- Recent research and interest, especially from the U.S. government, has pushed a solar geoengineering idea known as SAI, or stratospheric aerosol injection, to the top of lists of potential ideas to cool the planet. SAI would use fleets of high-flying aircraft to disperse sunlight-reflecting particles, including sulfates, into the stratosphere.
- As climate change worsens and carbon emissions continue to rise, researchers say we must be ready with other potential tools to stave off total catastrophe, such as the disintegration of the West Antarctic Ice Sheet with sea-level rise that would drown coastal cities.
- But critics say any geoengineering is too unpredictable and could allow governments to lower their ambitions on cutting emissions. They also argue that there’s no global governance policy to implement geoengineering justly.
- As debate intensifies, however, some feel it’s only a matter of time, given the current and projected trajectory of warming, until the world seriously considers geoengineering deployment.
This is Part One of a two-part story. Find Part Two here.
A 2023 study published in JGR Atmospheres found that implementing a global solar geoengineering project could slow the melting of the increasingly threatened West Antarctic Ice Sheet. That’s important because last year scientists determined that climate change has made the collapse of that ice sheet “unavoidable” — with geoengineering posing a plausible last-ditch chance to save it and the world from eventual catastrophic sea-level rise.
Researchers modeled how deploying tiny sulfur droplets in the stratosphere at multiple points around the globe, a technology dubbed stratospheric aerosol injection (SAI), could reflect sunlight back into space and buy humanity time by keeping Antarctica cool.
A similar study published in Nature Climate Change found that SAI could significantly slow melt in Antarctica with a chance, though less than a certain one, of preventing the ice sheet’s collapse if started early in a low-emissions scenario. (Global carbon emissions from fossil fuels hit a record high in 2023, making a low-emissions scenario look increasingly doubtful.)
Why does all this matter? Because a total collapse of the West Antarctic Ice Sheet could ultimately raise sea levels by 3.3 to 4.3 meters (10.8 to 14.1 feet), remaking our civilization — and not in a good way.
“Yes, climate engineering can sound sci-fi, and it can be scary — it probably should scare us all. But climate change is scarier,” says Paul Goddard, lead author of the JGR Atmospheres study.
The total loss of the West Antarctic Ice Sheet (a process likely requiring several centuries to complete) would put southern Florida (including Miami), most of the Netherlands (including Amsterdam) and nearly half of Bangladesh underwater. The Bahamas wouldn’t exist; nor would many island nations. And that’s just a sampling. Hundreds of millions, or even billions, of people — as much as 40% of humanity — could be displaced, resulting in mass migrations, political unrest, and the potential failure of nation states. And that’s just sea level rise; add in this century’s, and future centuries’, escalating killer heat, drought, storms and wildfires.
Could we stop this from happening? Or slow it down? Some scientists say we can.
SAI: A hedge against catastrophe
Solar geoengineering, and in particular SAI, has become the most popular of the multitude of global geoengineering initiatives now under investigation. A preference for SAI is based on a few facts: First, we know that technically it would work. The approach mimics a volcanic eruption, which sends sulfur into the stratosphere, cooling the planet in the short term. 1991’s cataclysmic eruption of Mount Pinatubo in the Philippines cooled the globe by 0.5° Celsius (0.9° Fahrenheit) for more than a year.
Also, the technique is comparably affordable: SAI program cost estimates come in around $10 billion annually, environmental law professor Edward Parson, told CNBC. While not chump change, that’s a lot cheaper than putting giant mirrors in space (which experts say would cost trillions), another solar geoengineering idea. Also, SAI is far cheaper than the harm wrought by future unbridled climate change, estimated at a staggering $19 trillion to $59 trillion annually by 2050.
David Keith says SAI is a current favorite because most scientists agree it “could be done.” Keith, a professor in the Department of the Geophysical Sciences at the University of Chicago, is arguably the best-known SAI researcher, though admittedly that’s a small group. He’s been studying the idea since 1989.
Are there problems with SAI? There are many, ranging from potential environmental impacts, to concerns nations could weaponize the technology, to fears that even considering SAI could give fossil fuel companies and governments an excuse to shrug off climate change action.
Just as importantly, SAI raises monumental pragmatic policy questions as to who would govern a global effort to cool the planet.
