Introduction Hydrogen offers a promising avenue towards a cleaner and sustainable energy future. In its molecular form (H2), it is the simplest and most abundant element in the universe and possesses immense potential as an eco-friendly energy carrier. Its versatility, when harnessed efficiently, can significantly reduce greenhouse gas emissions and address pressing environmental concerns associated with traditional fossil fuel dependency. Between 2021 and 2050, the robust desire for H2 expansion and the embrace of greener production methods are projected to offset around 60 gigatonnes (Gt) of carbon dioxide (CO2) emissions, constituting 6% of the overall decrease in accumulated emissions. Techniques to Produce Hydrogen Hydrogen can be derived through various methods, each with distinct environmental footprints. The prevalent techniques predominantly include steam methane reforming, which utilizes fossil fuels, and electrolysis, a process that can be powered by renewable energy. These methods significantly diverge in their environmental consequences, between carbon-intensive and eco-friendly production pathways. The different methods are often indicated by a color (Table 1). Table 1: Main Processes of Hydrogen Production Color Method Details Green Biomass Gasification Biomass is converted into a gaseous product, which can be used to produce hydrogen. Green Water Electrolysis Electricity generated by renewable sources is used to split water into hydrogen and oxygen. Green Bioenergy Various biological processes can be utilized to produce hydrogen from organic materials. White Ground extraction Naturally occurring form of hydrogen gas generated within the Earth's crust is extracted. Turquoise Methane Pyrolysis Methane is decomposed into hydrogen and solid carbon. Blue Steam Methane Reforming The carbon dioxide produced during its creation is captured and stored underground through industrial carbon capture and storage (CCS). Grey Steam Methane Reforming Methane is heated with steam to produce hydrogen and carbon monoxide. Black/Brown Coal Gasification Coal is converted into a gaseous product containing hydrogen. Source: Adapted from J. Incer-Valverde et al. 2023. “Colors” of Hydrogen: Definitions and Carbon Intensity. Energy Conversion and Management. 291. 117466. Currently, hydrogen production mainly relies on fossil fuels, with natural gas and coal making up 95% of the total output. It is used in power systems, transportation, hydrocarbon and ammonia production, and metallurgical industries. Water electrolysis can produce hydrogen from 100% renewable sources (“green hydrogen”), but it only makes up less than 5% of global production currently. However, water electrolysis is becoming more popular because it has no carbon emissions, no harmful by-products like sulphates, carbon oxides and nitrogen oxides, and high hydrogen purity. Producing hydrogen in a clean manner can decrease emissions of greenhouse gases (GHGs), such as carbon dioxide (CO2) and methane (CH4), as well as other pollutants like carbon monoxide (CO), volatile organic compounds (VOCs), and nitrogen oxides (NOx), potentially resulting in improved air quality. It is important to note that any leakage of hydrogen could alter the atmosphere’s composition, affecting air quality and causing an indirect warming effect on the climate. This could partially negate the climate benefits of reducing CO2 emissions. As a result, it is crucial to ensure that hydrogen is produced, stored, and transported safely and efficiently to minimize any potential negative environmental impacts. Potential Impacts of Increased Hydrogen in the Atmosphere Hydrogen is currently present in the atmosphere with a mixing ratio[1] of about 0.55 parts per million (ppm). The sources of hydrogen in the atmosphere include fossil fuel combustion, biomass burning, and oxidation of methane and volatile organic compounds, in addition to its natural occurrence. In the lower atmosphere, H2 is mostly removed by soil uptake (“soil sink”), either by microorganisms or biogeochemical reactions. The size of the H2 soil sink may be difficult to determine due to the limited understanding of the processes involved and the challenges in extrapolating local measurements of H2 uptake to a larger geographical scale. The large-scale soil alterations and degradation as a result of human activities could