Last September, researchers in the UK launched a high-altitude weather balloon that released a few hundred grams of sulfur dioxide into the stratosphere, a potential scientific first in the solar geoengineering field, MIT Technology Review has learned.

Solar geoengineering is the theory that humans can ease global warming by deliberately reflecting more sunlight into space. One possible means is spraying sulfur dioxide in the stratosphere, in an effort to mimic a cooling effect that occurs in the aftermath of major volcanic eruptions. It is highly controversial given concerns about potential unintended consequences, among other issues.

The UK effort was not a test of or experiment in geoengineering itself. Rather, the stated goal was to evaluate a low-cost, controllable, recoverable balloon system, according to details obtained by MIT Technology Review. Such a system could be used for small-scale geoengineering research efforts, or perhaps for an eventual distributed geoengineering deployment involving numerous balloons.

The “Stratospheric Aerosol Transport and Nucleation,” or SATAN, balloon systems were made from stock and hobbyist components, with hardware costs that ran less than $1,000. 

Andrew Lockley, an independent researcher previously affiliated with University College London, led the effort last fall, working with European Astrotech, a company that does engineering and design work for high-altitude balloons and space propulsion systems.

They have submitted a paper detailing the results of the effort to a journal, but it has not yet been published. Lockley largely declined to discuss the matter ahead of publication, but he did express frustration that the scientific process was being circumvented. 

“Leakers be damned!” he wrote in an email to MIT Technology Review. “I’ve tried to follow the straight and narrow path and wait for the judgment day of peer review, but it appears a colleague has been led astray by diabolical temptation.” 

“There’s a special place in hell for those who leak their colleagues’ work, tormented by ever burning sulfur,” he added. “But I have taken a vow of silence, and can only confirm that our craft ascended to the heavens, as intended. I only hope that this test plays a small part in offering mankind salvation from the hellish inferno of climate change.”

European Astrotech didn’t immediately respond to an inquiry.

Test flights

The system included a lofting balloon filled with helium or hydrogen, which carried along a basketball-size payload balloon that contained some amount of sulfur dioxide. An earlier flight in October 2021 likely also released a trace amount of the gas in the stratosphere, although that could not be confirmed and the system was not recovered owing to a problem with onboard instruments, according to details obtained by MIT Technology Review. 

During the second flight, in September of 2022, the smaller payload balloon burst about 15 miles above Earth as it expanded amid declining atmospheric pressure, releasing around 400 grams of the gas into the stratosphere. That may be the first time that a measured gas payload was verifiably released in the stratosphere as part of a geoengineering-related effort. Both balloons were released from a launch site in Buckinghamshire, in southeast England. 

There have, however, been other attempts to place sulfur dioxide in the stratosphere. Last April, the cofounder of a company called Make Sunsets says, he attempted to release it during a pair of rudimentary balloon flights from Mexico, as MIT Technology Review previously reported late last year. Whether it succeeded is also unclear, as the aircraft didn’t include equipment that could confirm where the balloons burst, said Luke Iseman, the chief executive of the startup. 

The Make Sunsets effort was widely denounced by researchers in geoengineering, critics of the field, and the government of Mexico, which announced plans to prohibit and even halt any solar geoengineering experiments within the country. Among other issues, observers were concerned that the launches had moved ahead without prior notice or approval, and because the company ultimately seeks to monetize such launches by selling “cooling credits.”

Lockley’s experiment was distinct in a variety of ways. It wasn’t a commercial enterprise. The balloons were equipped with instruments that could track flight paths and monitor environmental conditions. They also included a number of safety features designed to prevent the balloons from landing while still filled with potentially dangerous gases. In addition, the group obtained flight permits and submitted what’s known as a “notice to airmen” to aviation authorities, which ensure that aircraft pilots are aware of flight plans in the area.

Some observers said that the amount of sulfur dioxide released during the UK project doesn’t present any real environmental dangers. Indeed, commercial flights routinely produce many times as much. 

“This is an innocuous write-up of an innocuous experiment, in the direct sense,” says Gernot Wagner, a climate economist at Columbia University and the author of Geoengineering: The Gamble.

