Understanding the Global Hydrogen Budget: Sources, Sinks, and Climate Implications

Hydrogen (H₂) has emerged as a critical component in global decarbonization strategies, particularly for hard-to-abate sectors like heavy industry and long-distance transport. When produced via electrolysis using renewable energy, it offers a pathway to near-zero carbon emissions. However, hydrogen's atmospheric behavior presents a complex environmental challenge. As an indirect greenhouse gas, H₂ interacts with methane, ozone, and stratospheric water vapor, contributing to climate warming. Understanding the global hydrogen budget—the balance between its sources and sinks—is therefore essential for assessing both the potential benefits and risks of a future hydrogen economy. This article synthesizes recent scientific findings to provide a comprehensive overview of hydrogen's atmospheric cycle and its climate implications.

The Global Hydrogen Budget: 2010–2020

Recent comprehensive analysis has quantified the global hydrogen budget for the decade 2010–2020, revealing a delicate balance between sources and sinks. Total global H₂ sources averaged 69.9 ± 9.4 teragrams per year (Tg yr⁻¹), while sinks averaged 68.4 ± 18.1 Tg yr⁻¹ during this period. This near-balance indicates that while atmospheric H₂ concentrations have been increasing, the Earth's natural and anthropogenic systems have been responding to these changes. The largest uncertainty in this budget comes from the soil sink, which demonstrates the complexity of measuring and modeling hydrogen uptake across diverse terrestrial ecosystems.

Major Hydrogen Sources

Photochemical oxidation represents the dominant source of atmospheric hydrogen, contributing approximately 38.4 Tg yr⁻¹ or 56% of total global sources. This process occurs through the oxidation of methane (CH₄) and various non-methane volatile organic compounds (NMVOCs) in the atmosphere. The oxidation of methane alone contributes an estimated 26.1 ± 3.5 Tg H₂ yr⁻¹, with this source increasing over recent decades primarily due to rising atmospheric methane concentrations from anthropogenic activities. NMVOC oxidation from biogenic, wildfire, and anthropogenic sources contributes an additional 12.3 ± 5.0 Tg yr⁻¹.

Direct emissions from combustion processes constitute another significant source category. Fossil fuel combustion contributes approximately 7.5 ± 3.9 Tg yr⁻¹, with automobile transportation being the primary contributor. Biomass burning, including both wildfires and biofuel combustion, contributes 11.6 ± 3.7 Tg yr⁻¹. Interestingly, emissions from fossil fuel combustion have shown a decreasing trend over recent decades, likely attributable to improvements in combustion efficiency that reduce incomplete combustion.

Biological and Industrial Sources

Biological nitrogen fixation (BNF) represents a natural but significant hydrogen source, with terrestrial and oceanic processes contributing approximately 8.0 Tg yr⁻¹ combined. On land, leguminous crops and natural vegetation release hydrogen as a byproduct of nitrogen fixation, with South America being the largest regional contributor due to extensive soybean and peanut cultivation. In oceans, cyanobacterial nitrogen fixation releases hydrogen that eventually reaches the atmosphere, though this process remains relatively poorly constrained compared to terrestrial sources.

Industrial hydrogen production currently contributes a relatively small but growing source through leakage. Using a leakage rate of 1 ± 0.5% for industrial hydrogen production, researchers estimate total H₂ leakage at 0.7 ± 0.4 Tg yr⁻¹ for 2010–2020. While this represents less than 1% of total sources currently, this figure is expected to increase substantially as hydrogen production scales up in future energy systems.

Hydrogen Sinks: Removal from the Atmosphere

Soil uptake represents the dominant sink for atmospheric hydrogen, removing approximately 50.0 ± 18.0 Tg yr⁻¹ or 73% of total sinks during 2010–2020. This process occurs through microbial activity in soils containing organic carbon, where specialized bacteria consume hydrogen. The efficiency of this sink varies significantly based on soil properties including texture, porosity, moisture content, and temperature. Tropical regions account for approximately 50% of global soil hydrogen uptake due to favorable conditions for microbial activity, though significant uptake occurs across all non-desert and non-frozen lands globally.

Reaction with hydroxyl radicals (OH) in the atmosphere constitutes the second major hydrogen sink, removing approximately 18.4 ± 2.2 Tg yr⁻¹. This chemical process occurs throughout the troposphere, with reaction rates dependent on temperature and the three-dimensional distribution of both OH and H₂. While better constrained than soil uptake, this sink still carries uncertainty related to variations in global OH distributions and trends, particularly as methyl chloroform concentrations decline below detection limits, reducing constraints on OH fields.

Regional Distribution of Hydrogen Sources and Sinks

The spatial distribution of hydrogen sources reveals significant regional variations, with hotspots of emissions concentrated in specific geographical areas. Southeast and East Asia show the highest density of emissions, though tropical regions collectively contribute approximately 60% of total global emissions. This pattern results from the combination of higher temperatures promoting methane and NMVOC oxidation, abundant vegetation producing biogenic NMVOCs, and frequent tropical fires in regions like Africa and South America.

