Startups want to cool Earth by reflecting sunlight. There are few rules and big risks

SAN JOSE, Calif. — One morning late last year, an RV pulled into a parking lot on the outskirts of Silicon Valley. Inside it was a blue Ikea bag with huge balloons, and metal tanks full of helium and sulfur dioxide gas.

The mission was led by Luke Iseman, a 41-year-old serial entrepreneur with a mohawk hairstyle and an orange t-shirt that read "Cool Earth." Inspired by a science fiction novel, Iseman had founded a company in 2022 called Make Sunsets. In the novel, a billionaire undertakes a type of "solar geoengineering": shooting vast amounts of reflective particles high into the atmosphere to reflect sunlight off Earth and counter global warming. Now, Iseman was trying to do it in the real world.

Iseman took a wrench and opened the tanks, releasing the sulfur dioxide first and then the helium. He siphoned the gasses into a long tube while his business partner, Andrew Song, stood just outside the RV using the tube to inflate a weather balloon. As the balloon grew to about 6 feet wide, some sulfur dioxide – which can be hazardous to human health in high concentrations – started to leak. I began to cough, and my eyes watered.

"Don't take big whiffs of air," Song said.

"Just don't breathe," Iseman said with a laugh.

The three balloons that Make Sunsets launched that day made it to the stratosphere – a layer of the atmosphere about six to 30 miles above Earth's surface. When the balloons popped, the sulfur dioxide gas turned to particles– reflecting enough sunlight, the company says, to offset the warming of 175 gas-powered cars for a year. But Make Sunsets and its balloons signal something bigger: a growing number of startups, research projects, and billionaire-backed nonprofits hoping to ready this tech to potentially cool the earth on a wider scale.

In the past year, the conversation around solar geoengineering as a climate solution has become more serious, says David Keith, geophysics professor and head of a new University of Chicago initiative to study a broad array of climate geoengineering ideas. "Suddenly we're getting conversations with senior political leaders and senior people in the environmental world who are starting to think about this and engage with it seriously in a way that just wasn't happening five years ago," Keith says.

But as money flows in – some from investors who hope to profit from this technology – regulations around outdoor experiments and possible broader deployments aren't keeping up, experts say. Because of the way the stratosphere works, a large-scale release of particles in one part of the world could impact a large part of the planet. Questions persist about possible risks of solar geoengineering for everything from global crops to droughts. And there are risks of unintended consequences that scientists and investors haven't yet imagined – the unknown unknowns of trying to engineer a cooler Earth.

Shuchi Talati, founder and executive director of the nonprofit The Alliance for Just Deliberation on Solar Geoengineering, says Make Sunsets' actions crystallize the need for urgent regulation of this tech.

"At some point down the road, they're going to do this at a big enough scale to trigger some sort of climate impact," Talati says. "It can be done in an effective, globally governed way, or it can be done by two crazy people in California, and it can look horrible for a lot of people."

"The allure of the techno fix"

Iseman, a former director at a tech incubator who used to build large art projects at Burning Man festival, acknowledges there's a performative aspect to his work. For his company to have a measurable cooling impact, researchers say it would need to release significantly more material, probably require equipping special planes, and is likely years away.

But Make Sunsets is attracting significant Silicon Valley investment, and is hoping to have that bigger impact. The company has raised more than $1.2 million from venture capital firms like Boost VC, Pioneer Fund, and Draper Associates.

It's unclear how many entities are exploring solar geoengineering, because some projects keep their work under wraps. But Make Sunsets isn't alone.

A U.S.-Israeli startup called Stardust Solutions that plans to someday launch reflective particles into the stratosphere has raised $15 million, according to its chief executive officer and co-founder, Yanai Yedvab. Stardust's investors include SolarEdge, an Israeli green energy company, and Awz Ventures, an Israeli-Canadian venture capital fund that highlights on its website a partnership with Israel's Ministry of Defense.

