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Why scientists have pumped a potent greenhouse gas into streams on public lands

Blacktail Deer Creek in Yellowstone National Park, seen here in a 2019 photo from the ecological study known as NEON, is one site where researchers have bubbled sulfur hexafluoride into the water.
Blacktail Deer Creek in Yellowstone National Park, seen here in a 2019 photo from the ecological study known as NEON, is one site where researchers have bubbled sulfur hexafluoride into the water.

A massive ecological study that's happening across the United States, and which is designed to track the impact of long-term changes like a warming climate, is deliberately releasing a highly potent and persistent greenhouse gas in national parks and forests.

The gas, sulfur hexafluoride, is "the most potent greenhouse gas known to date,"according to the Environmental Protection Agency. It's 22,800 times more effective at trapping heat than carbon dioxide, and lasts in the atmosphere for thousands of years.

So far, this ecology study has released around 108 pounds of the gas, which has about the same impact as burning more than a million pounds of coal.

That may not seem like a big deal in the grand scheme of global emissions, but government scientists working at federal parks and forests have objected to using this gas on public lands — especially since this major study is designed to go on for 30 years and alternative gasses are available.

This kerfuffle has so far played out quietly within government agencies. But it comes at a time when all kinds of researchers are thinking about the climate effects of past practices, with some saying that scientists who understand the urgency of the climate crisis have a special obligation to set an example to the public by reducing the greenhouse gas emissions of their own work.

The National Science Foundation (NSF), which funds this large ecology study, told NPR that it supports an evaluation that's now underway to see whether phasing out the use of this gas would affect the quality of the information that's being gathered.

That's not good enough for one watchdog group, which is calling for an immediate halt to the release of this gas on public lands.

"We're using just a tiny amount"

For decades, ecologists have sometimes burbled small amounts of sulfur hexafluoride into streams and rivers, in order to study how quickly gasses can move from the water into the air. One reason that's of interest is that, although inland waterways cover up only a small fraction of the Earth's surface, researchers believe these running waters could be an important source of greenhouse gasses, as rainfall can carry carbon from the ground into turbulent streams that later release it into the atmosphere.

Ecologists have always known that sulfur hexafluoride was itself a potent greenhouse gas, "but we always said, 'Well, we're using just a tiny amount of it," says Bob Hall, a professor of stream ecology at the University of Montana.

"The beauty of sulfur hexafluoride is we only have to add very tiny quantities, and it's really, really easy to measure and it's perfectly unreactive. We're not doing anything to the ecosystem by adding it, it's not reacting with anything, it's not poisoning anything," says Hall, who once calculated that the amount he used in one of his experiments had about the same climate impact as burning 35 gallons of gasoline.

Given the usefulness of this gas in stream studies, it's not surprising that tests involving sulfur hexafluoride were built into the standardized protocols of the National Ecological Observatory Network, orNEON, which is an ambitious government-funded effort to track ecological changes. Its mission is to use consistent methods to collect all kinds of data on 81 different locations across the nation--and to do this regularly for three decades.

"The idea is to understand the effects of things like climate change, land use change, and invasive species on these ecosystems," says Kaelin Cawley, who works at Battelle, the nonprofit applied science and technology organization that operates NEON for the NSF.

NEON workers sample fish in 2019 at Lower Hop Brook in Massachusetts, one of the streams being monitored as part of this long-term ecological study.
NEON workers sample fish in 2019 at Lower Hop Brook in Massachusetts, one of the streams being monitored as part of this long-term ecological study.

The planning for this half-billion-dollar ecological project, and the construction of its monitoring instruments, took around twenty years. It began operating at full tilt in 2019.

That's the same year when a scientist at Yellowstone National Park started to question why NEON was releasing sulfur hexafluoride at Blacktail Deer Creek, according to documents obtained through a public records request by Public Employees for Environmental Responsibility, a group that supports workers within the government who are concerned about activities that can harm the environment.

The environmental consequence

NEON's protocols called for it to annually release around 3.3 pounds of sulfur hexafluoride, or SF6, in Yellowstone National Park, hydrologist Erin White pointed out in a November, 2020 email to another National Park Service official. Over the 30-year lifetime of the project, White calculated, that meant the use of SF6 for research in Yellowstone National Park alone would be equivalent to burning over 1,139,000 pounds of coal.

"In short, the environmental consequence of a small SF6 application in the park is significant," noted White, who recommended that NEON immediately substitute an alternative gas, such as argon, even though NEON staffers thought making this switch would be problematic because of things like lab contracting constraints.

It is somewhat ironic to study carbon cycling using a tracer gas with that much greenhouse forcing.

