Iron and steel have been the dominant metal for the last millennium. Global steel production of 1.9 Gt/yr, including 1.5 Gt/yr of primary production, exceeds the production of all other metals combined. On average, steelmaking produces 1.9 t CO2/t steel, but emerging technologies aim to reduce these emissions by using low-carbon energy sources like hydrogen, biofuels, or renewable electricity, or, alternatively, by capturing and sequestering produced CO2. But each of these low-carbon approaches to steelmaking faces challenges. Green hydrogen is substantially more expensive than fossil fuels and cannot easily be integrated into blast furnaces, which require solid fuels for mechanical support and chemical transport. Production of steel from biofuels faces concerns about competition for agricultural land -the food versus fuels debate -and the high cost of making bio-oil and bio-syngas. Electrochemical production of steel by molten oxide electrolysis 12 is less technologically developed and will likely be more capital-intensive than thermal processes. Finally, carbon capture requires favorable geologic conditions and imposes costs and parasitic energy demands on existing steel mills.