From sucking carbon dioxide out of the sky to giant sun-reflecting space mirrors, geoengineering the planet to avoid the worst impacts of global warming is capturing the imagination of greater numbers of people. Coming out of the hottest year on record, with governments and industries failing to adequately transition away from fossil fuels, some people are looking for silver bullets. This attraction is especially felt in Silicon Valley, says Gernot Wagner, a climate economist at Columbia Business School.

"Climate tech is sexy," Wagner says, "because it's the allure of the techno fix. Look to D.C., and things are messy. Politics is messy. Wouldn't it be nice if we could cut through all of this with the ultimate techno fix that will solve this thing once and for all?"

Iseman says he's motivated by an urgency to act on climate change. World governments agreed to keep global temperatures from rising more than 1.5 degrees Celsius above pre-industrial levels by 2100. But temperatures have already risen about 1.2 degrees Celsius, and many scientists think the world will blow past 1.5 Celsius. Passing it would mean more catastrophic impacts like lethal heat waves and flooding, coral die-offs and melting ice.

"What we have done has not worked," Iseman says about current efforts to address global warming. "And we need to try many more broad approaches."

Iseman and other solar geoengineering advocates reject the idea of a so-called "moral hazard" with this technology. That's the idea that solar geoengineering will distract from the difficult - and scientifically necessary - work of reducing greenhouse gas emissions, and will give fossil fuel polluters continued license to pollute.

"I think we need to do solar geoengineering – hard stop – because the world is too hot. We need to cool it off," Iseman says. "I wouldn't say we should only do that after we start dropping global greenhouse [gas] emissions. 'Cause, frankly, I don't know when we're gonna do that."

Make Sunsets does not employ any scientists. The employees are just Iseman and Song, who met while Iseman worked at a tech incubator and Song worked as an outreach manager at a crowdfunding website. Iseman says if they scale up the company, they'll hire scientists.

Stardust's team includes 20 scientists and engineers, including chemists, aviation experts, and physicists, like the company's CEO, Yedvab. Yedvab wrote in an email that he started looking into the tech two and a half years ago and found that it "is the sole viable option humanity has to stop global warming in the coming decades."

Stardust notes their focus is research in anticipation of future government contracts for deployment.

But there is still a lot of uncertainty surrounding the science, and experts worry it's too dangerous for companies to have a financial motive to develop and deploy this tech on a larger scale, when there still isn't clear regulation.

"We do know it will reduce global temperatures. That is the one thing we know," Talati says. "We don't know almost everything else."

From droughts to "termination shock," the technology poses risks

The type of solar geoengineering Make Sunsets works on is often called "stratospheric aerosol injection," and much of what's known about how it could work comes from volcanoes. When Mount Pinatubo erupted in the Philippines in 1991, sulfur dioxide from the eruption spread across the global stratosphere. The particles ended up cooling the world about half a degree Celsius the following year.

Scientists predict the world would, on average, get cooler with this type of solar geoengineering. Compared to other climate tech, it's also very cheap, and – once operational – could have a rapid cooling impact.

But there are lots of unpredictable risks. Computer modeling has limits, says Jonas Jägermeyr, who models impacts of solar geoengineering on global crops at NASA's Goddard Institute for Space Studies. "It is difficult to know what we are getting ourselves into unless we actually did the experiment on the whole planet," he wrote in an email.

If the world engages in stratospheric aerosol injection, it wouldn't go back to some pre-global warming climate, says Alan Robock, climate scientist at Rutgers University. This type of solar geoengineering could weaken the summer monsoon, meaning some regions – where billions of people live – would see less rain. And it could end up changing the ozone layer and ultraviolet radiation, which could affect crop growth and global food supplies, Jägermeyr says.

Changing temperatures and rainfall patterns could mean new risks for infectious diseases, says Christopher Trisos, chief research officer in the Climate Risk Lab at the University of Cape Town. ​​"Solar geoengineering could shift the population at risk by nearly a billion people for malaria across developing countries," Trisos says.

Then there's "termination shock," which is what it sounds like: the shock of suddenly halting a huge experiment. After injecting particles into the stratosphere, the particles only stay there for a year or two and then fall back to earth. Depending on the material, those falling particles can sometimes create their own environmental and health risks.

