Aluminum is the second most produced metal after steel. Because aluminum is more expensive than steel (roughly $2700/t compared to $600/t in our production cost model, or $2500/t versus $700/t in representative U.S. prices 13 ), it is not used as much: global annual production of aluminum in recent years is 85 Mt/y, including 70 Mt/y of primary production. Despite higher emissions per ton of metal, lower levels of aluminum production mean that aluminum CO2 emissions are an order of magnitude less than that of steel (270 Mt/y CO2, or 0.8% of anthropogenic CO2). Also, whereas steel is mainly produced thermally, often using coal, aluminum is produced electrochemically, often using hydroelectricity. All of this would seem to suggest that decarbonization of steel is a priority for emissions reductions in the coming decades, and decarbonization of aluminum is of secondary importance. Yet this narrative does not survive scrutiny from the standpoint of emissions and cost. As renewable electricity prices decline and untapped mineral reserves become harder to access, light metals like aluminum and magnesium may become more important as their costs fall fastest and they benefit most from simpler transitions to fully electrified production. Any widespread substitution of aluminum for steel could be either beneficial or harmful for total metal CO2 emissions, and this depends most critically on process developments in the aluminum industry.