Helium is produced by radioactive decay of uranium and thorium in the Earth’s crust and co-released during human extraction and usage of fossil fuels. This should lead to an atmospheric build-up of helium but so far direct observations have not detected a change in helium.

I will present a record of changes in the atmospheric helium-to-nitrogen ratio (δ⁴He/N₂) which is sensitive primarily to atmospheric helium variability because N₂ is essentially stable. Using a novel mass spectrometric technique, we analyzed 46 archived air samples from three different locations (La Jolla [U.S.A.], Trinidad head [U.S.A.], and Cape Grim [Australia]) and find a 1.9 per mil increase in δ⁴He/N₂ from 1974 to 2020. This corresponds to an annual increase of 39 ± 3 billion mol of ⁴He per year.

Taken together with previous observations of a near constant atmospheric helium-3/helium-4 (³He/⁴He) ratio, the increase in ⁴He implies a comparable increase in atmospheric ³He. Currently known sources of ³He are insufficient to explain the implied ³He trend, which is notable because ³He is an important, yet very scarce, fuel resource for nuclear fusion reactors.

Picture: Benni Birner in front of some of the air samples he used in his work. Copyright Eric Jepsen at SIO.