Because the particles don't stay up forever, if the world doesn't simultaneously reduce the amount of planet-heating gasses in the atmosphere while doing large-scale solar geoengineering, suddenly stopping the experiment poses a big risk.

"You get a whole rush of global warming and climate change in a very short period of time," Trisos says. "That would be very dangerous for ecosystems, for biodiversity, in many cases, very dangerous for crops and food supplies as well."

New companies raise fears of regulatory gaps

A growing number of legal scholars say national and international regulations are inadequate to cover potential large-scale deployments of solar geoengineering. As global warming gets worse, "there's going to be a strong desire by someone somewhere to do something," says Tracy Hester, a law professor at University of Houston who studies climate geoengineering. "There will be a temptation to grab the throttle and push ahead."

Hester worries that right now there's no regulatory strategy if that happens: "We need to know what we're doing. The consequences here are pretty massive."

In the U.S., the interactions Make Sunsets has with U.S. government agencies are minimal. Before each balloon launch, Iseman calls up the Federal Aviation Administration and alerts them that he'll be launching weather balloons. Iseman also files a yearly report with the National Oceanic and Atmospheric Administration (NOAA) listing all of the company's "weather modification activities". In one NOAA report in the box marked "project or activity designation," Iseman wrote: "Cooling Earth."

While Make Sunsets was launching balloons in the parking lot, on the edge of a county park, a park ranger drove up to us, got out of the car, asked Iseman and Song a few questions about the balloons, and drove away after less than three minutes.

Iseman spends much of his time in Baja California, and the company has done a number of experiments in Mexico. Last year the Mexican government released a statement saying that they would ban solar geoengineering in their national territory, referencing the activities of Make Sunsets.

But outside of Mexico, things are unclear. There are no international conventions that specifically deal with this type of technology, Hester says. The United Nations Convention on Biological Diversity – a treaty that protects wildlife – has a moratorium on geoengineering that affects living species. But the moratorium is guidance, and non-binding. And "the U.S. is actually not a party" to the treaty, says Daniel Bodansky, a climate legal expert and law professor at Arizona State University.

The Montreal Protocol, a treaty to address ozone depletion, could potentially cover the ways that stratospheric aerosol injection affects ozone. But that treaty does not currently address all the impacts of the tech, nor does it cover all types of solar geoengineering, Bodansky says.

Last month Hester, Talati and others filed a petition to NOAA. They asked the federal agency to consider making a rule requiring solar geoengineering companies like Make Sunsets to file more information about what they're doing, and consider regulating American citizens when they do solar geoengineering internationally.

A NOAA spokesperson wrote in an email, "We are currently reviewing the request for rulemaking and will provide a response to the petitioners."

Stardust is doing outdoor small-scale testing "only to the extent required ... and in full compliance with all relevant regulations," Yedvab wrote.

Yedvab, who used to work for Israel's atomic energy commission, adds: "Decision making regarding whether to deploy [stratospheric aerosol injection], when and to what extent should only be taken by governments."

A White House official wrote in an email that solar geoengineering activities in the U.S. must comply with applicable local, state, and federal laws. The email notes that new tech "may present unforeseen circumstances that require new guidance and/or governance mechanisms."

The official did not clarify what that new guidance or governance might look like.

Iseman of Make Sunsets says, "I think regulations have to be holistic to be meaningful, or people, including me, will just game them."

A test on San Francisco Bay

As the private sector grows, not-for-profit entities are speeding up their research.

Earlier this month, a handful of scientists and engineers gathered on deck of a decommissioned aircraft carrier on the San Francisco Bay to test a large machine. After about a decade of work, the researchers were readying the machine for one of its first tests outdoors, creating tiny salt water particles that could – someday – reflect sunlight and cool earth.

An engineer scooped salt into a large plastic container, mixing it with water. Then he turned the machine on, letting forth a giant hissing spray of salt water particles down the aircraft runway.

The test represents a different type of solar geoengineering than Make Sunsets' balloons. "Marine cloud brightening" involves brightening ocean clouds to reflect more planet-heating sunlight.

