A New Roadmap Tries to Chart the Path to a Decision on Solar Geoengineering

Imagine a decision so consequential it could reshape rainfall patterns on multiple continents, yet so poorly understood that scientists still argue about the basics of how it would work. That is the situation the world faces with solar geoengineering, and a new roadmap released today wants to change it.

Reflective, a San Francisco nonprofit that funds solar geoengineering research, has published what it calls the SAI Research Roadmap. SAI stands for stratospheric aerosol injection, the idea of releasing reflective particles high in the atmosphere to cool the planet, essentially mimicking what a large volcanic eruption does naturally. Scientists have studied the concept for half a century. Even so, after hundreds of studies, huge gaps remain in our understanding of how well it would work and what side effects it might cause.

"Our mission is to equip the world with the data and tools required for informed decision-making about sunlight reflection fast enough to matter," says Dakota Gruener, Reflective's cofounder and chief executive. "Our sense is the world may need to make very consequential decisions on timelines far shorter than our research system is prepared for."

Think of the roadmap as a construction schedule for a bridge nobody has agreed to build yet. It lays out what needs to happen before anyone can make a responsible call on whether to use this technology at all. Done in a coordinated way, Reflective estimates the full research program would take about a decade and cost around $370 million. Left disorganized, it would stretch to roughly 20 years and nearly $1.4 billion.

That price tag buys a lot of uncertainty reduction, but not universal agreement that the work should happen. Since 2002, hundreds of academics have signed an open letter calling for a ban on outdoor experiments and an international non-use agreement, arguing that a technology this powerful could never be governed fairly across the globe.

"The first-order questions, from my perspective, are not technical," says Aarti Gupta, co-initiator of the non-use initiative and a professor of global environmental governance at Wageningen University in the Netherlands. "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."

What the Roadmap Actually Proposes

Reflective has moved fast since Gruener incorporated it in late 2023. The group has raised more than $20 million from prominent charities and individuals, distributed around $4 million to dozens of research teams, and built tools including an open-source solar geoengineering simulator and an online research hub. Earlier this year it released a database cataloging the scientific and engineering unknowns standing in the way of even a small-scale test. The new roadmap builds directly on that work.

The plan unfolds in stages. The first, called "foundational knowledge," relies on computer models and lab experiments to study potential effects on ocean circulation, ice sheets, and crop yields. It also calls for building better tools to observe baseline stratospheric conditions, since you cannot measure a change if you never established what normal looks like. This phase would run two to three years and cost $30 million to $75 million.

The next stage gets more hands-on. Modified aircraft would release 10 metric tons of sulfur dioxide into the stratosphere, four separate times across two seasons. Gruener notes that amount is less than 2% of what the global aviation industry already releases into the atmosphere each day, a comparison meant to put the scale in perspective. This phase could take four to eight years, cost $70 million to $150 million, and shave roughly 25% off the uncertainty surrounding "cooling efficacy," which is essentially the question of how much cooling you get per ton of material released.

After that comes a much bigger step: releasing 25,000 tons of sulfur dioxide over a single season, potentially twice. This phase would last four to 11 years, cost between $270 million and $1.1 billion, and could cut efficacy uncertainty by around two-thirds. The final phase envisioned in the roadmap is not really an experiment at all. It is ongoing monitoring of full-scale deployment, should the world ever choose to go there, gathering real-world data and watching for unexpected consequences.

Gruener describes this as a first draft, "concrete enough for people to argue with," and says Reflective will update it as feedback rolls in. She also points to built-in stopping points between the later stages. If experiments reveal the approach does not deliver the hoped-for cooling, or turns up worrisome side effects, the plan calls for pumping the brakes rather than pushing forward. "Our road map has these gates precisely because there may be points where the answer is 'You should stop,'" she says.

Getting from a roadmap to actual outdoor testing is its own challenge. Harvard's SCoPEx project and the UK's SPICE experiment both collapsed under pressure from environmental groups and policymakers before releasing any material. Wil Burns, a research professor and legal scholar at American University who signed the non-use agreement, doubts that even successful experiments at these scales would deliver answers that matter. He worries that the true risks, particularly to the ozone layer and regional rainfall, may only become visible once the technology is deployed at full scale, and by then it is too late to treat it as a controlled trial. He also raises the specter of "termination shock," the sudden burst of warming that could hit if a future generation, having no say in the original decision, decides to stop using the technology after the world has kept emitting greenhouse gases in the meantime.

"What that would do, in my mind, is put a sword of Damocles over future generations," Burns says. "So even if you could, quote-unquote, 'prove it works,' I don't think from an intergenerational perspective it would ever be tenable." Other researchers have pushed back on this framing, arguing termination shock is less likely than commonly assumed and that any deployment could be wound down gradually rather than halted abruptly.

Not everyone in the research community shares Burns's skepticism about experimentation itself. Ilan Gur, former CEO of the UK's Advanced Research and Invention Agency, which funded 21 geoengineering research projects last year, calls Reflective's approach the right one. "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," he says. Sebastian Eastham, an associate professor in sustainable aviation at Imperial College London who leads a separate ARIA-funded project on engineered cooling, agrees that outdoor tests cannot resolve every unknown, but argues that refusing to run them at all risks trapping researchers in an endless loop of computer simulations. "Every hard decision that has ever been taken has been in the context of unresolved uncertainty," he says. "That's just the nature of things."

Why It Matters

The debate over this roadmap is really a debate about how societies should handle irreversible risk under time pressure. Gruener worries that rising climate damage will push governments toward solar geoengineering out of desperation, whether or not the science is ready. "We don't think the alternative is decisions not happening at all," she says. "We think the alternative is decisions being made in a panic or on lack of evidence." Whether more experiments can ever settle questions about fairness, control, and who bears the risk is a separate matter entirely, and one no amount of funding can fully resolve. For communities already living with climate disruption, the stakes of getting this wrong, in either direction, could hardly be higher.