Atmospheric hydrogen is rising, which may be a problem for the climate

Levels of hydrogen in the atmosphere have jumped by 60 per cent since pre-industrial times, underscoring the dramatic impact fossil fuel burning has had on the planet’s atmospheric composition. Although hydrogen is not a greenhouse gas, it has an indirect warming effect through reactions with other molecules.

The findings come from the first ever long-term record of atmospheric hydrogen, compiled using data from ice cores extracted in Greenland in 2024. “The ice core record is incredible,” says Alex Archibald at the University of Cambridge.

Hydrogen is a small, lightweight molecule which escapes easily into the atmosphere. That means hydrogen has usually leaked from ice cores before samples make it back to laboratories – which are often thousands of miles away – making it very difficult to compile a long-term time series of its atmospheric levels.

To overcome this problem, John Patterson at the University of California, Irvine and his colleagues took their laboratory equipment into the field, analysing the ice cores immediately after extraction. “We took instruments out onto the ice, and as soon as we had drilled our samples, we were working to get them cleaned and get them sealed up into our melt chambers so that we could do our analysis right there on the ice,” he says.

The team was therefore able to build up a long-term record of atmospheric hydrogen going back 1100 years. It marks a huge advance on the previous longest time series of around 100 years, which was reliant on observational records and snowfall analysis. “It is really impressive from a logistical point of view, getting those measurements out,” says David Stevenson at the University of Edinburgh, UK.

Hydrogen concentrations have increased from about 280 parts per billion in the early 19th century to about 530 parts per billion today, the team found. That is not surprising, according to Patterson, given the steep increase in fossil fuel burning since the pre-industrial era. Hydrogen is released as a byproduct when fossil fuels or biomass are burnt.

Patterson and his colleagues combined information from the ice core records with modelling to try to build a picture of why hydrogen levels have fluctuated over the last millennium. “Our data gives us how the atmosphere has changed, but it doesn’t tell us why the atmosphere changed,” Patterson says. “So we try to use these biogeochemical models to explore why it might have changed.”

For example, the ice core records reveal hydrogen levels in the atmosphere dipped by 16 per cent during the so-called Little Ice Age, a period of lower temperatures between the 16th and 19th centuries. A reduction in wildfire emissions during this time does not fully explain this sharp dip in hydrogen concentrations, Patterson says. “That’s telling us that natural hydrogen biogeochemistry is changing with climate in a way that we don’t really understand, [and] we didn’t really expect,” he says. That could have worrying implications for the future, suggesting hydrogen levels in the atmosphere could be much more sensitive to climatic changes than first thought, Patterson warns.

When in the atmosphere, hydrogen competes with methane to react with hydroxyl radicals, molecules crucial for removing planet-warming methane molecules from the atmosphere. “The more hydrogen there is in the atmosphere, the less hydroxyl there is to react with methane,” Patterson explains, prolonging the warming effect of methane in the atmosphere. “Right now there’s about half a part per million of hydrogen in the atmosphere. Based on our best estimates, that provides something like 2 per cent of the total anthropogenic warming effect.”

A better understanding of the hydrogen cycle is crucial for judging whether mass adoption of hydrogen fuel in the future as part of a move away from fossil fuels could bring unintended consequences. A sharp increase in atmospheric hydrogen concentrations could, for example, amplify the warming effect of methane. Methane emissions have been rising steadily since 2007 due to fossil fuel production, agriculture and warming temperatures triggering its release from wetlands and permafrost.

“Methane is the big reason we would be hesitant to go down the hydrogen economy route, because ultimately we will leak some hydrogen into the atmosphere,” says Archibald. “If we do leak hydrogen into the atmosphere, we will exacerbate the methane crisis.”

That could be an argument for using hydrogen sparingly where renewable power can’t replace fossil fuel use, Archibald suggests. But Patterson and other experts stress the warming effects of increased hydrogen use are still likely to be minimal compared to the huge warming effect of fossil fuels. “I don’t want to scare people away from hydrogen energy, because it’s so much better than the alternative,” Patterson says.