Can reflecting the Sun buy time for net zero? A startup has raised $US75m to try

Private firms are making plans to cool the planet by releasing reflective particles high into the atmosphere, an approach known as solar geoengineering or solar radiation modification (SRM).

With a war chest of private capital estimated at more than the entire globe's SRM research budget, the US-Israeli startup Stardust Solutions has unveiled a proprietary engineered particle it hopes to begin testing outdoors and is calling on governments to regulate the technology before it does.

Currently, there is no international regulatory framework for SRM, and many scientists have called for its ban altogether.

While computer modelling shows the technology might work to offset global heating in the short term, it also points to risks for global weather systems, particularly in South and South-East Asia, where billions of people depend on the annual monsoon.

Andy Parker, who leads an NGO supporting SRM research in the developing world, said Australia's neighbours faced disproportionate climate risks whether the technology was used or not.

"There are risks if you use SRM [or] reject SRM and choose warming," he said.

"The decisions around this are not going to be made in universities. They're going to be made in the messy real world of geopolitics."

Fearing disruption to crucial monsoon rains, India's government recently became the first in Asia to publicly outline plans to regulate SRM.

Reflecting the Sun

SRM may refer to measures as simple as painting white roofs on houses.

But reflecting the Sun using tiny particles in the stratosphere, about 20 kilometres above Earth, is the largest-scale and most controversial SRM proposal.

Large eruptions in the Pacific sparked the idea.

When volcanoes blast millions of tonnes of sulphur dioxide into the sky, scientists have measured meaningful reductions in global temperature for years afterwards.

When blasted skyward, the gas reacts naturally with compounds in the stratosphere to form tiny particles that reflect part of the Sun's heat before eventually falling back to Earth.

However, even if that effect can be replicated using balloons or planes, it would not remove greenhouse gases from the atmosphere or prevent the harm caused by new emissions.

That means that while SRM might temporarily mask the effects of global heating to buy time to achieve net zero emissions, it cannot replace that goal.

The particles would also need to be regularly replenished in the atmosphere for as long as the global energy transition lasts, at great cost and with untested effects on fragile weather systems such as the South Asian monsoon.

As little as a decade ago, the idea of testing it in the atmosphere was far-fetched.

Academic research has been kept strictly in the laboratory, with fears that even small-scale testing outdoors may derail decarbonisation efforts.

While the Israel-based Stardust Solutions said it had refrained from outdoor testing of SRM, another startup has openly commercialised it.

Make Sunsets sells what it calls "cooling credits" to private customers who want to offset their contribution to global warming by sending balloons filled with sulphur dioxide into the stratosphere.

It claims each $2 credit delivers enough sulphur dioxide into the atmosphere to offset a tonne of carbon dioxide emissions, a figure disputed by climate scientists.

"If all of the countries in the world but one outlaws this, and that country has a flag of convenience I can fly ships under, we can do this from the ocean," Make Sunsets chief executive Luke Iseman told the ABC.

Operating in the US with limited capital, the business has nonetheless raised the stakes for SRM globally.

Don't wait, regulate

NITI Aayog, the Indian government's public policy think tank, has become the first in Asia to publicly announce plans to regulate SRM.

Citing concerns over rainfall and crop yields, it has brought together scientists and policymakers to discuss how the world's most populous country will respond to SRM proposals abroad and oversee its own research.

NITI Aayog has been contacted for comment.

"Modelling [by] the Indian Institute of Science shows very clearly that it could have significant effects on Indian monsoons," Vaibhav Chaturvedi, senior fellow at the India-based Council on Energy, Environment and Water, said.

"If that ends up happening, it is actually a very scary scenario because most of India's livelihoods depend on the agriculture sector."

Research has found that SRM could significantly affect the amount, timing, and duration of monsoon rains through complex interactions among climatic effects.

That complexity makes it difficult to predict exactly how any SRM scenario might play out year to year, prompting NITI Aayog's concern.

Other global bodies have made statements concerning SRM, but lack targeted policies.

In 2023, Mexico announced it would ban SRM within its borders after Make Sunsets launched sulphur dioxide balloons from Baja California without government permission.

"The headline version is that Mexico banned solar geoengineering," Mr Iseman said.

