A research team led by the University of California, San Diego (UCSD) has employed an unprecedented technique to discover that levels of helium are rising in the atmosphere at a very small but, for the first time, clearly measurable rate. This increase in the 4-helium (4He) isotope is caused by the burning and extraction of fossil fuels. Although the 4He isotope does not contribute to global warming, measuring its atmospheric levels could serve as an indirect marker of fossil fuel use.
“The main motivation was to resolve a longstanding controversy in the science community about atmospheric helium concentrations,” said study lead author Benni Birner, a postdoctoral researcher in Geoscience at UCSD. While no reliable method for measuring atmospheric helium levels has previously existed, Dr. Birner and his colleagues have now developed a precise technique to compare the 4He isotope to levels of the common atmospheric gas nitrogen. Since atmospheric nitrogen levels are fairly constant, an increase in He/N2 is indicative of the rate of 4He buildup in the atmosphere.
The isotope 4He is produced by radioactive decay in the Earth’s crust, and accumulates in similar reservoirs as fossil fuels, especially those of natural gas. During the extraction and burning of fossil fuels, the helium isotope is coincidentally released. Thus, measuring its atmospheric concentrations is an important step forward in better assessing the scale of (harmful) industrial activity.
This study also provides a foundation for scientists to understand more about the highly valuable 3-helium (3He) isotope, which is very useful for nuclear fusion or cryogenics. Since the 4He isotope seems to exist in the atmosphere in an unvarying ratio with the 3He one, the rises in 4He atmospheric levels detected by the researchers imply that the 3He isotope must be increasing at a comparable rate.
“We don’t know for sure, but I wonder if there is more 3He coming out of the Earth than we previously thought, which could perhaps be harvested and fuel our nuclear fusion reactors in the future,” said Dr. Birner.
“The study lays in starker relief a controversy surrounding the rare helium isotope 3He,” added study co-author Ralph Keeling, a geochemist at UCSD. “The implications are far from clear, but it begs additional work.”
The study is published in the journal Nature Geoscience.