The warming impact of hydrogen has been “overlooked” in projections of climate change, according to authors of the latest “global hydrogen budget”.

The study, published in Nature, is the most comprehensive analysis yet of the global hydrogen cycle, showing how the gas moves between the atmosphere, land and ocean.

Hydrogen has long been recognised as a clean alternative to fossil fuels and an important component of the green energy transition.

However, while hydrogen is not itself a greenhouse gas, rising emissions are “supercharging” the warming effect of methane, the authors say.

Increasing levels of atmospheric hydrogen have led to “indirect” warming of 0.02C over the past decade, the study finds.

The authors say that limiting leaks from future hydrogen fuel projects and rapidly cutting methane emissions will be key to securing benefits from hydrogen as a clean-burning alternative to oil and gas.

The international team of scientists behind the study also produce the annual “global carbon budget”, which saw its 20th edition published last month.

‘Supercharging’ methane

Hydrogen is the lightest and most abundant element in the universe. It is also an explosive gas that contains more energy per unit of weight than fossil fuels.

The gas has long been recognised as a clean alternative to fossil fuels, because it only emits water when burned.

There are many ways to produce hydrogen. It is typically generated in a carbon-intensive process that relies on fossil fuels. However, renewable energy can be used to produce “green hydrogen” with near-zero carbon emissions.

Hydrogen “indirectly” heats the atmosphere through its interactions with other gases. This warming is mainly due to interplay between hydrogen and methane – a potent greenhouse gas that is the second biggest contributor to human-caused global warming after CO2.

This interplay involves molecules in the atmosphere called hydroxyl radicals. These naturally occurring molecules are known as the atmosphere’s “detergents” because they react with certain greenhouse gases, such as methane, converting them into other compounds that do not warm the planet.

Prof Rob Jackson is a scientist at Stanford University and an author on the study. He explains that hydrogen also reacts with hydroxyl radicals, effectively “using up” these detergents and leaving less to react with methane.

This effectively “extends the lifetime” of methane in the atmosphere, Jackson tells Carbon Brief, leading to higher concentrations and greater warming.

There is also a reciprocal effect, where more methane in the atmosphere leads to more hydrogen. This occurs because methane reacts with oxygen in the atmosphere in a process called “oxidation”, which produces hydrogen.

Jackson tells Carbon Brief that interactions between hydrogen and methane have “not really been considered in climate circles”, adding:

“I think people don’t realise that the dominant source of hydrogen in the world today is methane in the atmosphere.”

Overall, the study estimates that increasing levels of hydrogen in the atmosphere led to global warming of 0.02C over 2010-20. This climate impact has been “overlooked”, the researchers say in a press release.

Jackson tells Carbon Brief that although this level of warming “looks fairly small”, it is still “comparable” to the warming caused by emissions of individual countries, such as France.

The hydrogen cycle

The global hydrogen budget brings together a range of observed data and models to quantify sources of hydrogen emissions as well as “sinks”, which absorb the gas from the atmosphere.

The authors find that hydrogen levels in the atmosphere increased from 523 parts per billion (ppb) in 1992 to 543ppb in 2020.

The graphic below shows the main sources (up arrows) and sinks (down arrows) of hydrogen over 2010-20.

As the figure shows, the largest single contributor to rising hydrogen emissions over 2010-20 is from the oxidation of human-produced methane. Methane emissions are on the rise due to human activity, such as from the fossil fuel industry, livestock and waste.

According to the study, 56% of atmospheric hydrogen over 2010-20 was caused by the oxidation of methane and non-methane volatile organic compounds (NMVOCs) reacting with oxygen to produce hydrogen.

(NMVOCs are chemicals that are released naturally from vegetation and more rapidly during wildfires. Human-produced emissions of NMVOCs – for example, from oil refineries or car tailpipes – are also on the rise, according to the study.)

The study also points to leakage from industrial hydrogen production as another driver of rising atmospheric hydrogen levels.

Jackson tells Carbon Brief that hydrogen leakage is on the rise “not because manufacturing is getting dirtier, but because we’re making more hydrogen from coal and natural gas”.

Hydrogen can also be produced as an unintentional byproduct from the combustion of fossil fuels. The study finds that these emissions of hydrogen are decreasing.

At the same time, natural sources of hydrogen emissions have not shown any increasing or decreasing trend over time, the authors say.

One of the largest natural sources of hydrogen is through “nitrogen fixing” – a chemical process in which nitrogen is converted into ammonia, which releases hydrogen as a byproduct. This process locks down nitrogen into the soil and ocean, where it is used by plants and algae to grow.

Meanwhile, hydrogen sinks have “increased in response to rising atmospheric hydrogen” over the past three decades, the study says.

