-intensive sources of energy.  These are the four—and the only four—means of reducing carbon dioxide emissions. All policies being discussed as climate policies must influence these levers if they are to have an effect. So debates about carbon taxes, cap-and-trade programs, offsets, energy innovation, personal carbon allowances, and on and on must eventually arrive exactly the same place.  Kaya Identity (1) Carbon Emissions = Population * Per Capita GDP * Energy Intensity * Carbon Intensity a. P = Total Population b. GDP/P = Per capita GDP (2) GDP = Economic Gr...