This technology could someday significantly reduce many of the impacts of global warming, says Sarah Doherty, a University of Washington professor who manages this program. But it also has risks. If the particles are too big, they can make the clouds less reflective, and actually warm the planet. An imbalance of cloud brightening off West Africa could cause a drought in the Amazon. For Doherty, these unknowns are a big reason for this research.

"You could see in 20 years from now, people saying, 'Oh my God, we're really in trouble. We've got major climate disruption. Let's do this thing that we know exists.' Well, if we haven't done the research to look at the ways to not do it properly, to not do it in a way that's going to cause more damage, then we're really in trouble," she says.

The risks are why university researchers, not for-profit companies, should guide studies of this tech, argues Kelly Wanser, executive director of SilverLining, the nonprofit that led fundraising for Doherty's program. The University of Washington program – which also aims to improve current climate modeling – has raised $16 million, mostly from philanthropic climate funds and Silicon Valley scientists. Doherty says there's no strings attached. "They're not gonna ever get anything back out of it other than more science," she says.

Wanser contrasts this with what she calls the "misaligned incentives" of private companies entering the space. In addition to its venture capital fundraising, Make Sunsets sells "cooling credits" – a single $10 credit offsets the warming of 1 metric ton of carbon dioxide, the company says. For Wanser, for-profit companies like Make Sunsets have a monetary incentive to keep releasing balloons, even if the effects could be harmful.

Iseman responds in an email that, "All change is scary, and we can't use 'someday maybe' as an excuse to avoid the bold actions that the climate crisis demands."

But even with university-based solar geoengineering research, there's a need for regulation, says Imran Khalid, a climate policy researcher based in Islamabad. While the aircraft carrier is a museum open to the public – a group of elementary school children came aboard a little while after the test – there are no rules requiring future research projects to be so transparent. And while this study released a small amount of particles, there are no specific regulations to limit a future study from making a larger release.

Given the stakes, Khalid says there should be global input into research frameworks. "Research leads to deployment at some time," he says. "There needs to be a global discussion around this issue."

"I have an obligation to do what I can"

Right now, most solar geoengineering research is funded and carried out by people in the Global North, in places like Silicon Valley. That worries Khalid.

"When we're talking about solar geoengineering, it's important to contextualize it from this perspective," he says, "of somebody who's sitting here in Pakistan, who's recently seen the 2022 floods."

Those floods, which scientists found were made worse because of global warming, left almost a third of the country underwater at their peak, and left hundreds of thousands of displaced people in camps. Just as global warming's impacts are often felt more in developing countries, some scientists fear developing countries could also be particularly vulnerable to solar geoengineering's risks.

"You can get some strong kinds of winners and losers," Trisos says. "And that's especially concerning because a lot of these developing countries in the tropics, such as in Africa, Asia, and parts of Latin America, right now don't have a strong voice in the solar geoengineering conversation."

Some nonprofits are trying to change that. In the past year The Alliance for Just Deliberation on Solar Geoengineering has started consultations and workshops in South Africa, India and Pakistan to educate local scientists and civil society groups about solar geoengineering. The U.K.-based Degrees Initiative has awarded $900,000 to scientists from Brazil, Indonesia, Nigeria and other countries to study solar geoengineering.

During the United Nations Environment Assembly in Nairobi this past February, Switzerland put forth a proposal for more global research on solar geoengineering. While it didn't pass, Talati, who was at the talks, says they left her encouraged. "That was a glimmer of hope for me," she says. "To see so many new countries offering their comments and wanting to engage in this conversation."

From his vantage point on a Zoom call in Silicon Valley, Iseman says it's unfair that he has privileges to act on solar geoengineering while others don't. "I think that it is unfair that I was born as a lower middle class, white-ish American male," he says.

But he says that isn't stopping him from scaling his company: "I have an obligation to do what I can to cool the planet, as does anyone else who actually reads the science."

"We're two guys playing with balloons," Iseman says. "In an ideal world, this shouldn't be something left to dudes with a startup to be doing. But that's the world we're in, for now."