Climate change is an increasingly dangerous antagonist, and we\u2019re 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\u2019s now\u00a0a growing possibility\u00a0we\u2019ll build machines that will, in a manner of speaking, darken the Sun.\r\n\u00a0\r\nThere are several variations on the so-called\u00a0solar geoengineering\u00a0theme, 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\u2019s 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?\r\n\r\nTo 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\u00a0diamond 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.\r\n\u00a0\r\n\u00a0\r\nVolcanic 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\u2019s\u00a01815 outburst, or Pinatubo\u2019s\u00a01991 furious firework show), we know sulfur-rich eruptions can briefly chill the planet by a degree or more, and sometimes even\u00a0rob the world of a summer or two. This, crudely speaking, is natural SRM.\r\nAll we have to do to postpone the apocalypse, then, is reproduce this artificially\u2014and that\u2019s 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.\r\nThe quantity of aerosols they\u2019ll deploy will be in the millions of tonnes. Unlike carbon dioxide emissions, which\u00a0linger\u00a0up there for decades or centuries at a time, sulfur sifts out from the atmosphere in\u00a0just a handful\u00a0of years. Our SRM machines, then, would have to operate perhaps perpetually, continually refueling the shield.\r\n\r\nWith few localized\u00a0field-testing experiments\u00a0having ever taken place, machine designs that match these criteria are little more than\u00a0concepts\u00a0at present, ranging from buoyant inventions to ballistic projectiles.\r\n\u00a0\r\n\u00a0\r\n\u201cI believe delivery of the sulfide gas into the stratosphere is envisaged either by artillery shell, high-altitude weather balloon or aircraft,\u201d\u00a0Ian Stimpson, a senior lecturer on geophysics at Keele University, told Earther.\r\nThe idea for artillery shells makes a prominent appearance in a\u00a02009 study, which looks at a range of SRM methods, but it can be traced back to a\u00a01992 US government-sanctioned publication\u00a0on the subject. Using \u201cnaval rifles,\u201d as the report calls them, we\u2019d need to fire roughly 8,000 shells skyward each day to achieve sufficient coverage, which would cost as much as $30 billion per year\u2013probably, far too expensive.\r\nArtillery shells aren\u2019t the only militaristic options, though. Missiles are investigated in a\u00a02012 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\u2019t 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.\r\nThose high-altitude balloons Stimpson mentions are also explored in the 2012 study in various forms, including ones loaded with sulfur compounds. But you\u2019d 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.\r\nWhy not squadrons of geoengineering aircraft? Easily one of the more popular SRM deployment ideas, aircraft also aren\u2019t without problems.\u00a0Gernot Wagner, the co-director of Harvard\u2019s Solar Geoengineering Research Program, told Earther that most planes can\u2019t fly into the stratosphere, and \u201cthose that can\u2014like the\u00a0civilian version\u00a0of the U2 spy plane, for example\u2014have no payload.\u201d\r\n\r\nStill, Wagner thinks aircraft might be the best option we\u2019ve got, suggesting that \u201ca little bit of mix and match\u201d of pre-existing designs is all that\u2019s required to create a plane optimized for SRM.\r\n\u201cThe most prominent delivery mechanism would be newly designed, newly built airplanes,\u201d ones \u201cwith a fairly large body to deliver a few tonnes of [sulfur] material at a time\u201d into the stratosphere, Wagner said.\r\nWith that in mind, Stimpson opined that adapted KC-135s, mid-air refueling tankers belonging to the US military, may work. Nicknamed\u00a0Stratotankers, they can, as the moniker suggests, already reach the stratosphere, so is a little tweaking all that\u2019s needed to turn them into our SRM machines?\r\n\u201cYes, I think it is certainly possible in the not too distant future,\u201d a spokesperson for the UK Civil Aviation Authority told Earther. \u201cDon\u2019t forget though that a new aircraft type will have to go through a rigorous certification process that will add to the overall timeline.\u201d\r\n\r\n\u201cWill there be pilots involved? If you ask me, probably not,\u201d Wagner noted, suggesting that an uncrewed vehicle\u2014a network of drones, essentially\u2014is perfectly feasible. Either way, Wagner estimates the scheme would cost \u201csingle digit billions of dollars\u201d which, for the significant effect it\u2019ll have on the planet, is relatively cheap.\r\n\u00a0\r\nThe 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\u00a0leading suggestion\u00a0was 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.\r\nThe concept was\u00a0due to be tested\u00a0on 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\u00a0cancelled\u00a0in 2012. At this stage, it\u2019s 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\u2019s not clear precisely how many balloons around the world would be required to deliver enough sulfur.\r\nMatthew Watson, SPICE\u2019s former principal investigator and an expert in natural hazards at Bristol, told Earther that it\u2019s not all about the design. \u201cControl is a key question\u201d too, he said. \u201cI don\u2019t think anyone knows how it would work in practice.\u201d\r\nIndeed, it\u2019s easy to see that those who\u00a0control the machines\u00a0control the fate of the planet\u2019s climate, which invokes some unsettling thought experiments. What if one or several countries wanted to\u00a0go rogue\u00a0and act unilaterally? What if the\u00a0developing world\u00a0is left behind?\r\nSketchy governance isn\u2019t the only danger here. Yes, a sulfur shield will very likely ensure that global warming would slow down or stop if it\u2019s sustained. Studies disagree, however, on what other unintended effects SRM may engender, and dramatic changes to precipitation patterns remain particularly enigmatic and\u00a0potentially devastating.\r\n\u201cMake no mistake, deployment of SRM will be unambiguous proof of our miserable failure as a species to act as responsible planetary stewards,\u201d\r\nThere\u2019s also the risk of the governing body intentionally pulling the plug, or a natural disaster or even a\u00a0terrorist attack\u00a0taking the machines down, bringing about something known as a \u201ctermination shock,\u201d where the Earth could in theory warm rapidly and suddenly. A paper out earlier this year suggested this could lead to unprecedented ecosystem\u00a0upheaval, but another recent study\u2014one that\u00a0advocates\u00a0for a more gradual approach\u2014points toward ways in which we can avoid such a shock.\r\n\u00a0\r\nWagner said that if he was given the choice to deploy SRM machines tomorrow or never use them, he\u2019d opt for the latter. They could have a productive role to play in climate policy, but there\u2019s so much we simply don\u2019t know about these machines yet.\r\n\u201cMake no mistake, deployment of SRM will be unambiguous proof of our miserable failure as a species to act as responsible planetary stewards,\u201d he said. It would, however, be \u201ca last resort to reduce risks while we sort ourselves out,\u201d stressing that it\u2019s not, nor should it ever be, considered a replacement for carbon-cutting mitigation.\r\nWhatever you think of these machines, it\u2019s 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?