Green hydrogen is produced by the electrolysis of water. The electricity required for this comes from renewable energy sources such as solar, wind and hydroelectric power. For this reason, production is CO2-neutral and therefore climate-friendly. Gray hydrogen is produced from fossil fuels such as natural gas, coal or oil by steam reformation. Ten tonnes of CO2 are produced as a waste product for when producing one tonne of hydrogen. It is not climate-neutral. Blue hydrogen is produced, as with grey hydrogen, by steam reformation. However, the CO2 released is stored underground (CCS technology: Carbon Capture and Storage). Since the CO2 is not released into the atmosphere, blue hydrogen is effectively CO2-neutral. Turquoise hydrogen is produced during methane pyrolysis. Natural gas (methane) is split into hydrogen and solid carbon in a thermochemical process. The solid carbon can be stored as granulate and reused at a later date. Red/pink hydrogen is produced by electrolysis, just like green hydrogen. However, the necessary electricity comes from nuclear energy. Therefore, while this type of hydrogen is CO2-neutral, it produces radioactive waste that has to be stored safely.