also affect the soil’s capacity to metabolize hydrogen. In the upper atmosphere, chemical reactions include hydrogen oxidation by free hydroxide (OH-) radicals (“chemical sink”). The methane lifetime would increase linearly with the increase in hydrogen even if methane emissions remain constant. For every 1 ppm increase in hydrogen, methane lifetime would increase by about a year, and its concentration by about 12%, according to mathematical modelling. If hydrogen emissions continue to increase, it could have a significant impact on the concentration of methane in the atmosphere. This could have some important implications for the methane global warming potential (GWP) due to its longer persistence in the atmosphere. However, due to the dominance of the soil sink over the OH chemical sink, the feedback mechanism involving OH is supposed to have a relatively minor impact on the total atmospheric lifetime of H2. The concentration of hydrogen in the atmosphere has also a direct impact on the mixing ratios of ozone (O3) in the upper stratosphere. As the H2 concentration increases, the O3 mixing ratios in the upper stratosphere, above 40 km altitude, decrease. For instance, a 1.5 ppm increase in H2 concentration results in a decrease of about 5% in O3 mixing ratios. Other significant reactions that involve H2 and water vapor might occur, as shown in the Table 2. Table 2: Potential Effects of Increased Hydrogen and Water Vapor in the Atmosphere Increase in Water Vapor Increase in the abundance of water vapor in the stratosphere, potentially by as much as 1 ppm. Water vapor is a significant greenhouse gas, but its direct contribution to global warming is complex.[2] Stratospheric Cooling Increase in water vapor in the stratosphere could lead to stratospheric cooling, potentially impacting the temperature of the lower stratosphere. Colder temperatures could have implications for atmospheric dynamics, including the formation of polar stratospheric clouds and the persistence of the ozone hole. Enhancement of Ozone Destruction Increase in the stratosphere’s water vapor could enhance the heterogeneous chemistry[3] that destroys O3, potentially leading to changes in stratospheric O3 levels.[4] Changes in Noctilucent Clouds Increase in the mesosphere’ water vapor derived from hydrogen could lead to more noctilucent clouds, potentially impacting the earth's albedo, and mesospheric chemistry. Impact on Tropospheric Chemistry and Atmosphere-Biosphere Interactions Changes in the stratosphere could have downstream effects on tropospheric chemistry and atmosphere-biosphere interactions, potentially affecting ecosystems and air quality. Source: N. Warwick et al. 2022. Atmospheric Implications of Increased Hydrogen Use. The increase in atmospheric hydrogen can lead to variations in tropospheric ozone, water vapor, and methane, which could increase radiative forcing and partially offset the climate benefits of switching to the hydrogen economy. The net radiative forcing at the top of the atmosphere will strongly depend on the rate of hydrogen leakage, any associated reduction in methane emissions, and the extent of co-emission benefits. Hydrogen Global Warming Potential Hydrogen, in its molecular form, is not able to absorb infrared radiation, so it does not have a direct Global Warming Potential. However, for the reasons discussed above, the presence of H2 can have an indirect effect on the atmospheric chemistry, increasing the CH4 lifetime and water vapor and ozone concentration, that would then be impacting the planet’s energy balance. A combined (indirect) Global Warming Potential for H2 can be calculated by adding the Global Warming Potential arising from changes to CH4, tropospheric ozone, and stratospheric water vapor. Hydrogen’s atmospheric lifetime is another key, but unclear, factor to be included for its Global Warming Potential. Two main routes destroy H2: reacting with OH and going into soils (around 82% estimated). The timing of both these mechanisms is still very unclear. Recent estimates of hydrogen Global Warming Potential (100 years’ time horizon) is 11 ± 5, more than double of what was previously calculated. Hydrogen fugitive emissions should be precisely measured to prevent canceling out the advantages of using hydrogen for lowering GHG emissions. The lack of clarity in how much hydrogen may escape, how