What if climate change became so intense that one country broke with international protocol to protect its people? In fiction, that scenario has already played out. Kim Stanley Robinson’s 2020 novel Ministry for the Future opens with a catastrophic heatwave in India sparking a climate disaster of unmatched scale: 20 million people die as extreme temperatures take a horrific toll. Unwilling to let such a threat recur, the country’s leaders decide to take drastic action: by unilaterally dimming the Sun. 

Day after day for seven months, fleets of India’s planes pump vast plumes of aerosols into the stratosphere. From there, the mix of sulphur dioxide and other chemical particulates slowly spread across the northern hemisphere and “eventually everywhere”. 

By reflecting sunlight back into space, the particulates act as a planetary parasol, mimicking the effect of large-scale volcanic eruptions. The sky turns whiter, sunsets redder and the planet cools. The contentious move flies in the face of international law, as the book imagines it, and risks disrupting monsoon rains – but it also reduces global temperatures by “one degree, for three years”.

In Robinson’s imagined scenario, India’s rogue deployment of solar geo-engineering turns out to be broadly benign, and buys time to scale-up emissions reductions. But in the real-world, the idea that such a deus ex machina technology could ever be safely deployed remains highly speculative, with many risks and unknowns.

So if one rogue nation did decide to dim the Sun for real, what environmental and geopolitical consequences might unfurl? And is the safe deployment of such a technology even a conceivable goal? 

This January, more than 440 scientists signed an open letter calling for a non-use agreement on solar geoengineering – including for small-scale outdoor experiments, like the unauthorised test conducted by a San Francisco start-up in Mexico earlier this year. They argue that the side-effects are unpredictable, the current global governance system is “incapable” of guaranteeing fair and effective control, and that development might encourage “normalisation” of the technology as part of the world’s climate policy. Its cooling effect could create a “moral hazard”, numerous researchers and civil society organisations warn, by taking pressure off efforts to cut the underlying CO2 emissions.

Such concerns have so far resulted in a de-facto moratorium on deployment, while a planned field test over Sweden was cancelled in the wake of objections. “When you’re in a hole, stop digging”, is how Greta Thunberg expressed the fear that solar geoengineering will reinforce a planetary relationship based on extraction and exploitation. “A crisis created by lack of respect for nature will most likely not be solved by taking that lack of respect to the next level.”

Andreas Malm, associate professor of human ecology at Lund University in Sweden and author of How to Blow Up a Pipeline, agrees. “The worst-case scenario for the deployment of geoengineering,” he says, “is that you have it and then business as usual just continues with investment in fossil fuels and their infrastructure – and emissions continue to rise.” You’d then have to keep increasing the injections, he argues, which would only exacerbate the risks.

As Malm wrote in 2021, Robinson’s novel introduces a global armed struggle against fossil fuel capital at the same time as the solar geoengineering experiment. Without this added incentive to decarbonise, Malm fears the world would be too tempted to simply delay emissions cuts. And it’s a fear he still holds today. “The more I’ve read on this, the more I’m convinced this technology has such extraordinary potential for harm and destruction, that I don’t think I will ever personally support or advocate,” he says.

Yet in the three years since the publication of Robinson’s novel, global emissions have continued to rise, exacerbating everything from unprecedented heatwaves to deadly floods. This summer in the Northern hemisphere was the hottest on record, and September’s global temperature jumped again by a huge margin.

Amid these rises, some researchers suggest there could be a case for emergency solar geoengineering as an accompaniment to decarbonisation. In the US, the National Academies of Sciences, Engineering and Medicine last year recommended a national research programme on Solar Radiation Modification [SRM] – as the technology is also known. This year, the White House issued a report that examined what a federally-funded research programme might entail. And in the private sector, money from US tech giants and billionaires is flowing towards further investigation.

This February, dozens of scientists published an alternative open letter, organised by Sarah Doherty, an atmospheric scientist at the University of Washington, arguing more research is needed. And there are growing calls for a clearer international consensus on rules – one way or the other. The UN Environment Program has noted a “dearth of data” on impacts, and the EU has called for international talks about its risks.

As for Robinson, whose novel has helped foment so much discussion, he stressed to BBC Future that his book was not proposing a plan: “I abjure prediction, or even prescriptions”. Yet he also pointed to the need to take “emergency actions” to reduce the damage caused by burning carbon – from finding new ways to pay for decarbonisation, to exploring options for non-solar geoengineering. “I object to anyone saying ‘Oh we can’t try to fix things because it will encourage fossil fuel promoters to keep breaking things!’ We’re past that moment now. The sense of emergency is intensifying year by year.”

So what might it look like in reality?

Alternative methods of solar geoengineering are all now competing for their moment in the spotlight, from marine cloud brightening (which would inject sea salt aerosols into low-lying clouds to increase their reflectivity), to cirrus cloud thinning (which would inject ice nuclei into high clouds, shortening their life-span and allowing more heat to escape into space). But stratospheric sulphate aerosol injection by aeroplane remains the best-studied proposal for global impact. The technology could potentially lower temperatures at speed, and with relatively low finance. One 2013 estimate equates start-up costs to “the price of a Hollywood blockbuster”. A more recent calculation of running costs comes in at around $18bn (£16bn) a year.