“I cannot imagine any global governance scheme that can effectively and fairly govern the deployment of such technologies under current political circumstances,” says Frank Biermann, a social science researcher at the Global Sustainability Governance at Utrecht University in the Netherlands. Biermann is one the initiators of an open letter calling for a solar engineering ban signed by 500 experts and endorsed by 2,000 civil society organizations.
The only reason some scientists are urging aggressive geoengineering field research today is their concern over the looming existential cliff imposed by 40 years of failure to effectively tackle climate change. This failure is both an argument for geoengineering (we’ve created an increasingly unstable planet and it’s only going to get worse), and against it (if governments can’t even cut CO2 emissions, how are they going to equitably deploy and regulate a global effort to cool the entire planet?).
The world’s goal, agreed to in Paris in 2015, of keeping temperatures from rising more than 1.5°C (2.7°F) since the preindustrial period is likely dead. And if emissions aren’t massively cut very soon, we’re going to blow past 2°C (3.6°F) too, assuring an apocalyptic future.
That’s why what once sounded like a mad scientist’s scheme — sending fleets of jumbo jets daily into the stratosphere to dump polluting sulfate particles — is getting rising attention from researchers, the media, and even the U.S. government.
A hundred planes for a hundred years
“Geoengineering mostly means deliberate, large-scale human intervention in the environment to achieve some goal,” Keith says. “Solar geoengineering is the deliberate large-scale manipulation of the Earth’s radiative balance to reduce the impacts of CO2 in the atmosphere.”
SAI in particular would put aerosols into the stratosphere to reflect perhaps about 1% of sunlight away from Earth. The task would be performed by hundreds of huge airplanes taking off from various points around the Earth, flying about 20,000 meters (66,000 feet) up, and releasing sulfates or some other material.
These flights would need to fly continuously, maybe for a century, just to maintain the baseline temperature. That’s because sulfates only stay in the stratosphere for weeks to months. Once humanity significantly cuts its carbon emissions and sequesters sufficient excess carbon dioxide, we could taper off the SAI.
No one doubts this being a Herculean endeavor, but Wake Smith, a climate researcher and author of the book Pandora’s Toolbox, says it would “cost roughly one-thousandth what any mitigation, adaptation, or carbon capture [strategy] would. So, cheap, cheap, cheap!”
But it’s also important to understand that SAI would not be a time machine, taking us back to the climate of 1700, before humans started burning coal for steam power. No researcher is advocating geoengineering as a replacement for emissions cuts.
“It is not a magic ‘undo’ button for climate change,” Smith warns. “It might do bad stuff we didn’t expect … It is not risk-free and doesn’t justify continuing emissions.”
We’ll still need to “decisively start bending the curve of [carbon] emissions” to near-zero, says Earth system scientist Johan Rockström, by investing in a rapid global sustainability transformation of the energy, transportation and agricultural sectors, while also deploying large-scale carbon-capture methods. It’s the biggest venture humanity has ever faced.
Solar engineering hope
SAI’s potential is simple: It could decrease the rate at which Earth is heating up. “SAI could cool the planet substantially and quickly,” Smith says, though he notes this can only happen after we produce the infrastructure to do it, which he estimates would take 20 years.
Think of the climate problem like a tug-of-war rope. Right now, the heat balance is tugging us one way, towards global heat catastrophe. SAI would add a team at the other end of the rope, pulling Earth back toward a relatively benign climate that could continue nurturing large-scale agriculture, cities, and a population topping 8 billion people.
Not only could SAI potentially slow Antarctic ice loss, but also curb Arctic melting, preserving Greenland’s ice sheet, thereby significantly slowing sea-level rise and saving millions from homelessness.
SAI could also save innumerable lives from heat-related deaths, prevent some of the megafires now ripping through ecosystems, and slow desertification and drought. It could even protect crops, though it could also cause crop failures. It might also prevent some extreme weather events — though it could cause some too.
Keith says SAI could make climate change “slower” and offer “pretty broad” benefits, especially to the “world’s poorest.”
“We have enormous amounts of econometric analysis [done] over the last few decades that shows that heat kills people, causes reductions in learning … and reduces people’s productivity,” Keith says, especially in the Global South.
