Climate scientists designed a road map to “dim the sun” from the stratosphere and potentially cut global warming uncertainty by two-thirds in just a decade for under $400 million, but the planet-altering plan comes with a built-in kill switch

For 50 years, scientists have explored whether releasing reflective particles into the stratosphere could cool an overheating planet.

Hundreds of studies followed, yet fundamental unknowns remain gridlocked—how well it works, what unintended risks exist, and whether science could be ready before climate impacts force humanity’s hand. No coordinated plan has ever existed to resolve these questions.

Half a century of research, still no road map

Scientists have long known volcanic eruptions cool the planet. When Mount Pinatubo erupted in 1991, global temperatures dropped by roughly 1°F for about a year. That natural experiment suggested humans might replicate the effect deliberately by injecting reflective particles into the stratosphere. Yet despite decades of study, researchers never produced a systematic plan for resolving core uncertainties, such as particle behavior in the dry atmosphere or regional weather shifts.

Hundreds of academics signed an open letter calling for an international non-use agreement, arguing the technology can never be governed equitably.

Reflective, a San Francisco nonprofit incorporated in late 2023, stepped in to fill that gap. Raising over $20 million, it funded dozens of research groups and published its SAI Research Roadmap. “Our sense is the world may need to make very consequential decisions on timelines far shorter than our research system is prepared for,” says cofounder and CEO Dakota Gruener.

What the road map actually proposes

The plan unfolds across four phases:

• Phase 1: Foundational knowledge. Running two to three years at $30 million to $75 million, this phase relies on computer simulations and lab experiments evaluating regional impacts, ocean circulation, ice sheets, and crop yields.

• Phase 2: Outdoor testing. Aircraft would release 11 U.S. tons (10 metric tons) of sulfur dioxide over two seasons to measure cooling per ton. Lasting four to eight years at $70 million to $150 million, it cuts uncertainty by 25%.

• Phase 3: Scaling up. Dispersing 27,500 U.S. tons of SO₂ over a season could cut uncertainty by 66%, spanning four to eleven years at $270 million to $1.1 billion.

• Phase 4: Ongoing monitoring. Tracks long-term impacts if deployment proceeds.

Coordinated efficiently, total costs drop to $370 million over ten years—versus 20 years and $1.4 billion uncoordinated.

Built-in off-ramps: The ‘stage gates’

The roadmap is no one-way ramp toward deployment. Reflective built firm “stage gates” between later phases—explicit decision points where research stops if experiments show severe downsides or fail to deliver results. “Our road map has these gates precisely because there may be points where the answer is ‘You should stop,'” Gruener says.

Reflective frames the proposal as “Version 1″—designed to invite peer criticism rather than force conclusions. The nonprofit stresses it advocates only for generating the evidence required to decide responsibly.

The deep fault lines: Governance, equity, and termination shock

Not everyone believes research resolves the hardest issues. Hundreds of academics signed an open letter calling for an international non-use agreement, arguing the technology can never be governed equitably. “The core question is: Who would control a planet-altering technology like stratospheric aerosol injection?” asks Aarti Gupta, professor of global environmental governance at Wageningen University. “This planet-altering technology will have winners and losers.”

Legal scholar Wil Burns at American University warns of termination shock: if emissions continue while geoengineering masks warming, any sudden shutdown could trigger a severe temperature spike, hanging a “sword of Damocles” over future generations. Others counter that risks can be managed by phasing down injections gradually.

Why some scientists say experiments are unavoidable

Sebastian Eastham, associate professor in sustainable aviation at Imperial College London, acknowledges outdoor experiments won’t resolve every unknown—but argues they answer questions simulations cannot. Without field data, researchers risk “spinning their wheels.”

Ilan Gur, former CEO of the UK’s Advanced Research and Invention Agency, calls Reflective’s approach “100% the right one” to buy down uncertainty. For perspective, 11 U.S. tons of sulfur dioxide is less than 2% of what commercial aviation emits daily.

Here is the central hook and ultimate answer behind this 50-year impasse: the roadmap does not exist to fast-track geoengineering, but to prevent a disaster born of ignorance. As Gruener reveals, humanity’s choice isn’t between acting and doing nothing—it is between deciding on solid scientific proof or making a panicked, uninformed gamble in an emergency.

Kelly Lippke

Kelly is an experienced writer with 15 years of experience exploring the big stories that shape our world, from tech breakthroughs and space exploration to climate, energy, and the fascinating quirks of science. She has a talent for turning complex ideas into sharp, memorable insights that stay with readers long after they’ve finished reading.