Research supports a slightly different technical vision than in Robinson’s novel, however, with scientists suggesting a much slower ramp up, deployment and decline. One paper recommends that if stratospheric injections begin around 2030, they should peak 50 years later before tapering-off over two centuries. Another study has injections starting in 2035 and running at least until 2100; another estimates a duration of 245-315 years.  

Particulates fall back to Earth after about 12 months, so too short a run-time could only see minimal cooling effect. But if longer releases end too abruptly, a “termination shock” could also follow – unleashing devastating pent-up warming from emissions whose effects have only been masked and not removed. 

Among the most prominent proponents of further research is David Keith, head of climate systems engineering at the University of Chicago (and founder of a Canadian company developing technologies for carbon removal). Like in the novel, Keith has stressed that solar geoengineering should not substitute mitigation, but instead be used to help the world maintain global temperatures rise below the crucial 1.5C above pre-industrial levels; a threshold the World Meteorological Organization gives a 66% likelihood of overshooting by 2027.

And, as in Robinson’s book, Keith and others have also stressed climate change’s increased threat to the world’s poorest – especially those in the Global South. They note “an obligation” to take steps to reduce such harm and a “moral obligation to conduct research on solar geoengineering”. As the former president of the sinking islands of Kiribati, Anote Tong, this year told the website Climate Home: “we are facing a catastrophe and we’re trying to survive. What other options do we have?”

It would not be a Global South nation that would likely deploy the technology, however, says Wake Smith, a climate researcher at the Yale School of the Environment and a former aviation industry expert. According to Smith, stratospheric aerosol injection would require “a fleet of several hundred large high-altitude jets of a sort that does not currently exist”. These specialist planes would collectively need to release millions of tonnes of chemicals at an altitude of roughly 20km (66,000ft). The only countries that could build such fleets, Smith claims, are the US, UK, France, Russia or China, and potentially Germany or Japan. “No other state is technologically capable, and this is vastly too big for individuals or companies to pursue.”

It would also not be possible for India alone to create the kind of global cooling that Robinson’s novel envisages, Smith contends. If aerosols were released over northern India, the planet’s rotation would spread them into a ring around the Earth, from where an atmospheric system called the Brewer Dobson Circulation would carry them to the North Pole and they would descend – directly cooling the northern hemisphere only. An equivalent southern hemisphere deployment would also then be necessary, Smith suggests, in order to attempt to counterbalance the resulting redistribution of weather systems. Such far-reaching geophysical impacts means any rogue nation would be unlikely to remain solo for long, Smith says, either being deterred from their efforts or joined by others.

Can we predict any side-effects?

Yet even if a future large techno-power could press ahead with deployment, that doesn’t mean they would or should. Aside from the possibility it could slow efforts to reduce emissions, there are also numerous other environmental hazards that could result. As Elizabeth Kolbert illustrated in her 2021 book Under A White Sky: The Nature of the Future, humanity’s attempts to control nature are littered with unforeseen disasters. There may come a point at which solar geoengineering’s benefits are necessary, she implies – but history suggests it would be naive to hope that fearsome consequences won’t result.

So can scientists predict what those environmental consequences might be? To date, models and simulations show that stratospheric aerosol injection could impact everything from the position of the jet stream to causing regional droughts. One study has noted a potential for extended Arctic summer sea ice loss, while another points to a “considerable reduction” in monsoon precipitation. And, even if deployed consistently, crops could still suffer. 

Among the most potentially consequential effects is damage to the atmosphere’s protective ozone layer. A 2022 UNEP report into its depletion noted “shortcomings” in the modelling on solar geoengineering’s impact. The technology would also do nothing to stop rising CO2 concentrations from acidifying the oceans.

How bad might such side-effects be? Karen Rosenlof, a senior scientist at the US National Oceanic and Atmospheric Administration’s Chemical Science Laboratory, is part of a group of researchers analysing the atmospheric impacts of various existing aerosol-based phenomena – from rocket emissions and satellite re-entry to volcanic eruptions. These instances can all be used to learn about “possible impacts” of deliberate aerosol injection, she notes, but different properties of various aerosols (sulphate, soot, organic carbon and metals) all have different effects. As do how high, where, when and in what volume the substances are released. 

There is also no analogue for continuous emission, as volcanoes are sporadic. Nor a complete knowledge of how various parts of the climate system interact. What there is, Rosenlof stresses, is “a lot of uncertainty”. 

How would the world react?

As the above suggests, not all solar geoengineering’s effects are distributed equally – and what might be welcome lower rainfall in one place could spell disastrous drought in another. Such uneven distribution of impacts means any unilateral attempt to dim the Sun would likely be squashed by more powerful neighbours, some suggest. Or that only nations geopolitically big, bold and brazen enough would ever dare to deploy in the first place. For Frank Biermann, professor of Global Sustainability Governance at Utrecht University in the Netherlands and founder of the Earth System Governance Project, the US is the most likely candidate in such a scenario: “If the majority of countries object to the deployment… the political cost for any country to do it unilaterally is extremely high.” 