“In a hot year in India, the economy grows slower and deaths go up. If solar geoengineering reduces temperatures pretty evenly — which is one thing every single model says it can do, (nobody serious disagrees about that) — then it’s pretty much axiomatic that the big benefits are going to go to the poor.”
But Biermann, the social researcher, says he strongly disagrees. He points to a recent call by apprehensive African nations for a ban on solar geoengineering. And to researchers like Keith, who say SAI is being done to benefit the Global South, Biermann says they “should think very carefully about the fact that African governments don’t want solar geoengineering.” Mexico banned solar geoengineering experiments last year.
Such bans raise a troublesome question: How can nations doing pro-geoengineering research impose a planetwide climate altering technology on countries that have laws against it?
Solar engineering problems
There’s no such thing as a free lunch, as the saying goes, and SAI, despite its potential, comes with many major drawbacks.
The number one question, Biermann says, is who would govern this planetwide undertaking, and how? “The current international governance system is not in a position to deal with [SAI],” he says, adding, “In the absence of a world government, I do not believe that one can effectively deal with the many governance challenges that come with such technologies.”
Who would decide where aerosols are released? How often, and how much? Different configurations would almost certainly lead to different outcomes for different places — and adverse outcomes could easily bring nation states into fierce conflict.
The ecological risks are high as well. Although SAI would likely cool the planet, it could unpredictably alter regional precipitation, maybe drastically decreasing rainfall over rainforests or vital agricultural areas, or causing extreme flooding.
“You’re going to change atmospheric circulation [with SAI]. How will that change precipitation patterns?” says Goddard, lead author of the 2023 SAI paper. He adds, though, that “there’s a lot of research going into how you can minimize that change.”
Drought and extreme weather are, of course, already happening due to global warming, and will get worse. But deployment of SAI would infinitely complicate matters, especially regarding causation: If, for example, there’s an extreme drought in India, was it triggered by climate change, SAI, or natural variability?
Would SAI make weather “worse than what would happen under climate change? Is it fairly equal to it, or is it improving it? All those are subjective questions,” Goddard says. And who would arbitrate these questions, oversee decisions, and pay for SAI-induced damage?
There are also concerns SAI might harm the ozone layer, though this depends on the material used and how it performs in the stratosphere. Critics have also raised concerns over acid rain, though research shows the wide dispersal of aerosols would mean the risk here could be tiny.
Other political concerns address humanity’s dark side: “There’s the possibility of using [SAI] for warfare, which luckily seems to be very small, but you can’t discount it,” Keith says. “Humans use technologies for destructive ends.”
But arguably one of the biggest issues arising from SAI or geoengineering discussions is whether studying it, or even mentioning it, will give nations and the fossil fuel industry an excuse to slow emission cuts.
Biermann says he and many others fear that “major industries, but also entire countries, might reduce their current and future efforts in mitigating climate change because they believe there’s a silver bullet somewhere that would save them.”
He says this is already happening “to some extent” due to the hype around carbon dioxide removal, and adds he worries that societal acceptance of SAI could worsen this existential threat. “Imagine the oil industry, gas industry, coal industry, and even an entire country using … solar geoengineering [as a carbon offset mechanism],” Biermann says.
Smith, the climate researcher, argues, however, that the reality isn’t that geoengineering research is slowing emissions cuts, but that those cuts are simply not ramping up fast enough — and that even with the impressive growth in green energy, we’re not achieving the scale of change we need.
“Far too many people — mostly the general public, rather than climate experts — believe that emissions reductions alone will produce an acceptable climate outcome. They won’t,” Smith says. “Renewable energy sources are growing impressively, but demand for fossil fuels is not shrinking.”
Smith says emissions will eventually plateau, but then stay at roughly the same level for decades, with the world not achieving net zero until 2100.
“If all of that proves true, then humanity will get far more climate change than it is currently expecting. We clearly need to reduce emissions as fast as possible, but we will need other tools by which to contend with the warming that is coming,” Smith warns. “Industrial carbon capture for sure, but probably solar geoengineering as well.”