much hydrogen may substitute fossil fuels in the future, and how much hydrogen may be absorbed by soils, make it hard to correctly forecast the alterations to hydrogen levels in the air in a hydrogen economy. Hydrogen Leaks Hydrogen tends to leak more than natural gas because of its small molecule size. A recent study on the US natural gas supply chain showed natural gas leaks of about 2.3% of gross gas production, which is about 60% more than what the US Environmental Protection Agency previously estimated. It is reasonable to expect that hydrogen would leak at a similar rate, even considering additional investment to control leaks. A recent study identified noncontinuous sources, such as flares, vents, and ship loading/unloading as the main contributors of methane emissions in onshore facilities. How much hydrogen escapes in different carriers, and by consequence the potential indirect Global Warming Potential of these emissions, can change based on the processes used for making, moving, and using them. A recent study compared four green hydrogen carriers: ammonia (NH3), toluene/methylcyclohexane (MCH), methanol (MeOH), and formic acid (HCOOH). It looked at different scenarios of hydrogen leakage during their production, transportation, and use of 1-ton H2-equivalent of each carrier, for both a 100-year period (GWP-100) and a 20-year one (GWP-20). While the Global Warming Potential of GHG is commonly represented as GWP-100, a shorter period is very relevant to achieve the Paris Agreement, considering the critical role of the next few decades in reducing GHG emissions.[5] The calculations indicate that the Global Warming Potential of hydrogen could be higher than those of fossil fuels in the next 20 years (Figure 1-A), but significantly lower in a 100-year period (Figure 1-B). The calculations have a high degree of uncertainty, as shown by the wide gap between the upper and lower estimates. An example of the estimations of H2 fugitive emissions for NH3 is provided in Table 3. Figure 1: Global Warming Potential (in kgCO2eq/GJe) of Fossil Fuels and H2 Energy Carriers (Based on H2 fugitive Emissions Estimates) A = GWP-20 years; B = GWP-100 years.NH3 = ammonia, MCH = methylcyclohexane, MEOH = methanol, HCOOH = formic acid.Source: Adapted from I. Dutta et al. 2023. The Role of Fugitive Hydrogen Emissions in Selecting Hydrogen Carriers. ACS Energy Letters. 8 (7). pp. 3251–3257. Table 3: Estimated Fugitive Emissions (FE) of NH3 Carrier Phase Method H2 loss (Fugitive Emissions) Today's technology H2 loss (Fugitive Emissions) Future scenario H2 production Electrolysis 3–10% 1–3% H2 transport Cooling + Compression 0.5–1% 0.5–1% NH3 production from H2 Haber-Bosch (H-B)[6] 4–5% 4–5% Conversion of NH3 to H2 after transport NH3 cracking[7] 15-20% 3–5% H2 compression for use Cooling + compression 5% 1–3% Source: Data (simplified) from I. Dutta et al. 2023. The Role of Fugitive Hydrogen Emissions in Selecting Hydrogen Carriers. ACS Energy Letters. 8 (7). pp. 3251–3257. Conclusions The future of atmospheric hydrogen levels in a hydrogen economy is hard to estimate accurately because there are uncertainties in how much hydrogen might leak, how much might replace fossil fuels, and how much might be absorbed by soils. The amount of H2 that escapes into the air as a result of a hydrogen economy will vary based on how much H2 is used in different energy sectors and how much is lost to the atmosphere during its generation, delivery, storage and use. The degree of any change depends on how much hydrogen is used and the fossil fuel technologies that are replaced, and an estimation of its Global Warming Potential should be considered in developing policies to increase the hydrogen use. Agencies are starting to recognize hydrogen leakage as an issue. A global standard for losses needs to be considered as an important priority if hydrogen is adopted as a major energy source. Training and certification will be required for logistics providers as we undertake the transition into new energy sources. [1] The mixing ratio in the atmosphere refers to the amount of one substance (like water vapor or carbon dioxide) compared to the amount of dry air per unit of measure. [2] The warming effect of increased water vapor is part of the feedback loops in the climate system rather than a direct consequence of emissions like other greenhouse