Hydrogen Is An Indirect Greenhouse Gas So Now Hydrogen Proponents Are Denying It – CleanTechnica

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Hydrogen has been touted as a clean energy source, a critical pillar of decarbonization efforts in sectors from heavy industry to transportation. Policymakers and industry leaders alike have been singing its praises for decades, envisioning a future powered by green hydrogen, electrolyzers, and pipelines feeding a global energy transition. But buried beneath the hype lies an inconvenient truth: hydrogen itself is an indirect greenhouse gas with a potentially significant warming effect.

With inconvenient truths comes denial, of course. An emerging feature of hydrogen proponent’s discourse is denial of hydrogen’s role in the atmosphere, denial of the science, minimization of the problem and deprecation of the scientists doing the work. Most recently, I ran into this on LinkedIn, where an engineer whose entire career had revolved around hydrogen, including work for NASA and on liquid hydrogen facilities, used exactly the same argument climate change deniers make, but specifically about hydrogen.

That argument is that there has been no direct atmospheric study proving that CO₂ is a greenhouse gas because there is no “control atmosphere” against which to compare its effects. Deniers argue that since Earth’s atmosphere has always contained CO₂ in varying concentrations, there is no experimental setup where CO₂ is entirely absent to serve as a baseline. This claim suggests that climate science relies solely on models rather than empirical atmospheric experiments.

This argument is fundamentally flawed. The greenhouse effect of CO₂ has been demonstrated through multiple lines of evidence, including laboratory spectroscopy, satellite observations, and real-world atmospheric measurements. John Tyndall’s 1859 experiments showed that CO₂ absorbs infrared radiation, and this was later confirmed with increasing precision. In the 20th century, experiments such as those by Guy Callendar and Charles Keeling’s long-term CO₂ measurements at Mauna Loa reinforced the link between rising CO₂ levels and global warming. More recent satellite observations from NASA’s AIRS and CERES programs directly show CO₂ trapping heat in the atmosphere.

The “no control atmosphere” claim also ignores natural experiments, such as past climate records showing how CO₂ levels correlated with temperature changes over millions of years. Volcanic eruptions and ice core data provide clear evidence of how CO₂ influences global temperatures. Additionally, the controlled physics of radiative transfer, used in engineering and remote sensing, further validates CO₂’s warming effect.

This denialist tactic mirrors those used to cast doubt on hydrogen’s indirect warming effect, where critics like the engineer claim a lack of “real-world” confirmation despite extensive atmospheric chemistry modeling, laboratory studies, and empirical observations supporting the science.

A 2022 article from RMI titled Hydrogen Reality Check #1: Hydrogen Is Not a Significant Warming Risk leaned into this, downplaying the concern substantially, claiming that:

even at high rates of leakage, green hydrogen has an undeniably positive climate benefit in the short- and long-term

That was part of the reason I wrote a 14,000 word analysis of everything RMI had published related to hydrogen pointing out the errors in their assumptions and modeling across multiple domains, and provided them guidance on a strategic reset of their hydrogen position in the piece RMI Has Fallen Into The Hydrogen For Energy Pit Again. I’m gratified to report that their publications since that time have been far less full of hydrogen maximalist positions, and much more aligned with hydrogen reality. I consider it unfortunate that they still have the apologia and misinforming hydrogen warming piece up, and would recommend that they retire it or rewrite it.

To be blunt, where direct electrification is viable — vastly more of the economy than hydrogen maximalists assert — the climate benefits are far better than if the same energy use cases were served by hydrogen. Comparing hydrogen only to the worst case is disingenuous at best and hinders useful comparison of policies, and it’s unfortunate RMI allowed and allows that to be its position.

Back to the science. While hydrogen’s direct radiative forcing is negligible, its interaction with atmospheric chemistry—specifically its impact on methane decomposition and hydroxyl radicals—amplifies climate change in ways that are now receiving serious attention. Understanding hydrogen’s role as an indirect greenhouse gas requires a journey through atmospheric chemistry, laboratory experiments, global warming potential calculations, and the evolution of policy responses. The story begins decades ago, but only now is it shaping decisions at the highest levels of climate governance.

