The release of carbon dioxide (CO2) during the extraction and burning of fossil fuels has contributed to major changes in Earth’s atmosphere in the centuries since humans realized their value as an energy source. Often accompanying CO2 are benign gases such as helium (He) that can be used to trace such emissions.
Scientists have long speculated that the amount of 4He—an isotope of helium—in the atmosphere is increasing because it is found in the same reservoirs as natural gas and other hydrocarbons. But measurements have so far been conflicting and imprecise. Now, researchers have developed a new way to measure the noble gas, shedding light on the decades-old conundrum.
“With our measurements, for the first time, we’re able to demonstrate that [the theory is] actually true, that helium concentrations in the atmosphere are increasing,” said Benjamin Birner, an atmospheric chemist and postdoctoral researcher at Scripps Institution of Oceanography.
The new discovery could lead scientists to better identify sources of CO2 in the atmosphere, which could guide policies to curb emissions. The increase in 4He also raises questions about its isotopic companion, 3He, and a potential undiscovered reservoir of the gas—a critical resource in some research and commercial industries.
Helium Pairs with Fossil Fuels
Some minerals naturally contain uranium and thorium. These radioactive elements decay to stable ones over millions of years, releasing 4He in the process. Because 4He is a noble gas, it does not readily bond with other elements and slowly leaks out of its host crystal over time. Rogue helium in Earth’s crust percolates toward the surface before escaping to the atmosphere.
In some cases, the rising gas gets trapped beneath an impermeable cap rock. Natural gas, escaping from buried source rocks, also rises through the subsurface and becomes trapped along with helium. “If you have a geological setting that’s suitable to contain [natural] gas, it’s probably also suitable to trap the helium,” Birner said.
When humans come along and extract the gas from these reservoirs, 4He is also liberated. With the growth of fossil fuel use since the beginning of the industrial era, 4He should be flooding the atmosphere. And scientists have been looking for it. Unfortunately, conflicting data have so far muddled any evidence of a long-term rise in atmospheric helium—some studies measured an increase, whereas others showed little to no change.
A Precise 4He Measurement
Birner and colleagues developed a new way to calculate 4He that boasts a precision higher than that achieved by any previous studies.
First, they obtained samples. Because of helium’s leaky nature, air samples are difficult to store, and scientists have had to mine creative sources of old air. One past study extracted air from inside carburetors and sealed metal pétanque game balls. “[Helium] doesn’t diffuse through metals. So you had to find some good metal boxes,” said Bernard Marty, a geochemist at the University of Lorraine who was not involved with the study. Birner and colleagues used gas stored in metal tanks sporadically collected by scientists for other experiments since the 1970s.
Then the group measured the change in the ratio of 4He and nitrogen (N2) through time. Nitrogen levels in the atmosphere remain relatively constant over the years; therefore, any change in the ratio between samples indicates a change in the amount of 4He. The researchers discovered a significant increase in 4He in air samples dating back to 1974—two orders of magnitude more than what is expected from Earth’s natural processes, according to the study. The increase is also larger than the small amount released by commercial and research applications.
Because 4He can now be precisely measured and is demonstrably increasing, scientists can trace the origins of associated greenhouse gases such as carbon dioxide. 4He concentrations are highest in natural gas compared to other fossil fuels such as coal and petroleum. By measuring the amount of both 4He and carbon in an air sample, scientists hope to determine how much of the total emissions comes from natural gas burning as opposed to automobiles or a coal power plant, Birner said.
Surprisingly, scientists also still have a lot to learn about Earth’s natural carbon emissions. Having a precise way to trace carbon with helium could help them determine how much is being pumped into the atmosphere by nature, said Marty.
“I think we’ll learn a lot more about how the world works from helium,” Birner said.
A 3He Mystery
The new data settle the long-standing debate about 4He in the atmosphere. “They are great measurements,” said Marty. But, he added, they pose an interesting problem.
Earlier studies, including some by Marty and colleagues, investigated the ratio of 3He to 4He in air samples to get at the 4He concentration in the atmosphere. 3He is a naturally occurring, stable isotope of helium. The most precise 3He/4He measurements available have shown that ratio is unchanging in the atmosphere over time. The fact that the researchers in this study independently observed an increase in 4He means that 3He must also be increasing.
3He is rare on Earth; it is released primarily from a mantle reservoir remnant from the formation of our planet. It is also produced from cosmic ray bombardment, solar wind, and interstellar gases and in the manufacture of nuclear weapons. But none of these sources can account for the amount entering the atmosphere. “The signal is about 10 times the geological fluxes, and we don’t know how to explain the source of this additional 3He,” Birner said.
3He is used in applications such as cryogenics, nuclear fuel, and medical imaging. In recent decades, as demand on the world’s supply has increased, it has become a scarce resource. The prospect of an undiscovered source of 3He is therefore intriguing. “People have thought about flying to the Moon to mine 3He there. That’s how important that resource is,” Birner said. “It will become even more important in the future because nuclear fusion reactors are theorized to run on 3He,” he added.
—Jennifer Schmidt (@DrJenGEO), Science Writer