Climate change is an increasingly dangerous antagonist, and we’re not doing a great job curtailing it. Which is why, in the not too distant future, we may have to undertake a new, Manhattan Project-style endeavor to hold back the rising mercury. Once a fringe idea, there’s now a growing possibility we’ll build machines that will, in a manner of speaking, darken the Sun.

There are several variations on the so-called solar geoengineering theme, but they all have the same end-goal: using aerosols to blanket our atmosphere with reflective particles in order to quickly lower global temperatures. There’s been a lot of discussion of how this might go wrong, but much less on the technology needed to make it work. So, what would our hypothetical, sky-altering, solar radiation management (SRM) machines look like?

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To answer that question, we first need to understand what these machines would actually be putting in the sky. The ingredients required to create a nebulous skyward mirror range from table salt and aluminum oxides to obliterated diamond dust. The one that receives the most attention, however, is sulfur. There are several reasons for this, but perhaps most importantly, we know with near-absolute certainty that this aerosol would work.

Volcanic eruptions are known to sometimes effuse vast amounts of sulfur aerosols into the stratosphere, a layer of our atmosphere that starts at a height of nine miles up. Once there, the aerosols transform into droplets of reflective sulfuric acid. From geological records and from present-day observations (see, for example, Tambora’s 1815 outburst, or Pinatubo’s 1991 furious firework show), we know sulfur-rich eruptions can briefly chill the planet by a degree or more, and sometimes even rob the world of a summer or two. This, crudely speaking, is natural SRM.

All we have to do to postpone the apocalypse, then, is reproduce this artificially—and that’s where things get complicated. Machines capable of directly depositing a payload of sulfur gas into the stratosphere will need to be perfectly situated, operated and designed.

The quantity of aerosols they’ll deploy will be in the millions of tonnes. Unlike carbon dioxide emissions, which linger up there for decades or centuries at a time, sulfur sifts out from the atmosphere in just a handful of years. Our SRM machines, then, would have to operate perhaps perpetually, continually refueling the shield.

Crudely speaking, solar geoengineering would attempt to mimic the effects of a large volcanic eruption, such as the 1991 eruption at Mt. Pinatubo pictured above.
Photo: U.S. Geological Survey

With few localized field-testing experiments having ever taken place, machine designs that match these criteria are little more than concepts at present, ranging from buoyant inventions to ballistic projectiles.

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“I believe delivery of the sulfide gas into the stratosphere is envisaged either by artillery shell, high-altitude weather balloon or aircraft,” Ian Stimpson, a senior lecturer on geophysics at Keele University, told Earther.

The idea for artillery shells makes a prominent appearance in a 2009 study, which looks at a range of SRM methods, but it can be traced back to a 1992 US government-sanctioned publication on the subject. Using “naval rifles,” as the report calls them, we’d need to fire roughly 8,000 shells skyward each day to achieve sufficient coverage, which would cost as much as $30 billion per year–probably, far too expensive.

Artillery shells aren’t the only militaristic options, though. Missiles are investigated in a 2012 study, but even retrievable and reusable projectiles are estimated to cost tens of billions each year. The thought of using coilgun systems that fire relatively light, ferromagnetic slugs is also floated in the 2012 paper, but sulfur-adapted systems don’t yet exist. In both cases, the authors note the public would (quite reasonably) be wary of any major expansion in the production of war-like systems.

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Those high-altitude balloons Stimpson mentions are also explored in the 2012 study in various forms, including ones loaded with sulfur compounds. But you’d need tens of millions of them per year. Not only would this scheme be as pricey as the artillery shells, the plastic debris raining down on the planet would be decidedly unwelcome.

Why not squadrons of geoengineering aircraft? Easily one of the more popular SRM deployment ideas, aircraft also aren’t without problems. Gernot Wagner, the co-director of Harvard’s Solar Geoengineering Research Program, told Earther that most planes can’t fly into the stratosphere, and “those that can—like the civilian version of the U2 spy plane, for example—have no payload.”

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Still, Wagner thinks aircraft might be the best option we’ve got, suggesting that “a little bit of mix and match” of pre-existing designs is all that’s required to create a plane optimized for SRM.