Public engagement

But some are still concerned that the effort proceeded without broader public disclosures and engagement in advance.

Shuchi Talati, a scholar in residence at American University who is forming a nonprofit focused on governance and justice issues in solar geoengineering, fears there’s a growing disregard in this space for the importance of research governance. That refers to a set of norms and standards concerning scientific merit and oversight of proposed experiments, as well as public transparency and engagement.

“I’m really concerned about what the intent here is,” she says. “There’s a sense of them having the moral high ground, that there’s a moral imperative to do this work.”

But, she says, forging ahead in this way is ethically dubious, because it takes away any opportunity for others to weigh in on the scientific value, risks, or appropriateness of the efforts before they happen. Talati adds that part of the intent seems to be provocation, perhaps to help break what some perceive to be a logjam or taboo holding up stratospheric research in this area. 

David Keith, a Harvard scientist who has been working for years to move ahead with a small-scale stratospheric balloon research program, questioned both the scientific value of the effort and its usefulness in terms of technology development. In an email, he noted that the researchers didn’t attempt to monitor any effect it had on atmospheric chemistry. Nor did the work present a feasible “pathway to use this method for deployment at reasonable cost,” he wrote.

“So in some deep sense, while it’s much more thought out, much less cowboy than Make Sunsets, I see it [as] similar,” Keith said.

When asked if being provocative might have been a partial goal of the effort, Keith said: “You don’t call something SATAN if you’re playing it straight.”

Lockley stressed that the effort was “an engineering proof-of-concept test, not an environmentally perturbative experiment,” and that they obtained the standard approvals for such flights. 

“I’m unaware of any prior approval process which should have been followed but was not,” he wrote in an email. “A review body may be useful, if it was able to provide good-faith and practical feedback on similar low-impact experimental proposals in future.”

Moral hazards and slippery slopes

There are a variety of concerns about deploying solar geoengineering, including the danger that carrying it out on large scales could have negative environmental side effects as well as uneven impacts across various regions. Some fear that even discussing it creates a moral hazard, undermining the urgency to address the root causes of climate change, or that researching it sets up a slippery slope that increases the chances we’ll one day put it to use.

But proponents of research say it’s crucial to improve our basic understanding of what such interventions would do, how we might carry them out, and what risks they could pose, for the simple fact that it’s possible that they could meaningfully reduce the dangers of climate change and save lives. To date, though, not much has happened outside of labs, computer models and a handful of efforts in the lower atmosphere.

Several earlier proposals to carry out research in the stratosphere have been halted or repeatedly delayed amid public criticism. Those include the SPICE experiment, which would have tested a balloon-and-hose stratospheric delivery system but was halted in 2012, as well as the Harvard proposal that Keith is involved with, known as SCoPEx. 

The National Oceanic and Atmospheric Administration has begun conducting stratospheric flights, using balloons and more recently jets, as part of a growing US geoengineering research program. But its stated intention is to conduct baseline measurements, not to release any materials. One hope behind the efforts is to create an early detection system that could be triggered if a nation or rogue actor moves forward with a large-scale effort.

The challenges in conducting even basic, small-scale outdoor experiments that carry minimal environmental risks has increasingly frustrated some in the field—and left at least a few people willing to move forward without broad public disclosures in advance, perhaps in part to force the issue.

Scientists routinely conduct outdoor experiments without seeking up-front public permission, when doing so doesn’t present clear dangers to public health or the environment, and reveal their studies and peer-reviewed results in journals only after the fact. 

The question is whether solar geoengineering research demands greater up-front notification, not because the experiments themselves are necessarily dangerous but because of the deep concerns about even discussing and researching the technology.

Columbia’s Wagner says the field should err on the side of transparency. But he also says it’s important to strike the right balance between how much researchers must reveal in advance, how easily carefully designed projects can be blocked, and how much support major research institutions provide for an important area of inquiry. 

“This sort of thing is a direct response to other institutions’ reluctance to proceed with even seemingly innocuous research,” he says.

Correction: This story has been updated to clarify Lockley’s research affiliation. According to University College London, he is no longer an honorary research assistant at the institution.