Early research established the hydroxyl radical (OH) as the atmosphere’s primary “cleaning” agent, responsible for oxidizing trace gases like carbon monoxide (CO), methane (CH₄), and hydrogen (H₂). In 1969, Bernard Weinstock used radiocarbon-labeled CO to show that CO’s atmospheric lifetime is on the order of only a few months (~0.1 year), far shorter than expected. He proposed that an unknown tropospheric sink – likely reaction with OH (e.g. CO + OH → CO₂ + H) – must be rapidly removing CO. Building on this, Hiram Levy II published a landmark study in Science (1971) that predicted surprisingly large concentrations of OH radicals in the “normal” unpolluted atmosphere​. Levy’s model demonstrated a radical chain reaction initiated by ozone photolysis forming OH, which could swiftly oxidize CO and CH₄ (methane), in the lower atmosphere​.

Dieter Ehhalt’s 1974 analysis of The Atmospheric Cycle of Methane (published in Tellus) demonstrated that methane is removed primarily by OH attack in the troposphere. He noted that ~90% of CH₄ destruction occurs via the reaction CH₄ + OH → CH₃ + H₂O in the lower atmosphere (with only ~10% destroyed in the stratosphere)​. This was a critical realization: it identified OH as the chief sink for the potent greenhouse gas methane, implying a methane lifetime on the order of a decade controlled by OH abundance.

The first scientific discussions about hydrogen’s potential role in atmospheric chemistry surfaced in the 1970s and 1980s, but they remained largely theoretical. Researchers hypothesized that hydrogen, when released into the atmosphere, could interact with the key chemical processes responsible for breaking down methane, a potent greenhouse gas. These early insights suggested that hydrogen might extend methane’s atmospheric lifetime, indirectly increasing its warming impact.

The key reaction of interest is: H₂ + ·OH → H₂O + ·H. This hydrogen-abstraction reaction converts molecular hydrogen into water vapor and a hydrogen atom radical. OH is the central agent of the atmosphere’s self-cleansing capacity. If OH concentrations decline, the lifetimes of CH₄, CO, H₂ and many volatile organic compounds will increase correspondingly. More hydrogen in the atmosphere means less OH in the atmosphere to cleanse it of CH4, methane.

Decades of laboratory experiments have confirmed that hydroxyl radicals play a crucial role in atmospheric chemistry by reacting with methane and hydrogen. These reactions are essential for regulating greenhouse gases and determining the lifespan of atmospheric pollutants.

One of the earliest proofs came in the 1970s when researchers used flash photolysis to generate OH radicals and measure their interactions with methane. Scientists like David J. Howard and W. A. Payne demonstrated that OH radicals rapidly break down methane molecules, forming methyl radicals and water. Their work, published in The Journal of Chemical Physics, laid the foundation for understanding OH as the atmosphere’s “detergent.”

In later years, more precise techniques emerged. Pulsed laser photolysis with laser-induced fluorescence (PLP-LIF) allowed scientists to track OH radicals in real time. A 2006 study by Orkin et al. in The Journal of Physical Chemistry A measured the reaction of OH with hydrogen, confirming how the process influences atmospheric composition. Similarly, researchers at NASA’s Jet Propulsion Laboratory employed chemical ionization mass spectrometry (CIMS) to detect intermediate products, reinforcing that OH radicals are actively involved in methane breakdown.

While these findings are widely accepted in atmospheric chemistry, students typically encounter them at different levels. Basic concepts of OH chemistry appear in undergraduate physical and environmental chemistry courses, but hands-on laboratory experiments are reserved for graduate-level physical chemistry and atmospheric science programs. Advanced research teams now use flow tube reactors and real-time laser spectroscopy to refine reaction rate measurements and assess their climate impact.

As with carbon dioxide and methane’s global warming potential being proven over and over again in laboratory circumstances, so too with hydroxyls, methane and hydrogen. This is all proven and settled science and now only recreated in educational settings as part of pedagogical exercises around chemistry, atmospheric science and lab methodologies and tools.