Bobby Hensley, who works on NEON for Battelle, told NPR that the climate impact from the scientific use of this gas has to be kept in perspective.

"I don't want to sort of criticize Yellowstone National Park, but, I mean, there's hundreds of thousands of vehicles driving through that park every single day," says Hensley. "They can't tell people, 'Hey, you can't come visit the park.' But they can say, 'You can't use SF6.'"

Soon, government officials shared the concerns raised at Yellowstone with others who oversaw sites where NEON had been releasing this gas. Emails went out to Great Smoky Mountains National Park, the Bureau of Land Management, and the United States Forest Service. About half of the NEON sites with streams where sulfur hexafluoride was released were on forest service lands, records show.

"SF6 is a potent greenhouse gas and over the 30 year NEON program the release will be equivalent to burning millions of pounds of coal," wrote Bret Schichtel of the National Park Service's Air Resource Division to Linda Geiser, the National Air Program Manager for the Forest Service. "We would like to know if you are aware of this issue and share similar concerns."

In December of 2020, representatives of the park and forest services held a virtual meeting with NEON employees. Emails sent after that meeting make it clear that the public land officials felt a "strong desire" to discontinue the use of this gas.

NEON subsequently convened agroup of expert advisers to figure out if they could stop using the gas without disrupting the science.

One of those advisors was Hall, who says that he had already moved away from using SF6 in his own studies of streams, in part because of its extreme potency as a greenhouse gas. "It is somewhat ironic," Hall and a colleague wrote in one paper, "to study carbon cycling using a tracer gas with that much greenhouse forcing."

"This doesn't fit with the mission"

It turns out that the physical features of streams that affect turbulence and gas exchange don't change much over time. So NEON's expert advisers basically felt it would be okay to just make sure this study had some baseline measurements using SF6 for each site and then leave it at that, rather than switching to an alternative gas that would require new instruments and training so that measurements could be taken year after year.

"We have discontinued it recently at several of our sites, but not all of them," says Cawley, who notes that the water level in streams might currently be too low to get the data they want. "Some of the sites, we still need to get certain flow ranges that we haven't covered yet."

The NSF's Program Director for NEON, Charlotte Roehm, told NPR in an emailed statement that Battelle was evaluating the impact of phasing out the use of SF6 and that "the NSF team that manages NEON is supportive of conducting that evaluation."

In 2021, according to one memo sent from NEON to Roehm, NEON used approximately 18 pounds of the gas, which is the equivalent of greenhouse gas emissions from driving an average car over 460,000 miles. That memo stated the new plan was to use the gas to take measurements "up to three times per year at up to 10 sites," likely for one to three more years.

"Eventually we will stop using SF6 when all sites have enough data to draw conclusions about gas exchange rates across a wide range of flows at a site," the memo states.

Public Employees for Environmental Responsibility, however, wants them to stop using SF6 immediately.

"They're doing these experiments on public lands like national parks and national forests," says Chandra Rosenthal, who directs the non-profit's Rocky Mountain office. "This doesn't fit with the mission of these agencies at all."

The government workers who manage those federal lands are unhappy about the use of this gas, she says, "but they haven't really had the authority to do anything about the fact that this stuff is being used."

This week, her group sent a letter to the director of the NSF, asking the agency to stop funding projects that use SF6 on federal lands, and to assess the value of using SF6 and other greenhouse gasses in all the research it funds. A similar letter, sent to U. S. Department of the Interior Secretary Deb Haaland and U. S. Department of Agriculture Secretary Tom Vilsack, calls on them to stop allowing the use of SF6 on the lands they manage. "It is our understanding," the letter says, "that similar research projects have switched to argon without issue."

One researcher who uses small amounts of SF6 for studies of gas exchange in the ocean, rather than streams, says he thinks NEON's protocols could have been set up differently, to minimize the use of this greenhouse gas.

"If I was to do what they're doing, I would do it differently. I wouldn't be bubbling it in, because that does use a lot," says David Ho, an oceanographer with the University of Hawaii, who explains that he might infuse a small amount of the gas into a bag of water and then release that into the stream. "They haven't thought this through, in terms of the best way to do it."

And even if the amount that's been released by NEON and other scientific studies is essentially nothing compared to the amount of SF6 released globally from industrial sources, the concerns about it still seem reasonable to streams researcher Walter Dodds of Kansas State University, who served on NEON's technical advisory panel.

"It may be an overreaction of sorts, but it's completely understandable as well. We all are worried about what our own footprints are," says Dodds. "Certainly we should be cognizant of the potential for that harm and at least minimize the amount of times we use it and the amount of gas that we use in each experiment."

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Nell Greenfieldboyce is a NPR science correspondent.