If one nation or more were to push ahead regardless of such tensions, however, the worst-case scenarios are numerous. Countermeasures ranging from economic sanctions, to UN intervention and potentially armed conflict could all be deployed, says Biermann, with the ultimate result “difficult to predict”. There is also a possibility that an arms-race develops, with nations developing the technology simply because rival superpowers are doing likewise. 

To prevent this, Biermann argues you would need to stop the technology from getting to the level of development reached by nuclear bombs, such as via the non-use agreement he and others have proposed. The Chemical Weapons Convention, which aims to prevent both the development and use of chemical weapons, is a Nobel Peace Prize-receiving precedent.

But what if a group of nations, or even the entire world, agreed to act in concert? For Smith, there might come a scenario where deployment serves a majority of interests. The “most plausible scenario” for deployment, he suggests, is an escalating climate emergency leading to mass global migration from south to north, with the north unwilling to provide the relief the south needs. “While multiple, competing deployment programmes are a possibility, the incentives in this case actually favour a single, global programme,” he says. “Existing political structures are not well-suited to govern this, but neither are they well-suited to trapping hundreds of millions of people in faltering economies against their will.”

How far such a “single, global programme” is realistic is another thing. Keith has noted potential precedents in global central banking, the Internet, and air traffic control. Others point to the United Nations framework Convention on Climate Change, and the resulting Paris Agreement, under which nations have pledged to limit global temperatures. (Though the world is currently on track to miss the very targets this agreement has set).

For sceptics, meanwhile, a consistently globally united effort is simply not a likely prospect. In the case of any longer-term deployment, avoiding “termination shock” – where stopping too fast unleashes catastrophic warming – would mean ruling out a whole host of all-too-viable eventualities: from terrorists shooting down planes, to war, pandemics or even natural disasters. 

There simply isn’t yet enough modelling of the geopolitical consequences, says Olaf Corry, professor of global security challenges at Leeds University in the UK. “The scientists are good people, and they’re modelling [the physical impacts], but in their model-world, there isn’t any geopolitics.” In contrast, military planners and retired generals have told Corry that testing could be seen as a potentially hostile act and that they are concerned about other powers acquiring it. Think Russia invading Ukraine, but with the potential to switch off a global weather-creating technology.

Deployment could also risk opening up a whole new arena of disinformation about why the weather was changing, he suggests, and “infect” the rest of climate politics. “The whole relationship between science and society is already strained after Covid,” says Corry. “So you would have an extraordinary potential for conspiracy theories and for misinformation, and an incredibly difficult environment for science to do its job.”

Plus there are also the questions of justice and equity. Shuchi Talati of The Alliance for Just Deliberation on Solar Geoengineering, a US non-profit, is working to help spread awareness and “ensure climate vulnerable communities and countries – mostly in Global South – have access to information, knowledge and resources” about the technology. This in turn, she hopes, will allow “robust governance frameworks” to be built collaboratively.

But even with more awareness, Global South nations have less likelihood of developing the technology because of the scale and technology required, and already have less representation in international bodies like the UN Security Council. The voices of the most vulnerable nations would likely still be muted, warns Silvia Ribeiro of the Action Group on Erosion Technology and Concentration, which monitors the impact of emerging technologies. If one Global South country were to deploy at scale, it “would cause so many geopolitical conflicts that no Global South country would be in a position to handle”.

For Lili Fuhr, a director at the Center for International Environmental Law, a non-profit organisation based in Washington DC, the technology “requires governance systems much more sophisticated than anything that has ever existed and that would have to function over centuries or millennia – an impossible requirement.”

Others, however, still see room for optimism – including Kim Stanley Robinson himself. In his novel, Ministry For the Future, the severity of the climate emergency is such that a violent global resistance movement against fossil fuels also emerges. Yet the world feels “less dark” now than when he was writing the novel, Robinson said recently on a Bloomberg podcast, and there the impetus for such extreme measures might not arise.

Robinson told BBC Future that though climate disasters are increasing, so too is the international momentum to act. “My impression is that the pandemic shocked people into a new world of awareness that the biosphere matters and can severely disrupt civilisation – we saw it happen.” 

Since the chances of overshooting the 1.5C limit are still high (but not yet inevitable), “all possible means” are going to be needed to draw down CO2 from the atmosphere, Robinson adds. “We might have to try various methods grouped under the word ‘geoengineering’ to cool things if we can, while the decarbonization proceeds. Groups like C2G and Silver Lining and TerraPraxis, and I assume many others, are trying to stimulate these discussions to prepare the public for possible interventions, and I think the widespread hope is that if we ever do such things it will be by way of international agreements and general consensus.” 

Meanwhile, on solar geoengineering specifically, Robinson is not convinced by arguments it would cause decarbonisation efforts to slow: “I think we’re past that moment. The imperative to decarbonise ASAP is just too clear now. Any climate sceptics left are frauds or fools.”