The carbon intensity of aluminum and steel can be divided into contributions from energy sources like electricity, natural gas, and coal, as well as process emissions from aluminum anode oxidation or blast furnace coke reduction. The emissions intensity of a metal production route can be shown as a product of the carbon intensity of energy and the energy intensity of metal (Figure ). As with electricity cost, we assume an average U.S. carbon intensity of industrial electricity 14 as a representative example. The true carbon intensity of electricity for aluminum production could be substantially lower if production is co-located with, for instance, a hydroelectric dam, or higher if powered purely by coal or natural gas. The ratio of carbon intensities between aluminum and steel -roughly four or five to one -is close to the ratio of costs.