A San Francisco-based nonprofit, Reflective, has unveiled a comprehensive roadmap designed to guide informed decisions regarding the deployment of solar geoengineering. This detailed plan outlines the necessary experiments, studies, and infrastructure required to understand the potential of stratospheric aerosol injection (SAI), a method that aims to counteract climate change by releasing reflective particles into the stratosphere, mimicking the cooling effects of volcanic eruptions. Scientists have explored this concept for half a century, yet significant gaps remain in understanding its efficacy and potential unintended consequences. Reflective’s SAI Research Roadmap aims to systematically address these uncertainties.

Dakota Gruener, co-founder and CEO of Reflective, stated, "Our mission is to equip the world with the data and tools required for informed decision-making about sunlight reflection fast enough to matter. Our sense is the world may need to make very consequential decisions on timelines far shorter than our research system is prepared for." The hope is that this roadmap will steer scientific efforts and encourage funding from philanthropies and government agencies to accelerate research responsibly.

The report estimates that a coordinated effort to complete all the proposed work could take approximately a decade and cost around $370 million. Conversely, a less organized approach could extend to 20 years and require nearly $1.4 billion. While Reflective does not advocate for the use of SAI, their roadmap makes a case for conducting outdoor experiments. These would involve gradually releasing increasing amounts of sulfur dioxide, or materials that convert to it, into the stratosphere to observe the effects.

This proposal is contentious. Since 2002, hundreds of academics have signed an open letter calling for a ban on outdoor experiments and an "international non-use agreement," citing concerns about the equitable governance of such a powerful technology. Critics argue that no amount of study can resolve fundamental questions about who would control and deploy SAI, as it would inevitably create winners and losers. Aarti Gupta, a professor of global environmental governance and co-initiator of the non-use initiative, emphasized, "The first-order questions, from my perspective, are not technical. The core question is: Who would control a planet-altering technology like stratospheric aerosol injection? Who would develop it, and who would deploy it, and to what end? To serve what purposes, and whose purposes? Those questions are very fundamental, because this planet-altering technology will have winners and losers."

Since its incorporation in late 2023, Reflective has become a significant player in solar geoengineering research, raising over $20 million from prominent charities and individuals. The organization has funded dozens of research groups and developed tools like an open-source solar geoengineering simulator and an online collaborative research hub. Earlier this year, Reflective released its SAI Uncertainties database, cataloging scientific unknowns and engineering challenges crucial for even small-scale SAI deployment. Key uncertainties include identifying the optimal gases or particles and understanding their atmospheric behavior, such as whether they would disperse effectively or clump and fall out of the stratosphere.

The roadmap builds upon this database, outlining a phased approach to address these uncertainties. The initial phase, "foundational knowledge," involves more computer simulations and lab experiments to assess potential impacts on various regions, ecosystems, and phenomena like ocean circulation, ice sheets, and crop yields. This phase also calls for developing observational tools to better understand baseline stratospheric conditions. This stage is projected to last two to three years with an estimated cost of $30 million to $75 million.

The subsequent phase proposes using modified aircraft to release 10 metric tons of sulfur dioxide into the stratosphere over two seasons, four times. This research stage could take four to eight years and cost $70 million to $150 million, potentially reducing uncertainty about SAI’s cooling efficacy by about 25%. A more significant step involves releasing 25,000 tons of sulfur dioxide over one season, potentially twice. This stage, including other research, would span four to 11 years, costing $270 million to $1.1 billion, and aims to decrease efficacy uncertainty by around 66%. The final phase would involve ongoing monitoring of full-scale SAI deployment, if it occurs, to gather real-world data and identify any unforeseen consequences.

Gruener emphasizes that the roadmap is a "Version 1" intended to provoke discussion and will be updated based on feedback. The plan includes "stage gates" between phases, ensuring that research progresses only if experiments show promising results, acceptable downsides, and sufficient resolution of key uncertainties. "Our roadmap has these gates precisely because there may be points where the answer is ‘You should stop,’" Gruener stated.

While many agree that the proposed studies could reduce uncertainty about SAI’s effectiveness and feasibility, the political and ethical hurdles to conducting outdoor experiments remain significant. Past proposals, like Harvard’s SCoPEx and the UK’s SPICE project, faced opposition and were halted. Wil Burns, a professor at American University and signatory to the International Non-Use Agreement, fears that the most critical downsides, such as impacts on the ozone layer and regional precipitation, will only become apparent during full-scale deployment, which he deems "never tenable." He also raises concerns about intergenerational equity, arguing that continued reliance on SAI to mask warming caused by ongoing greenhouse gas emissions would place a "sword of Damocles" over future generations, who would face severe consequences if the technology were suddenly stopped, leading to "termination shock." Some researchers, however, argue that the risks of termination shock are less likely and that SAI could be gradually phased out.

Ilan Gur, former CEO of the UK’s Advanced Research and Invention Agency (ARIA), praised Reflective’s roadmap, stating, "Whether you’re a scientist or a policymaker or just a concerned citizen, our goal should be as quickly and efficiently as possible to answer the biggest questions scientifically that would tell us [whether] this is an approach that might work or that would never work. We should all want to spend the effort and money to buy down that uncertainty, so my view is 100% the approach that Reflective is taking is the right one."

Sebastian Eastham, an associate professor at Imperial College London, agrees that outdoor experiments won’t resolve all unknowns but believes the roadmap initiates a vital conversation about making decisions in the face of escalating climate dangers. He argues that moving beyond computer simulations is essential, as well-designed experiments can provide invaluable insights, making it "almost irresponsible to say, ‘Well, there cannot be ever any experiment.’" He warns against the risk of "spinning our wheels running the same computational simulations over and over again," potentially hindering progress in understanding SAI’s effectiveness or dangers.

Gruener acknowledges the risks of SAI exacerbating inequality but points out that unchecked warming also disproportionately harms developing regions. She stresses that while outdoor experiments won’t eliminate all scientific unknowns, they can address many critical questions with minimal environmental risk, noting that the proposed releases of sulfur dioxide are a fraction of what the global aviation industry emits daily. "Some people will be uncomfortable with any discussion of any outdoor experiment, but if we want decisions made on good science… then these are questions that an experiment will be necessary to address," Gruener stated. She fears that escalating climate change will pressure nations to adopt SAI prematurely, without adequate research, leading to decisions made in panic or based on insufficient evidence. "We don’t think the alternative is decisions not happening at all. We think the alternative is decisions being made in a panic or on lack of evidence."