This road map could help us decide whether to deploy solar geoengineering

Solar geoengineering, also called stratospheric aerosol injection (SAI), is a proposed method to counteract climate change by releasing reflective particles into the stratosphere—the second layer of Earth’s atmosphere, above where weather occurs. These particles would mimic the cooling effect of volcanic eruptions by reflecting sunlight back into space, potentially lowering global temperatures. Scientists have studied SAI for half a century, but major uncertainties remain about its effectiveness and potential side effects. A San Francisco nonprofit called Reflective, founded in late 2023, aims to fill these knowledge gaps by funding research and publishing a detailed road map for experiments and studies. Reflective has raised over $20 million and provided $4 million to dozens of research groups. The organization does not advocate for deploying SAI but argues that more research is needed to make informed decisions about its use.

Reflective’s SAI Research Roadmap outlines the experiments, studies, and infrastructure required to reduce uncertainty about solar geoengineering. The road map estimates that a coordinated research effort would take about 10 years and cost around $370 million. Without coordination, the process could take up to 20 years and cost nearly $1.4 billion. The road map is designed to guide scientific efforts and encourage funding for high-priority work. It includes four phases: foundational knowledge, small-scale outdoor experiments, larger outdoor experiments, and ongoing monitoring. Each phase has specific goals, timelines, and costs. For example, the first phase would last 2 to 3 years and cost $30 million to $75 million, while the third phase could cost $270 million to $1.1 billion and run for 4 to 11 years. The road map is intended to be updated based on feedback from researchers and other observers.

A key part of Reflective’s road map involves outdoor experiments, where small amounts of sulfur dioxide (a gas that converts into reflective particles) would be released into the stratosphere to observe the effects. This is controversial because hundreds of academics have signed an open letter calling for a ban on such experiments, arguing that solar geoengineering could never be governed fairly or safely. Critics, like Aarti Gupta, a professor of global environmental governance, question who would control and deploy this planet-altering technology and for whose benefit. Opponents also warn that even small-scale experiments could have unintended consequences, such as damaging the ozone layer or altering rainfall patterns. Previous proposals, like Harvard’s SCoPEx and the UK’s SPICE project, were canceled due to opposition from environmentalists and policymakers.

Major scientific uncertainties surround solar geoengineering, including what particles to use, how they would spread in the stratosphere, and their long-term effects. For example, particles might clump together and fall out of the stratosphere too quickly, reducing their cooling effect. Other concerns include impacts on ocean currents, ice sheets, crop yields, and regional weather patterns. Some researchers, like Wil Burns, argue that outdoor experiments won’t resolve these uncertainties because the technology would need to be deployed at scale to reveal its true effects. Burns also warns of termination shock, a sudden warming surge if solar geoengineering is stopped abruptly, which could disproportionately harm future generations. Others, however, suggest that termination shock risks are overstated and that the technology could be phased out gradually.

Supporters of Reflective’s road map, like Ilan Gur, former CEO of the UK’s Advanced Research and Invention Agency (ARIA), argue that reducing uncertainty through research is essential to determine if solar geoengineering is viable. ARIA funded 21 geoengineering research projects in 2025. Sebastian Eastham, an associate professor at Imperial College London, agrees that outdoor experiments are necessary to answer critical questions, stating that simulations alone cannot provide all the answers. However, critics like Burns believe that research cannot address fundamental governance issues, such as who controls the technology and who bears the risks. Reflective acknowledges these concerns but argues that unchecked climate change also poses severe risks, particularly for developing regions, and that research is needed to make informed decisions.