gases. [3] Water vapor can participate in heterogeneous chemical reactions on polar stratospheric clouds, leading to the release of reactive chlorine and bromine species. These reactive species contribute to the catalytic destruction of ozone. [4] If the atmospheric concentration of hydrogen increases with no change in other emissions, there would be a subsequent increase in tropospheric ozone. The relationship between the increase in hydrogen and the change in tropospheric ozone burden is linear and a 1.5 ppm increase in hydrogen would lead to a 1.8% increase in the tropospheric ozone burden. [5] The Paris Agreement aims to “hold the increase in the global average temperature to well below 2°C above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5°C above pre-industrial levels, recognizing that this would significantly reduce the risks and impacts of climate change.” To stay below 1.5°C of global warming, GHG emissions need to be cut by roughly 50% by 2030. [6] The Haber-Bosch method combines nitrogen (N2) and hydrogen (H2) in a special reactor at high temperature and pressure with the help of a catalyst to form ammonia (NH3). [7] NH3 cracking involves the thermal decomposition of ammonia (NH3) into nitrogen (N2) and hydrogen (H2). This process is typically carried out at elevated temperatures (800 to 1000°C), with or without catalysts, to facilitate the breaking of chemical bonds within ammonia molecules. Resources A. I. Osman et al. 2022. Hydrogen Production, Storage, Utilisation and Environmental Impacts: A Review. Environmental Chemistry Letters. 20 (1). pp. 153–188. F. Innocenti et al. 2023. Comparative Assessment of Methane Emissions from Onshore LNG Facilities Measured Using Differential Absorption Lidar. Environmental Science & Technology. 57 (8). pp. 3301–3310. G. Pieterse et al. 2013. Reassessing the Variability in Atmospheric H2 Using the Two‐Way Nested TM5 Model. Journal of Geophysical Research: Atmospheres. 118 (9). pp. 3764–3780. I. Dutta et al. 2023. The Role of Fugitive Hydrogen Emissions in Selecting Hydrogen Carriers. ACS Energy Letters. 8 (7). pp. 3251–3257. J. Incer-Valverde et al. 2023. “Colors” of Hydrogen: Definitions and Carbon Intensity. Energy Conversion and Management. 291. 117466. N. Warwick et al. 2022. Atmospheric implications of Increased Hydrogen Use. R. A. Alvarez et al. 2018. Assessment of Methane Emissions from the U.S. Oil and Gas Supply Chain. Science. 361 (6398). pp. 186–188. R. Conrad and W. Seiler. 1981. Decomposition of Atmospheric Hydrogen by Soil Microorganisms and Soil Enzymes. Soil Biology and Biochemistry. 13 (1). pp. 43–49. T. K. Tromp et al. 2003. Potential Environmental Impact of a Hydrogen Economy on the Stratosphere. Science. 300 (5626). pp. 1740–1742. T. S. Rhee, C. A. M. Brenninkmeijer, and T. Röckmann. 2006. The Overwhelming Role of Soils in the Global Atmospheric Hydrogen Cycle. Atmospheric Chemistry and Physics. 6 (6). pp. 1611–1625. United States Environmental Protection Agency. Hydrogen in Transportation. Ask the Experts Francesco Ricciardi Senior Environment Specialist, Office of Safeguards, Asian Development Bank Prior to joining ADB, Francesco Ricciardi worked for about 10 years as a researcher focusing on the impact of environmental contamination on natural ecosystems and wildlife. After leaving academia, he worked as an environment and ecology consultant in several projects around Asia, including renewable energy plants, large coastal infrastructures, and natural resources development and protection. He is a passionate underwater and wildlife photographer. Some of his photos have been published in international magazines and papers. Follow Francesco Ricciardi on Stephen Peters Senior Energy Specialist (Waste-to-Energy), Sectors Group, Asian Development Bank Steve Peters is responsible for developing the knowledge base, project development and implementation in waste to energy and supporting projects across waste, the circular economy and ocean impacts. He holds a Master of Engineering Science degree and a Bachelor of Engineering degree from the University of Melbourne, Australia. He is a past Board Member of Austcham Singapore. He was previously the Managing Director of the Stratcon group of companies and past chairman of the Asia Pacific Biogas Alliance. Leave your question or comment in the section below: View the discussion thread.