Africa and South America stand out as the largest regional sources of hydrogen, contributing approximately 16% and 11% of global totals respectively. In Africa, biomass combustion—primarily from wildfires—constitutes the largest source, followed by NMVOC oxidation. South America shows a similar pattern with NMVOC oxidation as the dominant source, followed by biomass combustion. Both continents contain extensive tropical areas with frequent wildfires and abundant vegetation that produces NMVOCs.

East Asia and North America, as the world's largest economic regions, contribute the most hydrogen emissions from fossil fuel combustion—approximately 32% and 15% of global totals respectively. Europe represents the smallest regional source of hydrogen at less than 2% of the global total. Sink distribution follows different patterns, with Africa also being the largest regional sink (approximately 23% of global total), followed by South America (approximately 13%), while Southeast Asia represents the smallest regional sink (approximately 4%).

Climate Impacts of Atmospheric Hydrogen

Despite its relatively short atmospheric lifetime of 1.9–2.7 years, hydrogen acts as an indirect greenhouse gas with significant climate consequences. By consuming hydroxyl radicals (OH)—the primary sink for methane—hydrogen extends methane's atmospheric lifetime, leading to additional warming. Hydrogen also contributes to the production of ozone and stratospheric water vapor, both potent greenhouse gases. Recent studies estimate hydrogen's 100-year global warming potential at 11 ± 4, meaning it's approximately 11 times more effective than carbon dioxide at trapping heat over a century when indirect effects are considered.

Historical analysis indicates that rising atmospheric hydrogen concentrations between 2010 and 2020 contributed to an increase in global surface air temperature of 0.020 ± 0.006°C. While this represents a relatively small contribution compared to other greenhouse gases, it demonstrates that hydrogen's climate impact is non-negligible and should be considered in climate models and policy decisions.

Future Projections and the Hydrogen Economy

Future climate impacts of hydrogen will depend critically on how hydrogen economies develop, including production methods, leakage rates, and interactions with methane emissions. Under IPCC marker Shared Socioeconomic Pathway (SSP) scenarios, projected hydrogen contributions to global warming range from 0.01–0.05°C, depending on hydrogen usage, leakage rates, and methane emissions. In low-warming scenarios with high hydrogen usage (such as SSP1-1.9 and SSP1-2.6), methane emissions are substantially mitigated, which reduces hydrogen formation from methane oxidation. In these scenarios, hydrogen-induced warming could potentially decrease slightly from current levels if leakage rates remain low (around 1%), though it would increase under higher leakage scenarios (around 10%).

In medium-warming scenarios like SSP2-4.5, where methane emissions decline only slightly, hydrogen's climate impact depends primarily on leakage rates—potentially similar to today under low leakage but increasing substantially under higher leakage. In higher-warming scenarios (SSP4-6.0 or SSP5-8.5), where hydrogen use is relatively low but methane emissions remain largely unmitigated, additional hydrogen formed from methane oxidation can outweigh leaked hydrogen, increasing hydrogen-induced warming even under both low and high leakage scenarios.

Research Gaps and Future Directions

Despite significant advances in understanding the global hydrogen budget, substantial uncertainties and research gaps remain. The largest uncertainty arises from soil uptake, which is sensitive to both model parameterization and intermodel variation of soil attributes. Refined quantification of soil characteristics and better understanding of microbial hydrogenase activity are needed to improve modeling of this critical sink. More in situ measurements across different ecosystems and seasons would help validate and constrain process-based hydrogen uptake models.

Uncertainties also persist regarding hydrogen emission factors for various sectors, particularly for minor sources. Limited data exist for leakage rates in production, transport, and end-use facilities; emission factors for fossil fuel and biofuel combustion; and processes like biological nitrogen fixation in oceans. The development of more sensitive and portable hydrogen measurement instruments—similar to those available for methane—would significantly advance empirical data collection and reduce these uncertainties.

Conclusion

The global hydrogen budget represents a complex system with significant implications for climate policy and the development of sustainable energy systems. While hydrogen offers promising pathways for decarbonizing challenging sectors, its indirect climate effects necessitate careful management and monitoring. Current estimates indicate that rising atmospheric hydrogen has contributed measurably to global warming, and future impacts will depend critically on how hydrogen economies evolve—particularly regarding leakage rates and interactions with methane emissions. As nations invest in hydrogen infrastructure and technologies, incorporating these atmospheric considerations into planning and policy will be essential for realizing hydrogen's full climate benefits while minimizing unintended consequences. Continued research to reduce uncertainties in the hydrogen budget, particularly regarding soil uptake and emission factors, will provide crucial guidance for developing a climate-safe hydrogen economy.