A potential global commitment to SAI comes with another warning: Scientists caution that any major interruption in the hundreds of daily flights shielding a globally warmed world from the full effect of the sun’s rays (due to war, an economic downturn or other causes), could result in something they call a termination shock: With our sun shield abruptly removed, the full force of accumulated carbon emissions would suddenly be unleashed, with super-destructive heat waves, storms and drought. Indeed, recent cuts in sulfate emissions by transoceanic shipping, followed by the 13 warmest consecutive months on record, could be evidence of humanity conducting an inadvertent SAI experiment on itself.
Where, when and how
Effectively heading off a meltdown of the West Antarctic Ice Sheet sounds like a good goal for SAI, with Goddard’s research showing that multiple SAI deployments focused in the Southern Hemisphere would be the best way to stabilize the ice sheet.
But should Antarctic stabilization be the priority of SAI? What would the 195 nations of the world, all with diverging opinions, want? And who would decide?
And despite the research that’s been done, no one can be sure of real-world results during an SAI deployment. Keith says, for example, we still don’t know a lot when it comes to Antarctica’s ice sheets. SAI would “clearly make things better in Greenland. I think I’d be very confident it would make West Antarctica less likely to collapse … But how much less likely, I just think we really don’t know very well.”
To understand better, we’d need more research — and especially research in the field, scientists say. But field research in geoengineering, and especially SAI, has run time and again into roadblocks posed by apprehensive activists, communities and governments.
Biermann, one of the initiators of the solar engineering non-use agreement, says he and his colleagues aren’t against research outright. But their letter does oppose any deployment of geoengineering, technology development or patenting. Biermann adds that the agreement signers also “object” to field research as it leads to technological development.
Meanwhile, climate impacts continue escalating: This year alone, the Amazon Rainforest burned as never before, the U.S. Midwest suffered devastating floods, while Southern Africa endured terrible drought, as record heat pummeled nearly 5 billion people during the hottest June in recorded history. Temperatures have now sporadically hit levels where unprotected human survival is no longer possible, with more than 1,300 people dying of extreme heat during the Islamic Hajj pilgrimage in Saudi Arabia.
This onslaught of extreme events is why some scientists say they’re absolutely convinced a global commitment to geoengineering research is required now.
“It’s inevitable that we’re going to need to do more because we are failing,” says Brad Ack, the CEO of Ocean Visions, which is currently investigating oceanic CO2 sequestration, along with other forms of geoengineering.
I asked Ack how likely it is that we’ll turn to some kind of geoengineering in the next 50 years: “It gives me enormous pain to say,” he responds. “I think it’s highly likely.”
Banner image: SAI would put aerosols into the stratosphere to reflect perhaps about 1% of sunlight away from Earth. The task would be performed by hundreds of huge airplanes taking off from various points around the Earth. Image by Jeremy Bishop via Unsplash (Public domain).
Read Part Two:
Don’t even study it: Geoengineering research hits societal roadblocks
Citations:
Goddard, P. B., Kravitz, B., MacMartin, D. G., Visioni, D., Bednarz, E. M., & Lee, W. R. (2023). Stratospheric aerosol injection can reduce risks to Antarctic ice loss depending on injection location and amount. Journal of Geophysical Research: Atmospheres, 128, e2023JD039434. doi:10.1029/2023JD039434
Sutter, J., Jones, A., Frölicher, T. L., Wirths, C., & Stocker, T. F. (2023). Climate intervention on a high-emissions pathway could delay but not prevent West Antarctic Ice Sheet demise. Nature Climate Change, 13(9), 951-960 (2023). doi:10.1038/s41558-023-01738-w
Biermann, F., & Gupta, A. (2024). A paradigm shift? African countries call for the non-use of solar geoengineering at UN Environment Assembly. PLOS Climate, 3(5), e0000413. doi:10.1371/journal.pclm.0000413
Tracy, S. M., Moch, J. M., Eastham, S. D., & Buonocore, J. J. (2022). Stratospheric aerosol injection may impact global systems and human health outcomes. Elementa: Science of the Anthropocene, 10(1). doi:10.1525/elementa.2022.00047
Yuan, T., Song, H., Oreopoulos, L., Wood, R., Bian, H., Breen, K., … Platnick, S. (2024). Abrupt reduction in shipping emission as an inadvertent geoengineering termination shock produces substantial radiative warming. Communications Earth & Environment, 5(1). doi:10.1038/s43247-024-01442-3
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