Should we block some sunlight to cool the planet?

In this episode, Dakota Gruener of Reflective walks me through her organization’s new project, which maps the gaps in our scientific understanding of stratospheric aerosol injection — currently the leading candidate for directly cooling the planet. We get into what we don’t know (including a factor-of-two disagreement on basic aerosol physics), who’s already doing this without oversight, and the unsettling governance question of who controls the Earth’s thermostat once humanity has grabbed it. (PDF transcript) (Active transcript) David Roberts Hello everyone. Greetings and salutations. This is Volts for March 27, 2026: “Should we block some sunlight to cool the planet?” I’m your host, David Roberts. For decades, human beings have been running a gigantic, unintentional geoengineering experiment. The combustion of fossil fuels has pumped millions of tons of aerosols (mostly sulfur particles) into the lower atmosphere, a layer called the troposphere. The particles don’t stay up there long — they react with water and fall as acid rain within days to weeks — but their sheer quantity has served to block quite a bit of sunlight from the surface of the Earth. Though estimates vary widely, our best understanding is that human-generated aerosol particles have suppressed around 0.5°C of warming. In other words, without them, we might already be experiencing the dreaded two degrees of warming. As fossil fuels are replaced with clean energy in coming years, aerosol particles will quickly fall out of the troposphere and the warming they’ve been “hiding” could come back with a vengeance, portending what could be a rapid leap in global temperatures. It’s pretty grim. But what if we put those sulfur particles into the stratosphere, the layer of atmosphere above the troposphere? There they stay longer, typically 12 to 18 months, and reflect considerably more solar radiation. We could put that sheltering blanket back in place, on purpose. That idea — that huge amounts of short-term global warming and all its ill effects could be avoided with reasonably inexpensive technology — is what animates the field of solar radiation management (SRM). My guest today, Dakota Gruener, runs a non-profit research organization called Reflective that aims to accelerate research into solar radiation management and ensure that it is conducted responsibly and transparently. Her group has just released a new database revealing gaps and uncertainties in the research — today we will discuss that research, those gaps, whether all of this can be done safely, who will coordinate it, and much more. With no further ado, Dakota Gruener, welcome to Volts. Thank you so much for coming. Dakota Gruener Thanks for having me. David Roberts Dakota, let’s start. Give us a little background. What is your professional training and background, and why did you feel the need to start this organization? Dakota Gruener Yeah, I spent most of my career in global health and development doing vaccine policy with the Gates Foundation, then worked in the CEO’s office at Gavi, the Global Alliance for Vaccines and Immunizations, then switched gears and worked on digital privacy and digital rights. In that work, we were doing a lot of work with UNHCR, the UN’s refugee agency, and with the government of Bangladesh, and these two projects were happening simultaneously. The work with UNHCR was sobering because it brought me face to face with how many people have already been displaced and how overstretched the international system already is with that displacement. In the work with the government of Bangladesh, we were working with them on forms of digital credentials that would work across institutional and international borders. The Bangladeshis were, frankly, quite unusual amongst governments in saying that that was a critical requirement. Usually most governments were focused on “How do you prove who someone is within their own borders?” When we pushed them on why, what they would say behind closed doors was, “We expect potentially hundreds of millions of Bangladeshis displaced in the coming century because of warming.” David Roberts They’re thinking already about how to just track them? Dakota Gruener It was a bit of “How do we ensure that they have continued access to the services that they’re going to need?” It wasn’t even a sense of tracking them, but “We expect these people to be displaced. If you can’t prove who you are, you’re really at a loss to access necessary services.” I just couldn’t square those two conversations. We are already beyond what the limits of the international system could handle, and people are planning for significantly more displacement. I started waking up every day thinking, “If I’m not working on climate, what am I doing with my time?” When I shifted then into climate, what really struck me was timelines. Even if we do everything we can and we need to decarbonize with maximum effort, it’s not going to be fast enough to address this. Even if we scale CDR as fast as we possibly can, that’s also going to take a really long time. David Roberts That’s carbon dioxide removal. Dakota Gruener Yes. With each of those trajectories, the question became, what happens in the near term? Of course, there is important stuff around adaptation. But SAI kept coming up — that stratospheric aerosol injection, one of the most researched forms of sunlight reflection. What was striking there was people were discussing it as a potential tool to buy time, but we do not have the evidence to evaluate that and we are not on a trajectory to learn enough, fast enough. That, to me, felt like it was something worth pursuing. David Roberts You looked around. I’m sure you are aware there are all kinds of geoengineering schemes, throwing stuff in the ocean and crushing up rocks. We’ve covered a few of them on the pod. Is the reason you went for stratospheric SAI — shooting the particles up into the sky — that the one that struck you as most promising or just most near to hand, closest to being ready? Most promising. Is it still, do you think, the leader of the pack in terms of the geoengineering possibilities? Dakota Gruener Major scientific assessments describe three main atmospheric sunlight reflection approaches: stratospheric aerosol injection, marine cloud brightening (which goes by MCB), and cirrus cloud or mixed-phase cloud thinning (which goes by CCT or MCT), alongside more limited surface albedo modification methods and more speculative space-based concepts. Different sunlight reflection approaches fit different goals. Some of those interventions are best understood as targeted risk reduction tools. Surface albedo modification, for example, could be valuable for reducing urban heat. There is interesting work happening in Australia about marine cloud brightening to protect coral reefs. David Roberts Just so all the listeners are keeping up, albedo is the reflectiveness of the surface. The idea is if you replace black or dark surfaces with white or more reflective surfaces, you’ll bounce more radiation out into space. Dakota Gruener Exactly. David Roberts You can get some local, as you say, local urban cooling with this. Dakota Gruener Others are candidates, at least in principle, for meaningful cooling at the global scale. If your objective is global cooling, SAI — stratospheric aerosols — is the leading candidate. David Roberts Interesting. I want to get into the research and some of the details and what we do and don’t know. I was going to ask this question later, but I might as well ask it first since I threw this out onto social media, that I would be doing this. You will not be surprised to hear that one question, above all — the top question I get back, I’m sure you have heard it a million times and we should just discuss it right off the bat — which is the moral hazard question. This is what strikes everybody when they hear about this: we have a way to mask the effects of climate change temporarily. Immediat…