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“The most prominent delivery mechanism would be newly designed, newly built airplanes,” ones “with a fairly large body to deliver a few tonnes of [sulfur] material at a time” into the stratosphere, Wagner said.

With that in mind, Stimpson opined that adapted KC-135s, mid-air refueling tankers belonging to the US military, may work. Nicknamed Stratotankers, they can, as the moniker suggests, already reach the stratosphere, so is a little tweaking all that’s needed to turn them into our SRM machines?

“Yes, I think it is certainly possible in the not too distant future,” a spokesperson for the UK Civil Aviation Authority told Earther. “Don’t forget though that a new aircraft type will have to go through a rigorous certification process that will add to the overall timeline.”

A modified version of the KC-135R Stratotanker, pictured above (left) refueling a fighter jet (right), might be used to deposit sulfate aerosols in the stratosphere for geoengineering.
Photo: Tech. Sgt. Angelique Perez (U.S. Air Force)

“Will there be pilots involved? If you ask me, probably not,” Wagner noted, suggesting that an uncrewed vehicle—a network of drones, essentially—is perfectly feasible. Either way, Wagner estimates the scheme would cost “single digit billions of dollars” which, for the significant effect it’ll have on the planet, is relatively cheap.

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The Stratospheric Particle Injection for Climate Engineering (SPICE), a 2010-2015 collaboration between the Universities of Bristol, Cambridge, Oxford and Edinburgh, considered some more exotic SRM machines. Its leading suggestion was a high-pressure, flexible pipe, tethered to a massive helium or hydrogen-filled balloon on one end and a pump on the other, reaching up 25 kilometers (15.5 miles) into the stratosphere. Some variants of this idea suggest using a pump running up through a tall, stationary tower, but SPICE opted instead for a balloon tethered to a ship, which allowed the scheme to be mobile.

The concept was due to be tested on a very local scale: a kilometer (0.62-mile)-wide pipe, that would have injected harmless water into the atmosphere. Sadly, due to various conflicts of interest and governance troubles, the test was cancelled in 2012. At this stage, it’s unknown what materials will be needed to construct a larger-scale device that could withstand the meteorological extremes up through the lower atmosphere, and it’s not clear precisely how many balloons around the world would be required to deliver enough sulfur.

Matthew Watson, SPICE’s former principal investigator and an expert in natural hazards at Bristol, told Earther that it’s not all about the design. “Control is a key question” too, he said. “I don’t think anyone knows how it would work in practice.”

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Indeed, it’s easy to see that those who control the machines control the fate of the planet’s climate, which invokes some unsettling thought experiments. What if one or several countries wanted to go rogue and act unilaterally? What if the developing world is left behind?

Sketchy governance isn’t the only danger here. Yes, a sulfur shield will very likely ensure that global warming would slow down or stop if it’s sustained. Studies disagree, however, on what other unintended effects SRM may engender, and dramatic changes to precipitation patterns remain particularly enigmatic and potentially devastating.

There’s also the risk of the governing body intentionally pulling the plug, or a natural disaster or even a terrorist attack taking the machines down, bringing about something known as a “termination shock,” where the Earth could in theory warm rapidly and suddenly. A paper out earlier this year suggested this could lead to unprecedented ecosystem upheaval, but another recent study—one that advocates for a more gradual approach—points toward ways in which we can avoid such a shock.

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Wagner said that if he was given the choice to deploy SRM machines tomorrow or never use them, he’d opt for the latter. They could have a productive role to play in climate policy, but there’s so much we simply don’t know about these machines yet.

“Make no mistake, deployment of SRM will be unambiguous proof of our miserable failure as a species to act as responsible planetary stewards,” he said. It would, however, be “a last resort to reduce risks while we sort ourselves out,” stressing that it’s not, nor should it ever be, considered a replacement for carbon-cutting mitigation.

Whatever you think of these machines, it’s clear that the technological, environmental and societal hurdles we have to overcome to deploy them pale in significance compared with a single question: What scares us more? Fleets of sky-coating, sulfur-emitting drones, or runaway climate change?

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Robin George Andrews is a volcanologist turned science writer with a penchant for extravagant tales, from stellar streams to climate change