Hydrogen is increasingly described as one of the building blocks of the energy transition.

Hydrogen is increasingly described as one of the building blocks of the energy transition. Green hydrogen. Blue hydrogen. Renewable hydrogen. Low-carbon hydrogen. Clean hydrogen. These labels can be useful. But from an engineering perspective, perhaps there is an even simpler place to begin: Where did the hydrogen come from? And immediately after that: Where did the energy used to produce it come from? Hydrogen carries energy — it does not create it Hydrogen is an energy carrier and industrial feedstock, not a primary source of energy. To produce hydrogen, energy has to come from somewhere else. Water can be split through electrolysis using electricity. Natural gas can be converted through steam methane reforming. Coal can be gasified. Other chemical and biological pathways are possible. The hydrogen molecule may ultimately be identical. But the physical pathway producing it can be very different. That means evaluating hydrogen solely by the fuel at the point of use can hide much of the system that matters. We have to follow the energy upstream. Follow the energy Consider renewable hydrogen produced by electrolysis. The simplified pathway is: renewable resource → electricity → electrolyser → hydrogen → storage/transport → end use At each conversion and handling step, there can be energy requirements and losses. IRENA notes that electrolysis, and particularly subsequent reconversion of hydrogen into electricity or other energy forms, involves inherent conversion losses. This is one reason direct electrification can be preferable where it is technically and economically practical. That does not make hydrogen inefficient in every application. It means the appropriate question is not: “Is hydrogen good or bad?” It is: “What function are we asking hydrogen to perform?” Hydrogen may be particularly valuable where direct electrification is difficult — including some industrial processes, chemical production, long-duration energy storage and production of hydrogen-derived fuels. The system boundary determines the answer. Then follow the carbon Hydrogen itself contains no carbon. But hydrogen production can have a substantial carbon footprint. The IEA reports that global hydrogen production still remains dominated by unabated fossil fuels. Low-emissions hydrogen production reached almost 1 Mt in 2025, while total hydrogen demand surpassed 100 Mt. So saying simply: “This process uses hydrogen” does not tell us its carbon impact. We need to know how that hydrogen was produced. For fossil-derived hydrogen, follow the geological carbon entering the production system. For hydrogen produced with carbon capture, follow both the captured carbon and the residual emissions across the defined boundary. For electrolytic hydrogen, follow the electricity. Then follow the carbon associated with producing that electricity. The colour assigned to hydrogen is secondary to the physical flows underneath it. Hydrogen can move carbon as well as energy There is another dimension that receives less attention. Hydrogen can react with carbon-containing molecules to produce fuels and chemicals. For example: CO₂ + 4H₂ → CH₄ + 2H₂O Here hydrogen becomes part of a carbon-management pathway. The important questions then become: Where did the CO₂ originate? Where did the hydrogen originate? Where did the energy originate? What happens to the carbon in the methane after use? Does it enter the atmosphere? Is it captured? Is it stored? Is it reused? And does the overall system require another unit of geological carbon to replace it? Now we are no longer simply discussing hydrogen. We are examining a carbon-and-energy system. Why low-emissions hydrogen is struggling to scale The physical system also helps explain some of today’s commercial difficulties. The IEA reports that low-emissions hydrogen production grew by about 20% in 2025, but persistent barriers remain: high costs, uncertain demand, regulation and insufficient infrastructure. Only around 20% of newly signed low-emissions hydrogen offtake volumes in 2025 were backed by firm contractual commitments. This should not necessarily be interpreted as hydrogen failing. It may instead indicate that hydrogen needs to be deployed where its system value justifies the additional conversion steps and infrastructure. The objective should not be to maximise hydrogen production. The objective should be to use hydrogen intelligently where it helps transform the underlying energy and material system. Net zero, decarbonisation and defossilisation Hydrogen also demonstrates why these three concepts should not be treated as synonyms. Net zero describes a balance between greenhouse-gas emissions and removals across a defined boundary. Decarbonisation describes measures that reduce emissions or emissions intensity. Defossilisation asks a different upstream question: Can we progressively reduce the requirement for newly extracted geological carbon? Renewable hydrogen can contribute to decarbonisation and defossilisation when it replaces fossil-derived hydrogen or enables industrial pathways that require less new fossil carbon. But simply introducing hydrogen into a system does not automatically accomplish either. The complete material and energy flows have to be examined. Beyond the colour of hydrogen Perhaps the hydrogen debate has become too focused on colours. Green. Blue. Grey. Pink. Turquoise. Those classifications can help describe production pathways, but they should not replace physical analysis. Instead, ask: Where did the hydrogen come from? Where did the energy come from? Where did the carbon come from? Where did the carbon end up? And finally: Did this system reduce the amount of new geological carbon that had to enter the economy? Hydrogen may become extremely important in the energy transition. But its value will ultimately be determined not by its colour, nor by the molecule alone. It will be determined by the system in which we use it. Follow the energy. Follow the carbon. Because molecules do not carry labels. They carry atoms.