As we move toward net-zero and low-carbon emissions, Hydrogen (H₂) is expected to play a crucial role as an alternative energy source. H₂ is considered a clean fuel, as it is produced through electrolysis, where water is split into H₂ and oxygen (O2). Its conversion to heat or power is both efficient and environmentally friendly. When combusted with O2, H₂ produces only water, without generating any pollutants.
H₂ production is categorized based on the source and production method, often classified by colour codes:
- Green H₂ – Produced from 100% renewable sources (such as wind or solar energy) through electrolysis, resulting in a low carbon footprint.
- Blue H₂ – Derived from fossil fuels but with carbon capture and storage (CCS) technology to reduce emissions.
- Grey H₂ – Produced from fossil fuels without CCS, emitting significant CO₂ (one tonne of H₂ production can release up to 10 tonnes of carbon) (Dvoynikov et al., 2021).
These classifications help the energy industry differentiate H₂ types. The various methods of H₂ production, storage, and applications are illustrated in Figure 1.
Figure 1. H₂ production routes, including renewables, fossil fuels, and nuclear, with H₂ being produced in power plants, pharmaceutical applications, synthetic fuels or their upgrades in transportation, ammonia synthesis, metal production, or chemical industry application (Osman et al., 2022).
H₂, being a small and highly diffusive molecule, can easily leak during production, storage, and distribution, which may reduce its climate benefits. When H₂ is released into the atmosphere, approximately 70%–80% is absorbed by the soil, while the remaining 20%–30% reacts with hydroxyl (OH) radicals. This oxidation process increases atmospheric concentrations of methane (CH₄), ozone (O₃), and water vapor (H₂O), leading to enhanced radiative forcing.
A key consequence of H₂ oxidation is the depletion of OH radicals, which in turn extends the lifetime of CH₄, increasing its atmospheric abundance (Derwent et al., 2001). Figure 2 illustrates the oxidation process of H₂ in the atmosphere.
Figure 2. The effects of H₂ oxidation in the atmosphere (Ocko & Hamburg, 2022).
Work done by N. J. Warwick et al. (2023) using the UKESM model by increasing H2 mixing ratios at the surface from 500 to 750, 1000, 1500, and 2000 ppb. The results showed a decrease in OH radical concentrations (Figure 3a), an increase in CH₄ lifetime (Figure 3b), and an increase in tropospheric O₃ concentrations (Figure 3c). These findings indicate that increased H₂ concentration could enhance radiative forcing (Figure 3d).
Fig3. (a) mass-weighted tropospheric mean OH, (b) CH4 lifetime concerning OH, (c) tropospheric O3, and (d) increasing ERF in UKESM1. In panels (a)–(d), Black circle (BASE scenario), Blue circle (the CH4 LBC remains fixed at 2014 levels), Red circle (CH4 LBC is adjusted to account for the change in CH4 lifetime), Orange circle (changes in emissions of ozone precursors). Green circle (changes in emissions of CH4 and ozone precursors, as well as adjusted CH4 LBCs). See N. J. Warwick et al. (2023).
Global Warming Potential (GWP) is a key metric that compares the warming effect of different greenhouse gases relative to CO₂ over a specified period, typically 100 years (GWP100). Since H₂ is an indirect greenhouse gas, its GWP is influenced by its effects on CH₄, O₃, and stratospheric H₂O. However, the main uncertainty while calculating H₂ GWP came from the soil sink process in different models. A multimodal study with a homogeneous soil sink of 59 Tg yr−1 in all models estimated that GWP100 of H2 is 11.4 ± 2.8 (Sand et al., 2023). A study by Weik et al. (2023) using the UKESM1 model with prescribed H₂ surface concentrations, estimated H₂ GWP as high as 12 ± 6. While H₂’s warming effects are potent, they are relatively short-lived compared to methane. Some of its effects occur within a decade after emission, but its influence on methane extends its climatic impact for roughly another decade (N. Warwick et al., 2022) .
At Reading University, I am investigating the impact of H₂ emissions on atmospheric composition and climate using the UKESM model. We have conducted multiple 40-year model simulations under present and future H₂ and CH₄ concentration scenarios, isolating the contributions of effective radiative forcing (ERF) from O₃, aerosols, and stratospheric H₂O. Additionally, we decomposed each ERF component under clean-sky (no aerosol) and clear-sky (no cloud) conditions.
This approach requires diagnostic calculations of top-of-atmosphere radiative fluxes under three conditions (Ghan, 2013): whole-sky, clean-sky (neglecting aerosol scattering and absorption), and clear clean-sky (neglecting both cloud and aerosol interactions). Preliminary results suggest that clouds play a crucial role in modulating radiative forcing by altering aerosol properties.
To further explore the complex interactions influencing CH₄, O₃, and H₂ evolution, we conducted pulse experiments in which a 1-month H₂ emission pulse was introduced globally and over specific regions. These experiments will help quantify the magnitude and timing of changes in H₂, CH₄, and O₃, improving our understanding of H₂’s indirect effects on atmospheric chemistry and climate.
References
Derwent, R. G., Collins, W. J., Johnson, C. E., & Stevenson, D. S. (2001). Transient behaviour of tropospheric ozone precursors in a global 3-D CTM and their indirect greenhouse effects. Climatic Change, 49(4), 463–487. https://doi.org/10.1023/A:1010648913655
Dvoynikov, M., Buslaev, G., Kunshin, A., Sidorov, D., Kraslawski, A., & Budovskaya, M. (2021). resources New Concepts of Hydrogen Production and Storage in Arctic Region. https://doi.org/10.3390/resources100
Ghan, S. J. (2013). Technical note: Estimating aerosol effects on cloud radiative forcing. Atmospheric Chemistry and Physics, 13(19), 9971–9974. https://doi.org/10.5194/acp-13-9971-2013
Ocko, I. B., & Hamburg, S. P. (2022). Climate consequences of hydrogen emissions. Atmospheric Chemistry and Physics, 22(14), 9349–9368. https://doi.org/10.5194/acp-22-9349-2022
Osman, A. I., Mehta, N., Elgarahy, A. M., Hefny, M., Al-Hinai, A., Al-Muhtaseb, A. H., & Rooney, D. W. (2022). Hydrogen production, storage, utilisation and environmental impacts: a review. In Environmental Chemistry Letters (Vol. 20, Issue 1, pp. 153–188). Springer Science and Business Media Deutschland GmbH. https://doi.org/10.1007/s10311-021-01322-8
Sand, M., Skeie, R. B., Sandstad, M., Krishnan, S., Myhre, G., Bryant, H., Derwent, R., Hauglustaine, D., Paulot, F., Prather, M., & Stevenson, D. (2023). A multi-model assessment of the Global Warming Potential of hydrogen. Communications Earth and Environment, 4(1). https://doi.org/10.1038/s43247-023-00857-8
Warwick, N., Griffiths, P., Keeble, J., Archibald, A., Pyle, J., & Shine, K. (2022). Atmospheric implications of increased Hydrogen use.
Warwick, N. J., Archibald, A. T., Griffiths, P. T., Keeble, J., O’Connor, F. M., Pyle, J. A., & Shine, K. P. (2023). Atmospheric composition and climate impacts of a future hydrogen economy. Atmospheric Chemistry and Physics, 23(20), 13451–13467. https://doi.org/10.5194/acp-23-13451-2023
