Green hydrogen is hydrogen gas produced using renewable energy sources, such as wind or solar. Unlike gray or blue hydrogen, which emit carbon dioxide from coal and natural gas inputs, the only byproduct of green hydrogen is oxygen. It is created by running renewably generated electricity through an electrolyzer, which splits water molecules into hydrogen gas and oxygen. Today, green hydrogen is used mainly for refining petroleum products and in the chemical industry for making fertilizers. But the gas is considered a general-purpose energy carrier, meaning its potential applications are vast—a robust green hydrogen industry could accelerate the decarbonization of steel and fertilizer production as well as balance electricity grids when extra power is needed. Most remarkably, green hydrogen has the potential to eliminate roughly a quarter of the world’s carbon dioxide emissions.