"What actually happened is that Mexico put out a press release saying that they would weigh the costs and benefits of solar geoengineering, and then they took no legislative action after that."

Since then, the European Parliament and ministers representing all 54 African countries have called for SRM to be banned, but have yet to develop specific regulations.

Climate Change Minister Chris Bowen did not respond to a request for comment on Australia's SRM policy but told parliament in 2024 there was no specific regulatory framework in place for geoengineering activities.

Even if countries do ban geoengineering within their borders, another country might still permit experiments that affect their local climate, a possibility that India is acutely aware of.

"It is entirely possible that some countries try to do this unilaterally,"

Mr Chaturvedi said.

"Any of the powerful countries or anybody who has got funds could start doing outdoor experiments, and without any regard for other countries and global norms, that could have devastating effects on other parts of the world."

Profit motive

Having raised $US75 million ($106 million) across two funding rounds, Stardust Solutions's pitch to research and eventually deploy an engineered SRM particle is almost certainly the best-funded worldwide.

The startup was founded by two former nuclear scientists for the Israeli government, Yanai Yedvab and Amyad Spector, and, for years, operated in Israel and the United States without revealing details of its research or the chemical design of its particle.

In 2025, public data reveals it spent at least $US370,000 lobbying US officials.

Speaking to the ABC, chief executive Yanai Yedvab said Stardust would now engage with policymakers in developing nations and actively seek regulation before taking its tests outdoors.

"Obviously we started with the US but the idea is to engage globally [including] India, definitely Australia and other prominent countries in the global south," he said, referring to developing countries," he said.

"These are the communities that, on one hand, will be most affected by the risks of climate change, but at the same time, their voice must be heard in any serious conversation that entails a global technology."

Shuchi Talati, a climate policy researcher and founder of the Alliance for Just Deliberation on Solar Geoengineering, has advocated against any corporate involvement in SRM research, citing poor transparency and profit incentives that work against common global interests.

"You would have to deploy SRM [in the atmosphere] for this company to make money, there's incentive for SRM to be used indefinitely [and] to see climate mitigation fail, so all of those elements together are extremely concerning,"

she said.

"I think the fact that Stardust does exist and that they've invigorated a venture capital interest in this space is indicative to me that they're definitely not the last."

She also criticised the company for not pre-registering its research or publishing raw data, steps she said were considered best practice in the scientific community to prevent the manipulation of results or the concealment of unfavourable findings.

"Releasing some preprints now does not take into account legitimate transparency or any version of public engagement," Ms Talati said.

"This idea that because they put out guidelines means that I should trust that they're following them, it's just kind of absurd in my opinion."

Having unveiled Stardust's particle design and taken steps to patent it, Dr Yedvab said the company would not cherry-pick the results it publishes, but did not commit to pre-registering its research.

He also dismissed criticisms of the incentives in for-profit geoengineering, pointing to the pharmaceutical industry and genome sequencing as examples of successful rollouts of privately developed technologies.

"You see companies developing [lifesaving] drugs, but you have all these layers of institutions who are doing both the regulation and eventually make the decisions [on] whether drugs could be distributed to the public," he said.

"We see the same case here. We will work hand in hand with governments, but at the end of the day, it will be for them to make the decisions."

No easy options

Dr Chaturvedi said India, by far the largest economy in South or South-East Asia, was well-placed to represent the developing world in global SRM discussions.

He also indicated India's response to any SRM proposal from private companies or a foreign government would be assessed on relatively equal grounds, with effects on the weather front of mind.

"We are talking about playing with Indian monsoons; it's a very different order of catastrophic impacts," he said.

"Whoever is the originator of that, they will all be responded to in a similar way."

Mr Parker said developing countries were in a vulnerable position and should take steps to fund their own research.

"You cannot opt out of a geoengineered world," he said.

"So if someone decides to use SRM, the entire planet will experience the consequences for good or for ill."

Dr Yedvab said Stardust was convening an advisory board and was firmly committed to including voices from the global south.

In a blog post reacting to Stardust's particle design, Mr Iseman from Make Sunsets said that if it were proven to be safer and more cost-effective for SRM than sulphur dioxide, his company would start deploying it in the atmosphere.

"Until then, we'll keep copying volcanoes … and won't be waiting for badges of approval from any random governments," he wrote.