Do we have the technology to cool the planet? - critical summary review

The question sounds like science fiction, but people are already raising venture capital to answer it. Can we lower the planet's temperature on purpose? The short answer is that nature has already done it, at scale, in front of everyone. The long answer is what separates a seductive idea from a dangerous bet.

the volcano that turned down the world's thermostat

On June 15, 1991, Mount Pinatubo in the Philippines produced the second-largest eruption of the century. It threw roughly 17 million tons of sulfur dioxide into the stratosphere, the layer where planes fly. Up there, the sulfur turned into a haze of tiny reflective particles called aerosols. Within weeks that haze circled the globe. It worked like a sunshade, bouncing part of the sun's light back into space before it reached the ground. The effect was measured. Global average temperature dropped by about 0.5 degrees Celsius and stayed down for nearly two years. Climate models predicted the drop with good accuracy, which made Pinatubo the most cited natural experiment in the debate. The math is simple, and it is exactly what excites investors: if a volcano cooled the world by accident, maybe we can do the same on purpose. The detail that usually gets left out is that during the same period, ozone concentrations at mid-latitudes fell to the lowest levels ever recorded.

the summer europe couldn't take

While the idea matures in laboratories, the reason for it grows more visible on the street. Europe spent the past several days under a heat wave that broke records well ahead of season. In Spain, the Carlos III Health Institute counted more than 1,000 heat-related deaths, more than double the roughly 400 from the same period in 2025. In the Netherlands, there were about 480 excess deaths in a single week, concentrated among people 80 and older. On June 30, France recorded its hottest June day on record. This is not an outlier summer by chance. The European climate service Copernicus ranks Europe as the fastest-warming continent, heating two to three times faster than the global average. A year earlier, the June 2025 episode had already left about 2,300 deaths in 10 days, according to a study by Imperial College London and the London School of Hygiene and Tropical Medicine, with 1,500 attributed directly to human-driven warming. It is this kind of arithmetic that moves the idea of a planetary thermostat from the fringe onto the agenda.

the el niño that warmer seas keep feeding

There is also an accelerant inside the ocean. El Niño is the name for the warm phase of a natural Pacific cycle, when equatorial waters heat above average and scramble rainfall and winds worldwide. The U.S. agency NOAA puts the odds of a strong El Niño by the end of 2026 at 60% to 63%, possibly among the most intense ever measured. The World Meteorological Organization projects anomalies above 2 degrees in the regions it monitors. The ocean stores heat and releases it slowly, so the peak of the event tends to push the following year's temperatures higher. And here is the point scientists keep repeating: with warmer seas, you no longer need a strong El Niño for the damage to be large. Background warming amplifies any version of the phenomenon. The baseline has risen, and it keeps rising.

who's trying to repeat the volcano's trick

It was in this setting that solar geoengineering stopped being a university-only subject. Founded in 2023, Stardust Solutions, based between Israel and the United States and run by physicist Yanai Yedvab, raised $60 million in October 2025, the largest round ever for a company in the field, reaching $75 million in total. The company says it has created its own reflective particle, described as inert and safe, capable of blocking 1% to 2% of sunlight. It will not say what the particle is made of, wants to patent it, and plans to begin outdoor tests as soon as April 2026, releasing the material inside modified jets flying about 11 miles up. Its stated goal is to have deployment capability by 2030. It is not alone. The American startup Make Sunsets has already launched more than 200 balloons carrying sulfur dioxide over the United States and Mexico and sells what it calls cooling credits. After one of those launches, Mexico banned the practice on its territory.

The strange part is that the barrier is neither technical nor financial. Releasing aerosols into the stratosphere is cheap and, on paper, simple. One climate economist frames the problem as a free driver: a single country, or even a company, could do it alone, and no one has agreed on the rules. There is no international treaty on the matter, and most governments do not have a single line of law about it. The most studied version of the technique uses sulfate particles, the same kind the volcano produced, which damage the ozone layer and can turn into acid rain. Stardust's particle would be different, but since the company will not say what it is, no independent scientist can assess whether the promise holds. Then there is the aiming problem. As University of Exeter professor James Dyke puts it, the cooling would arrive, about 0.5 degrees, but very unevenly, with unpredictable rainfall and extreme events shifting across regions. So far there have only been tiny tests. Moving from those to a real operation would require continuous flights for decades.

the challenges that don't show up in the lab

The bigger risks appear when you think about stopping. It is called termination shock: if the world comes to depend on the aerosols and the application is interrupted, all the pent-up heat returns at once, faster than it would have without the technique. There is also what the sunshade does not do. It reflects light, but it does not pull CO2 out of the air. The ocean keeps absorbing about a quarter of everything we emit, and keeps getting more acidic, which punishes marine life regardless of temperature. On top of that sits the political risk known as moral hazard: the existence of a plan B can become an excuse to delay what actually matters, cutting emissions. It is worth noting the argument is contested. Communication experiments run by economist Gernot Wagner suggest that sober descriptions of geoengineering do not make people ease off on emissions, only the most alarmist ones do. And that leaves the thorniest geopolitical question of all: who controls the planet's thermostat? Because the effect would be uneven, countries such as Pakistan and Fiji have already come out against it at U.N. forums, seeing more risk than benefit.

so how do you actually cool a planet

The honest answer is that the sunshade treats the symptom, not the cause. What lowers the fever for good is cutting fossil fuel burning and, in parallel, pulling carbon back out of the atmosphere, a path that also eases ocean acidity and carries far more scientific consensus. Among researchers, even those who defend studying geoengineering repeat the same condition: it would only make sense as an emergency bridge, never as a substitute, and never as an excuse to take the foot off decarbonization. So we can answer the headline. Yes, we have a technology that can turn down the thermostat. We do not yet have proof that it is safe at the dose that would solve the problem. And we do not have the rules to use it without creating new dangers. The volcano's trick exists. What's missing is the hard part, which is the same as always.

what to do with this information

If you make decisions or invest: treat solar geoengineering as a risk category, not a finished solution. The bottleneck is governance and trust, not engineering, and that is where a bet can sour.

If you want to follow the debate: the right question is not "does it work?" but "who decides, under what rules, and with which particle?" Without an answer to that, any outdoor test becomes news for the wrong reason.

If you just want a sense of the scale: the known way out is still the same, cutting emissions, and it does not depend on any technology that doesn't exist yet.

Pinatubo showed that the world can be cooled. It did not show how to do it on purpose, at the right dose, without side effects. That is the distance between an eruption and a public policy. Until it is closed, Europe's heat and the warmer ocean remain the reminder that the problem will not wait. The technology to cool is still a draft. The one to stop warming has been ready for decades. We just have to use it.