The Tennessee state House of Representatives passed a bill Monday designed to prevent geoengineering, the practice of intentionally modifying the atmosphere to counteract global warming.

The bill, which had already passed in the state Senate, covers a variety of technological interventions. They include theoretical ideas about cooling the climate by an approach known as solar radiation modification, as well as more limited practices that affect the weather, like cloud seeding, a technique used to increase rain and snowfall. 

Most geoengineering options are theoretical and untested. Federal researchers have taken only a few small steps toward studying their feasibility, and atmospheric scientists say there is no evidence of any large-scale programs.

On its face, Tennessee’s bill represents an attempt to prevent experimentation with or deployment of such technologies. 

However, lawmakers’ discussions of the proposal toed a line between fact and fiction, with several suggesting that solar geoengineering projects are already underway and others referring to fears and misunderstandings that appeared to stem from the “chemtrails” conspiracy theory.

“This will be my wife’s favorite bill of the year. She has worried about this, I bet, 10 years. It’s been going on a long, long time,” Republican Sen. Frank Niceley said at a hearing about the bill last month. “If you look up — one day, it’ll be clear. The next day they will look like some angels have been playing tic-tac-toe. They’re everywhere. I’ve got pictures on my phone with X’s right over my house. For years they denied they were doing anything.”

None of the six Senate sponsors responded to requests for comment. Niceley, who voted for the bill, also did not respond to a request for comment after the House vote. Rep. Monty Fritts, a Republican who sponsored the bill in the House, would agree only to an in-person interview, which NBC News was unable to arrange before the vote.

The chemtrails theory is a loose grouping of unfounded ideas that suggest planes are not making trails of condensation known as contrails but instead are spraying government-made chemicals to control people’s behavior or affect their bodies.

In recent years, some chemtrails conspiracy theories have evolved, with believers suggesting that contrails are actually aerosols designed to control the weather or the climate. The Tennessee Lookout, a nonprofit news organization, reported that Republican Sen. Steve Southerland, one of the sponsors, referred to the chemtrails theory when he presented his argument for the bill to a reporter. 

Justin Mankin, a climate scientist at Dartmouth University, said: “It’s conspiratorial nonsense. The challenge here is that the whole chemtrails conspiracy has blurred and subsumed all these distinct technologies with distinct aims, which makes it challenging to disentangle.”

The Tennessee Legislature is not alone in its effort to enact anti-geoengineering policy. Lawmakers in Illinois, Kentucky, Minnesota, New Hampshire, Pennsylvania, Rhode Island and South Dakota have introduced or previewed similar bills. 

The trend suggests that a mix of conspiracy theories, confusion and genuine concern about the possibility of climate modification has taken hold in the public consciousness — and among some Republican lawmakers. 

“There are people in places like Tennessee and Pennsylvania and New Hampshire who are fearful that the chemtrails theory is correct,” Mankin said. “Policymakers, instead of relying on science to appropriately debunk conspiratorial belief, have rendered it legitimacy through legislative action.”

Josh Horton, a senior fellow who studies solar geoengineering policy at the Harvard Kennedy School, said that as far as he is aware, Tennessee is the first state to pass such a bill through both chambers of its legislature. 

If the governor signs it into law, the bill would prohibit the “intentional injection, release or dispersion” of chemicals into the atmosphere for the “express purpose of affecting temperature, weather, or the intensity of the sunlight.” 

The bill claims that “it is documented that the federal government” or those working on its behalf can “conduct geoengineering experiments by intentionally dispersing chemicals into the atmosphere.”

A White House official said in an email that “the federal government is not involved in any outdoor testing or deployment of solar radiation management.” 

The official said the government “is engaged in a limited subset of research activities on this topic, including modeling, measurements and monitoring, and laboratory research.”

The Tennessee vote became contentious Monday evening. 

Rep. John Ray Clemmons, a Democrat, poked fun at the bill, attaching an amendment that mimicked its original language but suggested that geoengineering “may threaten the Sasquatch and its natural habitat.” His amendment failed. 

Another Democrat, Rep. Bo Mitchell, said: “It’s very appropriate this bill is on the calendar on April 1.”

However, Fritts, the House sponsor, pointed to federal funding for aerosol research as evidence of the government’s aims. 

“There is an intent and a plan,” he said. “I suspect that some of those that have these plans with the solar radiation modification intend to try to reflect the sun’s rays from the Earth by injecting these chemicals, chemical compounds, substances and apparatus into the upper atmosphere.”

It is not surprising that lawmakers would struggle with key concepts of geoengineering. The term is overarching and poorly defined, and many of the ideas that fall under its umbrella are little more than the back-of-napkin sketches of scientists dreaming of ways to reduce global warming. 

“It’s not fully formed. It doesn’t exist,” Horton said. “The terminology is all over the place.” 

The broad category includes solar geoengineering, which the Tennessee bill would ban. The term refers to activities like stratospheric aerosol injections, an untested theory that the planet could be cooled by spraying particles into the stratosphere from high-altitude aircraft. 

Other geoengineering concepts not mentioned in the bill include marine cloud brightening — using aerosols over the ocean in an effort to brighten clouds there — and cirrus cloud thinning, which refers to thinning certain icy clouds to allow more heat to escape Earth. 

Tennessee’s bill would also outlaw weather modification, like cloud seeding, a decades-old practice used in Western states to encourage precipitation.  

Some states regulate cloud seeding operations, which are usually small-scale efforts to increase snowfall in mountain ranges, but most other forms of geoengineering remain in a “regulatory Wild West,” Mankin said. 

Committee hearings about the legislation yielded a confusing mix of truth, innuendo and fiction.

Dr. Denise Sibley, an advocate for the bill who testified before both chambers, suggested that the federal government has been seeding chemicals in the atmosphere.

“There is no doubt that weather modification is taking place within our state,” Sibley said, adding: “We do not consent to the intentional blocking of the sun through the use of particulate aerosols and heavy metals.” 

She pointed Tennessee lawmakers to a 2023 White House report as evidence. The document discusses what a geoengineering research program could look like but does not outline an active program.

Sibley did not respond to requests for comment.

During the hearings, lawmakers also confused contrails with “chemtrails” and asked whether wildfires in Western states were caused by cloud seeding or whether geoengineering was causing a rise in cancer rates. 

Republican Rep. Bud Hulsey inquired about whether geoengineering was the reason for honeybees’ decline. 

“Absolutely — and it is the reason that the honeybees are going away,” replied David Perry, who was testifying in support of the bill and told the committee that he was a licensed health care provider of 40 years. “The microcosm that they live in is affected by these aerosols.” 

There is no evidence to support Perry’s assertion. A Tennessee chiropractor of the same name who matched the biographical information Perry gave during his testimony did not immediately respond to a request for comment. Bees do face dire problems, including threats from pests, pesticides, reduced habitat and climate change.

“What you see is the mixing up and conflation of all these things — geoengineering is the same as weather modification is the same as chemtrails or contrails,” said Horton, who reviewed video of recent legislative testimony. 

State Sen. Heidi Campbell, a Democrat, voted against the bill.

“It’s alarming although quite common around here to see people just wholesale buying into these conspiracy theories,” Campbell said, adding that she believed the bill was a distraction from core climate issues. 

Mankin and Horton agreed that there are important conversations to have about how to guide and regulate research into solar geoengineering, which is controversial even among many scientists. 

“Is the Tennessee state Legislature the place to have that conversation?” Horton said. “Probably not.”

Evan Bush

Evan Bush is a science reporter for NBC News. He can be reached at Evan.Bush@nbcuni.com.