Ellen Phiddian
Cosmos science journalist
Fossil fuels – natural gas in particular – are enriched in helium. So, it would make sense that as more fossil fuels are burned, the amount of helium in the atmosphere would increase.
But so far, there’s been debate among researchers about whether this is actually happening. As a nonreactive, non-greenhouse noble gas, helium hasn’t been consistently tracked like other gases in the atmosphere. And, because the total composition of the atmosphere has changed over the past century, simply checking absolute helium levels doesn’t necessarily generate accurate data.
Confusing the matter is the types, or isotopes, of helium around: natural gas has a lot of helium-4, specifically. The rarer isotope, helium-3, doesn’t appear in fossil-fuel supplies. This means that the ratio of helium-4 to helium-3 should be changing, but in fact it seems to have remained consistent.
Now, a team of US researchers believes it has settled the matter. Atmospheric helium-4 is increasing, according to the researchers’ paper in Nature Geoscience, and that means that helium-3 must be increasing along with it.
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The researchers figured this out by comparing helium-4 levels to a much more abundant gas in the Earth’s atmosphere: nitrogen. Nitrogen levels have stayed consistent over the past five decades, so it’s a good reference to use.
The researchers, who are based at Scripps Institution of Oceanography, at the University of California San Diego, US, analysed 46 air samples held at the university. The samples had all been collected between 1974 and 2020, at three different places: the Scripps Institution, Trinidad Head, also in the US, and Cape Grim in Tasmania.
The samples showed that helium-4 levels had been increasing relative to nitrogen. Between 1974 and 2020, helium had gone up by a total of 0.193% – plus or minus 0.014%. While that’s a very small number (helium only has a very low concentration in our atmosphere, after all), it does represent a significant change.
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Other than as an indication of fossil fuel use, the researchers don’t think this increase in helium-4 should be any cause for concern.
But they are interested in the corresponding increase in helium-3. Helium-3 is a very scarce resource and could be a key to making energy through nuclear fusion. There’s even speculation about mining it from the Moon because it’s so sought-after on Earth.
If helium-4 and helium-3 ratios have stayed roughly stable, and helium-4 is on the rise, that means helium-3 must be increasing too. But the researchers can’t explain where this extra helium-3 could be coming from.
“We don’t know for sure, but I wonder if there is more helium-3 coming out of the Earth than we previously thought, which could perhaps be harvested and fuel our nuclear fusion reactors in the future,” says lead author Dr Benni Birner, a postdoctoral researcher at Scripps.
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Co-author Professor Ralph Keeling, who also oversees the Keeling Curve first developed by his father, one of the first trackers of atmospheric CO2 says: “The study lays in starker relief a controversy surrounding the rare helium isotope 3He.
“The implications are far from clear, but it begs additional work.”