On a Tuesday in August, Luke Iseman drove two hours east of Oakland in drought-stricken California to a remote spot where he launched a handful of balloons filled with sulphur dioxide and helium high into the sky.
From there, he used GPS to try to track the balloons as they rose into the stratosphere, the layer of Earth’s atmosphere that begins about 12km high and contains the ozone layer that protects the planet from solar radiation. Once there, they would burst and release the gas.
Iseman’s start-up, Make Sunsets, is piloting small-scale stratospheric aerosol injections: the sulphur dioxide released by the balloons oxidises to form an aerosol, or fine mist, of sulphate particles that deflect some of the sun’s radiation. So far it has launched 28, each the size of a small weather balloon. “The company’s mission is to cool the Earth as quickly as we safely can,” he says.
It is a rudimentary version of a niche climate solution broadly known as solar radiation management — the idea that the Earth can be cooled by reflecting some of the sun’s rays back into space. Other SRM techniques range from painting the world’s roofs white to putting giant mirrors into orbit.
“Pretty much every country that has a climate scientist is interested [in SRM],” says Daniele Visioni, assistant professor in the department of Earth and atmospheric sciences at Cornell University, although the majority of research is concentrated in the US and Europe.
Proponents argue that humanity’s lack of progress in curbing carbon emissions means society will increasingly need to look at more drastic ways to limit the worst effects of warming as well as trying to prevent further rises in global temperatures.
July was declared the hottest month ever recorded, according to Copernicus, the European Union’s Earth observation agency. The Intergovernmental Panel on Climate Change warned earlier this year that the average global temperature would reach 1.5C above pre-industrial levels “in the near term”. The 2015 Paris Agreement pledged to “pursue efforts to limit” the increase to that amount.
In its most recent report on SRM, the UN Environment Programme described the technology as the “only” way to cool the planet in the short term.
Pascal Lamy, a former director-general of the WTO who now chairs the Climate Overshoot Commission, a civil society group working on ways to mitigate the effects of exceeding the Paris Agreement target, says governments should “open the box” and research solar radiation management.
“The risk of overshooting [1.5C] is higher and higher every day,” he adds. “We have to exhaust all the avenues that we know are the ones that are working and this is the known part of the solutions. And then there are other solutions [like geo-engineering] that are more in the unknown part of the solutions.”
Joshua Goldstein, professor emeritus of international relations at the American University, says that whatever efforts society makes to mitigate climate change “would be really helped [by] geoengineering because it would shave off some of the peaks of the temperature.”
“Everything else we do is really only pecking away,” he adds. “I don’t think [SRM] is going to be the big solution but it could be an important piece of a bigger solution.”
But for others, SRM is a wasteful, ineffective and even dangerous proposition. Little is known about the side effects of attempting such cooling on a planetary scale; some scientists argue that large-scale deployment of sulphur into the stratosphere might change the colour of the sky to a milky white.
Others suggest that such a project might have unpredictable effects on weather patterns across the globe and could potentially worsen extreme weather events such as droughts and monsoons in some regions. There are concerns that fossil fuel companies could promote it as a way to allow them to continue operating.
Diplomats and governance experts argue that the world lacks a meaningful way to discuss how to equitably unleash such technology, and that decisions with global consequences could end up being made by a few powerful governments.
Aarti Gupta, professor of global environmental governance at Wageningen University in the Netherlands, is part of a group of over 300 academics calling for a “non-use agreement” around solar radiation management.
“Our concern is that there has been very little discussion, and this risky idea is getting normalised — even though it’s only a small group of people, primarily in the global north, and some tech investors pushing this idea,” she says.
Raymond Pierrehumbert, professor of physics at the University of Oxford, acknowledges that while people are “legitimately in a panic about the climate crisis”, the unknowable side effects and governance issues mean that SRM is not an easy option.
“The discussion is a big distraction from the things we know we need to do, which is to get the carbon dioxide [emissions] down to [net] zero,” he says.
The sulphurous mirror
When Mount Pinatubo erupted in the Philippines in 1991 it produced the largest eruption cloud ever measured, according to the US Geological Survey, blasting around 17mn tonnes of sulphur dioxide into the atmosphere. Climatologists estimated that global temperatures dropped by about half a degree Celsius over the following year.
This is the basis of stratospheric aerosol injection, the most studied area of solar radiation management. It has been researched for decades but only recently has the idea gained some mainstream traction. Those working in the field largely agree that if done on a large enough scale, the Earth’s average temperature would drop just as it does following volcanic eruptions.
In 2021, the independent National Academies of Sciences, Engineering and Medicine recommended that the US should offer some funding to the research. Earlier this summer, both the European Commission and the White House released reports calling for more research into SRM.
The White House Office of Science and Technology Policy said that a research programme into the “scientific and societal implications” would “enable better-informed decisions about the potential risks and benefits of SRM as a component of climate policy, alongside the foundational elements of greenhouse gas emissions mitigation and adaptation”.
The authors of the UN report warned that the costs of aerosol injection could run into “tens of billions of dollars per year per 1C cooling” and that these made medium to large-scale deployment “unwise”.
The European Commission said it would support international efforts to assess “the risks and uncertainties of climate interventions, including solar radiation modification” and for research into how to regulate it globally.
Solar radiation management “remains a niche area of climate research with far, far less funding than other areas of climate modelling”, according to Andy Parker, the chief executive officer of Degrees, a fund that gives small grants to scientists from the global south to explore the possible effects of solar geoengineering on their climate.
