 We've arrived at a frightening point in the debate over climate change. Research shows that even sharp cuts in fossil fuel use will no longer be enough to avoid environmental catastrophes, likely including widespread famine, mass extinction, and hundreds of millions of climate refugees. But despite these growing risks, world leaders and politicians continue to resist hard commitments to change, or deny the problem even exists. It'll get cooler, it'll get warmer, it's called weather. Enter the Climate Hackers, a growing number of scientists and researchers who are taking matters into their own hands. They're exploring ways to seize control of nature itself to reverse climate change by altering clouds, changing ocean chemistry, making the atmosphere more reflective, and other forms of what's often called geoengineering. It sounds zany to some and reckless to others. But with millions of lives and entire ecosystems in the balance, ignoring any opportunity to limit these mounting threats could prove to be the most reckless action of all. Doing this could cut the actual risk we care about of climate change, cut them say in half this century at a low cost, and that's a big deal. Harvard scientist David Keith has a cheap and simple plan to prevent the Earth from overheating as global warming takes off in the decades ahead. The most basic idea is by making the Earth a little bit more reflective, by reflecting away some sunlight, you can reduce some of the warming and other climate changes that come from the buildup of long-lived greenhouse gases like CO2. The way to do this that scientists understand best is to fly planes into the stratosphere, where they would spray particles such as sulfur dioxide. Over time, these particles would combine with oxygen and water in the atmosphere to create sulfuric acid, which traps water vapor that would otherwise evaporate. A small droplet of water vapor, like a cloud droplet, would reflect sunlight back to space and cool the Earth, just like a thin cloud does. Done on a large enough scale, it could offset much of the warming in store this century. And Keith estimates it could be done for a few billion dollars a year. That sounds like a big number, until you consider the trillions in estimated climate change damages annually. Keith didn't invent this concept, known as solar radiation management, but he's arguably done the most work to date figuring out how it can be done safely and effectively. Using sophisticated computer modeling, he's looked at various particles in various quantities to test the climate reaction. Far more research is needed before anyone actually deploys this kind of technology at full scale. But Keith argues it's time to move from lab research to limited trials in the real world. It might or might not make sense to actually do solar geoengineering. At this point, all we're talking about is research. But if it makes sense to do it, it might make sense to do it quite soon. While we're also cutting emissions, because it actually reduces the risk substantially. We do know the basic science is sound because nature has already done the fieldwork. Major volcanic eruptions in the past have markedly cooled worldwide temperatures by blowing tens of millions of tons of sulfur dioxide into the sky. The most often cited example is the massive 1991 eruption of Mount Panatubo, which eased global temperatures by a full degree Fahrenheit the following year. But there are known risks to mimicking this natural phenomenon. The downside of shooting sulfur dioxide into the sky is that it also eats away at the ozone layer. As it is, the world has been struggling for decades to patch a gaping hole in this protective layer, caused by earlier chemicals used in things like refrigeration and hairspray. Some research has also suggested solar radiation management would reduce rainfall in certain areas. And that could have disastrous effects on food production, according to Will Burns, co-director of the Washington Climate Geoengineering Consortium. I have some serious concerns about solar radiation management approaches, because whereas they could potentially substantially cool the planet, they also could have some extremely serious negative impacts that would make the cure worse than the disease. You could potentially alter precipitation patterns, which could shut down the monsoon in South Asia on a regular basis. It could create large droughts in sub-Saharan Africa. It could create diebacks in the tropical Amazon region. In addition, geoengineering at the scale of an entire planet presents thorny international political challenges. Another concern with solar geoengineering is whose hand is going to be on the thermostat. It might be that Russia or Canada might prefer to see a little more global warming than in places like in India or equatorial countries. That gives potential for international conflict, and in a nuclear age, international conflict can be very dangerous. Keith first came across early proposals for solar geoengineering as a doctoral student in physics at MIT in the late 1980s. He had grown frustrated with his field's obsession with abstract issues and became drawn to the dawning real-world problem of global warming. He's published a series of papers on the subject throughout a rapid rise in the academic world that landed him at Harvard in 2011, where he's now a professor of applied physics and public policy. Much of Keith's recent work has focused on limiting the negative effects of solar radiation management. Last year, for instance, he and colleagues found that swapping sulfur dioxide for other materials, such as aluminum or diamond dust, would significantly lessen the ozone impact. In the case of diamond dust, it would also significantly increase the cooling effect. Keith also says that research following the work burn sites has found that solar geoengineering could actually increase food production, in part because it reduces heat stress on crops. If you take any modern climate model and put in a moderate amount of solar geoengineering, not enough to compensate away all the effects of CO2, which would almost certainly be crazy, but maybe enough to cut that in half, so imagine a gradual ramp in the amount of solar geoengineering that cut the rate of warming in half. If you do that in a model, essentially every single part of the world is, as far as we can tell, better off. As the threat level climbs, other scientists and policy makers are also calling for more geoengineering research. Jane Long, a senior consulting scientist at the Environmental Defense Fund, notes that major climate projections show that global temperatures this century will rise well above the 2 degree Celsius threshold that researchers of long-worn we shouldn't cross. The only models that predict the global void this danger zone include some form of climate intervention, which could include the method Keith has in mind, or new ways of removing carbon dioxide from the oceans and atmosphere. I think that means we better know more about intervention. We need to be able to decide whether these things are effective, whether they're advisable, whether it's a good idea to do them, and whether they're actually doable. If there's ever a real climate crisis, the pressure on a politician to do something now that can act within a year can be really intense. And so it's important that we do the research now so that when politicians are faced with this question, they have good scientific information and are not just engaged in wishful thinking. Burns, for his part, remains skeptical that lab research and field trials will ever tell us enough to be able to safely roll out the technology at full scale. We still won't know what its real impacts are until we fully deploy. But if you fully deploy, of course, and you shut down the monsoon, then it's going to have dire implications. Keith is the first to say that solar geoengineering isn't a magic bullet. It can address all the impacts of climate change, and nothing about it means that we don't still have to transition to a clean energy system as quickly as possible. There's a line of argument that says we can never understand solar geoengineering perfectly, we can never predict exactly what will happen, so we can never do it. I think that's absurd. It really misunderstands the choice that humans face. Both uncertainties matter. So you can't just focus on the uncertainty about solar geoengineering because you've got the uncertainty about CO2. And that's the thing that we know is a big risk. But I believe it's my job as a scientist both to warn about the foolish choices the world might make if it sees this as a get out of jail free card, but it's also my job to say what the science seems to say. What the science seems to say is that moderate amounts of solar geo reduce climate change in almost every way that we can project.