Nearly three-quarters of the global hydrogen sink comes from hydrogen getting trapped in soil – for example, by microbes taking in hydrogen to use for energy, or hydrogen seeping into the soil through diffusion.

Dr Zutao Ouyang is an assistant professor at the University of Harvard and lead author on the study. He tells Carbon Brief that soil uptake is “the main mechanism removing hydrogen from the atmosphere”, but adds that it also has “the greatest uncertainty” because there is “not much long-term data” on this component of the hydrogen budget.

Mapped

Drawing on data including observational measurements and emissions inventories, the authors map the sources and sinks of hydrogen and their relative strength.

The maps below show the sources (top) and sinks (bottom) over 1990-2020, where darker colours indicate a stronger source or sink.

The largest “hotspots” for hydrogen emissions are in “south-east and east Asia”, according to the research. More widely, it says that “tropical regions” contribute about 60% of total hydrogen emissions.

The authors explain that these “hotspots” occur because the oxidation of methane and NMVOCs – processes that happen in the atmosphere and produce hydrogen as a byproduct – happen more quickly at higher temperatures.

They also find that these regions have more vegetation, which leads to higher NMVOC emissions.

For emissions related to human activity, east Asia and North America “contributed the most hydrogen emissions from fossil fuel combustion”, the study says, due to the “intensive fossil fuel use”.

Hydrogen emissions due to nitrogen fixation – when plants draw down nitrogen and release hydrogen as a byproduct – are highest in South America. The report links these emissions to the region’s “extensive cultivation” of crops such as soybeans and peanuts.

Dr Maria Sand is a senior researcher at CICERO and was not involved in the study. She tells Carbon Brief that the paper “provides a valuable and much-needed assessment of the global hydrogen budget”. She adds:

“By better constraining the sources and sinks of hydrogen, this study helps reduce the uncertainty in the climate impact [of hydrogen].”

Dr Nicola Warwick is a researcher at the National Centre for Atmospheric Science and assistant research professor at the University of Cambridge. She tells Carbon Brief that the study “provides an important update to our understanding of the atmospheric hydrogen budget by better constraining the key sources and sinks of hydrogen”.

She adds that better understanding of hydrogen uptake by soil – including how it responds to “climate-driven changes in soil moisture and temperature” – are “essential for reliably assessing the climate impacts of any future changes in hydrogen emissions”.

Study author Jackson tells Carbon Brief that he hopes the study will “prompt people to evaluate some of these emissions and sources and sinks in new ways and new places”.

Hydrogen economy

In the pursuit of net-zero, hydrogen may play an increasingly important role in the global energy system.

There are many ways to produce hydrogen gas. Most hydrogen is currently generated through a process called steam reforming, which brings together fossil gas and steam to produce hydrogen, with CO2 as a by-product.

According to the study, more than 90% of hydrogen produced today uses this “carbon-intensive” method.

However, electricity can be used to split water into hydrogen and oxygen atoms, in a process called electrolysis. If renewable energy is used, hydrogen can be produced and consumed with near-zero carbon emissions.

Hydrogen can be stored, liquified and transported via pipelines, trucks or ships. It can be used to make fertiliser, fuel vehicles, heat homes, generate electricity or drive heavy industry.

This potential hydrogen “economy” is shown in the graphic below. The illustrations, with numbered captions from one to three, show how hydrogen could be made, moved and used

The graphic below, from Carbon Brief’s explainer, illustrates the elements of a potential hydrogen economy.

Jackson tells Carbon Brief that, in his opinion, hydrogen is a “brilliant” choice to replace fossil fuels on-site, for industries such as steel manufacturing. However, he says he is “concerned” about “a hydrogen economy that distributes hydrogen around the world in millions of users”, because there is potential for lots of the gas to leak.

He adds:

“We know that methane leakage is bad. Hydrogen is a smaller molecule than methane. So wherever you have methane and hydrogen together, if methane leaks, hydrogen is likely to leak even more.”

The authors model hydrogen emissions under a range of future warming scenarios over the coming century.

They find that in “low-warming scenarios with high hydrogen usage”, methane emissions are low, limiting the formation of hydrogen via the oxidation of methane. In this instance, changes in atmospheric hydrogen levels depend strongly on leakage.

Meanwhile, in higher-warming scenarios, the authors find that hydrogen use is “relatively low”, but methane emissions remain “largely unmitigated”. In this instance, they find that the additional hydrogen formed through the oxidation of methane can outweigh hydrogen released through leaks.

Overall, the authors suggest that hydrogen could cause additional warming of 0.01-0.05C by the year 2100. Study author Zutao tells Carbon Brief that this additional warming was not included in the climate projections in the last assessment report from the Intergovernmental Panel on Climate Change.

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Hydrogen emissions are ‘supercharging’ the warming impact of methane