“But it is clear to see that it is rising in prominence quite quickly, just as people start to [ask] — ‘what do we do if we can’t cut emissions fast enough?’” adds Parker, who led a 2009 report on geoengineering by the UK’s Royal Society. “It is not one’s first choice for dealing with global warming. It is recognition of how deep a hole we are in.”
Governments are not the only interested parties — wealthy individuals and philanthropic bodies have also been backing research into SRM. Harvard University has set up a solar geoengineering research programme, with financial backing from Bill Gates and a host of philanthropic trusts and funds.
“For the last 20 years or so, there’s been kind of a taboo against talking about this,” said professor David Keith, who was formerly of Harvard but now leads the climate systems engineering initiative at the University of Chicago.
Theory and practice
Although Keith is one of the leading researchers in the field, he acknowledges that there could be drawbacks and unforeseen side effects.
“If we deliver sulphuric acid to the stratosphere in a jet aircraft, which we know we can build, and you did it enough to reflect half a per cent of sunlight, then the Earth would cool,” he says. “That’s not to say that it’s a good idea.”
There are uncertainties, Keith adds, over how much the sulphur dioxide might damage the ozone layer or human health, and whether it would influence extreme weather events. Scientists have, for example, looked at models that suggest aerosols released in the northern hemisphere could cause severe droughts in sub-Saharan Africa.
Gupta argues that there are “things we cannot know through small scale experiments . . . we’re looking for a planetary scale effect, so we need planetary scale experiments”.
“In essence this is an untestable technology, because to test it would be to deploy it,” she concludes.
Researchers backed by Degrees funding are looking at how stratospheric aerosol injections might affect issues ranging from the spread of malaria to rainfall patterns and loss of mammal species in different regions.
Scientists are also wary of so-called termination shock — the idea that once the reflective aerosols in the stratosphere dissipate, the warming they prevented then takes place relatively suddenly, over a decade or so.
Pierrehumbert describes termination shock as the biggest drawback of solar radiation management. “You’re setting the world up for a real catastrophe,” he says. “There’s no precedent in all of our history for such a rapid warming on such magnitude — we’d be taking the Earth, way, way out of its known ways of operating.”
There is a mismatch, argues Pierrehumbert, between the year or so that aerosols reflecting sunlight will stay in the stratosphere, and the thousands of years that carbon dioxide lingers for.
“You’re trying to cancel out an essentially permanent climate effect with something that has to be renewed every year,” said Pierrehumbert. “We would be moving under the sword of Damocles, globally. That termination shock risk is always hanging over you.”
Managing the ongoing project of injecting aerosols into the stratosphere would be crucial. The White House report agreed that while SRM “offers the possibility of cooling the planet significantly on a timescale of a few years”, there should be international co-operation over its implementation.
Who’s in charge?
But as yet, no framework exists for such co-operation. Frank Biermann, professor of global sustainability governance at Utrecht University, says proponents of the technology are “extremely optimistic that we will have 1,000 aeroplanes flying around for 100 years, organised and managed by some international agency, and somehow everybody is happy.”
He is concerned about the consequences of countries deciding to act unilaterally. “What would happen if Vladimir Putin launched a unilateral geoengineering programme to compensate for continuing Russian fossil fuel use?” says Biermann. “Or what if the Americans wanted to do this, but the Chinese or the Europeans do not agree? Who would decide?”
There are also concerns that this planetary-scale technology may end up being controlled and run by wealthy countries, leaving governments from the global south excluded from decision-making.
In February, Ajay K Sood, the principal scientific adviser to the Indian government, warned that geoengineering could end up “concentrating power in rich countries”, despite the risks that the technology could worsen droughts or intensify hurricanes in countries not party to their decision-making.
Parker, at Degrees, believes countries on the front line of extreme weather events could be more tempted to jump to use geoengineering techniques without proper research.
“Desperate people do desperate things and the possibility remains that a country might decide that it is suffering disproportionately and it might want to take the option,” he says.
Pornampai Narenpitak, one of the researchers that Degrees supports at the National Science and Technology Development Agency in Thailand, agrees that the effects of using these methods to cool the Earth were not widely enough understood in her region.
“Countries that are impacted by climate change the most are those that are the more tropical countries or have lower incomes and so have less resources for whatever [research] they need,” she says.
She adds that it is important to have the voices of scientists from the global south “represented in discussions about geoengineering techniques.” But the first step is to “make sure the scientists or representatives from these countries understand what solar radiation management is and what we are talking about in terms of the risk-risk scenario”, referring to the process of comparing the risks of doing something with the risks of doing nothing.
DIY climate science
Another worry is the involvement of tech entrepreneurs acting on their own agendas. To some scientists and global governance experts, people like Iseman — with his DIY approach to trying to cool the Earth — are the planet’s worst nightmare.
Make Sunsets has just a handful of staff and has raised less than $1mn of venture capital funding, according to PitchBook data. But the existence of such companies conducting their own geoengineering experiments “shows you the amount of governance there is”, says Claudia Wieners, assistant professor in Earth system modelling at Utrecht University in the Netherlands. “We need to have governance, to stop crazy people doing crazy stuff.”
A report from the European Council’s research unit published in May also referred to “vast amounts of money swilling around Silicon Valley” being put to use by a private sector that believes “it can be more effective in delivering solutions than governments”.
“They see themselves as called to be free from government oversight in order to serve the interests of individual freedom and for the benefit of future generations,” the report said.
But Iseman, a serial entrepreneur with no science background who was inspired by science fiction novels to start his company, has the zeal of a political activist and is unapologetic about his efforts.
“This is an emergency,” he insists, referring to the seemingly unstoppable rise in global temperatures. “I’m not going to sit around and not take action because the responsible adults think there might be geopolitical consequences.”
Graphic illustration by Ian Bott