​In 1999, Patricia C. Novelli and her colleagues published a seminal paper titled Molecular hydrogen in the troposphere: Global distribution and budget in the Journal of Geophysical Research. This study provided a comprehensive analysis of molecular hydrogen (H₂) in the Earth’s lower atmosphere, offering valuable insights into its global distribution and the factors influencing its concentrations. ​

In 2001, Derwent and his colleagues sought to quantify the role of hydrogen as a greenhouse gas using STOCHEM, a global Lagrangian chemistry transport model. Their approach was straightforward but revealing. The model was initialized with real-world trace gas concentrations from October 1994 and simulated under actual wind conditions until early 1995. At that point, they introduced a controlled perturbation: an additional 40 teragrams (Tg) of hydrogen was injected into the modeled atmosphere by January 31, after which emissions returned to normal levels. The model then ran through to the end of 1998, allowing researchers to track how this hydrogen pulse affected atmospheric composition over time.

By comparing the control scenario—where no extra hydrogen was added—to the experiment with the hydrogen pulse, the researchers could isolate the impact of excess hydrogen on atmospheric chemistry. The differences, termed “excess concentrations,” illustrated how additional hydrogen influences the delicate balance of atmospheric gases. Although the size of the hydrogen pulse was somewhat arbitrary, its spatial distribution mirrored that of anthropogenic emissions, making the findings relevant to real-world hydrogen economy scenarios. The study Transient Behaviour of Tropospheric Ozone Precursors in a Global 3-D CTM and Their Indirect Greenhouse Effects, was among the first to provide a quantitative assessment of how hydrogen emissions could indirectly extend methane’s atmospheric lifespan by depleting hydroxyl radicals, reinforcing concerns that hydrogen leakage could have unintended climate consequences.

A landmark paper by Derwent and colleagues in 2006, Global Environmental Impacts of the Hydrogen Economy, detailed the consequences of widespread hydrogen use. The study used atmospheric models to estimate the potential increase in methane concentrations due to hydrogen leakage.

Hydrogen is therefore an indirect greenhouse gas with a global warming potential GWP of 5.8 over a 100-year time horizon. A future hydrogen economy would therefore have greenhouse consequences and would not be free from climate perturbations. If a global hydrogen economy replaced the current fossil fuel-based energy system and exhibited a leakage rate of 1% then it would produce a climate impact of 0.6% of the current fossil fuel based system. If the leakage rate were 10%, then the climate impact would be 6% of the current system

Note that was the first time a GWP was identified, but also that every time it was reassessed the GWP went up. Around the same time, Prather reinforced these findings, demonstrating that hydrogen emissions, even at modest levels, could exacerbate climate change. As experimental proof accumulated, scientists called for hydrogen’s inclusion in climate models.

Ehhalt and Rohrer examined the tropospheric cycle of hydrogen and published in 2009. Their comprehensive review, The tropospheric cycle of H₂: a critical review, synthesized decades of atmospheric data to establish a clearer picture of its distribution, budget, and isotopic composition. The study confirmed that hydrogen is relatively evenly distributed globally, with only slight latitudinal variations and seasonal shifts. It found that while hydrogen’s atmospheric lifetime is pegged at around two years, its role in atmospheric chemistry is more dynamic than previously understood, reinforcing the importance of accurate emissions tracking as hydrogen production scales up in the energy transition.

Despite these early warnings, hydrogen’s role in climate change remained a footnote. The world’s focus was elsewhere—on carbon dioxide, methane, and nitrous oxide—while hydrogen was seen as little more than an intermediary energy carrier.

By the 2010s, climate scientists began refining estimates of hydrogen’s global warming potential, which measures the relative warming impact of a gas compared to carbon dioxide over a specified period. Unlike methane or carbon dioxide, hydrogen’s impact is entirely indirect—stemming from its role in prolonging methane’s lifespan and its effects on stratospheric water vapor.

