A review of the hydrogen production Technologies’ global warming Potential: From the perspective of system boundary

• Hydrogen production carbon emissions are reviewed from a system boundary perspective. • Quantifying the impact of carbon emission of different system boundary. • Proposing a uniform carbon accounting framework for hydrogen production technologies. • Boundary selection can reverse technology rankings in carbon emission evaluations. Hydrogen energy, with the significant decarbonization potential as the clean secondary energy source, is driving the development of diverse production technologies. Here, we comprehensively evaluate technological advancements and environmental impacts of conventional, low-carbon, and emerging hydrogen production technologies. The results indicate that renewable energy-driven water electrolysis offers great promise for low-carbon hydrogen production. In contrast, steam methane reforming coupled with carbon capture and storage, although reducing emissions to 1.0–3.6 kg CO 2 eq/kg H 2 , continues to face lifecycle limitations due to methane leakage and ongoing fossil fuel dependence. Further analysis reveals that feedstock selection, process optimization, and the transition of energy structures serve as critical levers for green hydrogen production. Through systematic cross-technology quantitative analysis, we demonstrate how system boundary selection critically influences global warming potential assessments across different production methods. To address the methodological inconsistencies in current life cycle assessment practices, this work proposes a tiered and modular unified accounting boundary framework that reconstructs hydrogen production technologies into three core modules: Energy Supply, production, and application & end-of-life. This approach provides both standardized comparison and flexible adaptation for various technologies, significantly enhancing the transparency, comparability and policy relevance of environmental assessments for the global hydrogen transition.