Ocko and Hamburg in 2022 further solidified these concerns, showing that hydrogen’s GWP peaks at a 7-year time horizon, with a mean value of 40 and a range between 25 and 60, publishing their findings in Climate consequences of hydrogen emissions.

green hydrogen applications with higher-end emission rates (10 %) may only cut climate impacts from fossil fuel technologies in half over the first 2 decades, which is far from the common perception that green hydrogen energy systems are climate neutral.

Remember those emissions rates.

Most recently, Sand et al. in 2023 published research refining hydrogen’s global warming impact further, calculating a GWP20 of 37 and a GWP100 of 12. The 2023 study A Multi-Model Assessment of the Global Warming Potential of Hydrogen brought together leading atmospheric scientists from multiple institutions to refine estimates of hydrogen’s climate impact. The research team included Maria Sand, Ragnhild Bieltvedt Skeie, and Marit Sandstad from the Center for International Climate and Environmental Research (CICERO) in Norway, Gunnar Myhre from the Norwegian Meteorological Institute, and Didier Hauglustaine from the Laboratoire des Sciences du Climat et de l’Environnement in France. The study also featured contributions from renowned climate modelers such as Michael Prather from the University of California, Irvine, and Richard Derwent, a veteran atmospheric chemist. With expertise spanning atmospheric modeling, chemistry-climate interactions, and radiative forcing analysis, this collaboration produced one of the most comprehensive evaluations of hydrogen’s GWP, with findings critical for informing energy transition policies.

These findings were a wake-up call for the climate community. While hydrogen itself doesn’t trap heat like carbon dioxide or methane, its interference in atmospheric chemistry means that any hydrogen leakage from pipelines, storage facilities, refueling stations, or fuel cells could add to the climate burden. As policymakers ramped up hydrogen investments, concerns about leakage and unintended emissions gained traction.

Hydrogen’s indirect greenhouse impact is finally influencing climate policy. The IPCC’s Sixth Assessment Report in 2023 explicitly addressed hydrogen’s indirect warming effects, marking its official entry into mainstream climate discourse.

In response, the European Union has taken proactive steps. New regulations require projects to track and mitigate leakage rates, particularly in pipeline infrastructure. Hydrogen’s indirect effects are now included in climate modeling for energy planning. Policymakers are reconsidering hydrogen’s role in heating and transportation, favoring applications where leakage is minimal. The push for a hydrogen economy is unfortunately still strong, but the conversation has shifted. The industry can no longer afford to ignore hydrogen’s indirect warming effects. The challenge now is to minimize leakage through better infrastructure, advanced monitoring technologies, and smarter deployment strategies.

As I noted in a piece a few months on this fundamental challenge, How Much Does Hydrogen Leak And How Much Does It Matter?, hydrogen leaks a lot. It’s the smallest diatomic molecule in the universe and in order to get enough of it in one place for useful amounts of energy to be present, it has to be stored at pressures equivalent to kilometers under the surface of the ocean, at temperatures barely above the background temperature of deep space or both.

Empirical evidence from governmental reports and the peer reviewed literature make it clear that small electrolyzers leak, hydrogen refueling stations leak, hydrogen vehicles leak and hydrogen escapes every time it is transferred between containers. This is despite extraordinary machining tolerances and materials science intentionally designed to keep it on the inside. Maintaining the equipment so that it has minimal leakage requires disciplined and regular maintenance, and even then seals blow out. If supply chains for hydrogen for energy materialized, they would leak, 1% or more per touch point, leading to 5% to 10% leakage rates end to end.

This isn’t a significant concern when hydrogen is manufactured at the point of use as an industrial feedstock because the touch points are low and industrial engineers are charged with detecting leaks and maintaining the equipment. These aren’t hydrogen refueling stations staffed by bored teenagers.

Industry stakeholders must take hydrogen leakage seriously—not as a minor nuisance but as a factor that could undermine net-zero goals. Climate-conscious deployment will require better infrastructure to prevent leaks from production, storage, and transport, accurate lifecycle assessments to quantify hydrogen’s real-world emissions footprint, and policy mechanisms that reflect hydrogen’s indirect warming potential, ensuring it doesn’t become an unintentional climate liability. The lesson here is clear. Hydrogen is not a climate panacea and a low carbon world will use hydrogen only where necessary.