Executive Summary

Solar radiation modification (SRM) is a set of technologies proposed to cool the planet alongside greenhouse gas reduction by reflecting a portion of incoming sunlight or by helping Earth release heat. This primer explains proposed SRM technologies, current perspectives, evidence from research, and key questions.

SRM is controversial. Since 2024, many U.S. states have considered legislation to prohibit or ban outdoor SRM research or deployment, and Tennessee and Florida have enacted deployment bans. Some legislation reflects public confusion, conflating SRM with weather modification and “chemtrails.”

Computer modeling studies suggest SRM could cool Earth but would not restore preindustrial climate conditions and might introduce new risks, like shifting rainfall patterns and continued reliance on SRM.

Small-scale outdoor experiments and private-sector activities have caused public alarm. Community opposition has stopped many scientific experiments. Scientists and the public are critical of private companies deploying small-scale SRM without oversight.
 

Arguments for outdoor SRM research claim:

• SRM could quickly lower temperatures while we work to reduce emissions and could lower temperatures more quickly than cutting emissions alone.
• SRM is relatively inexpensive compared to the massive cost of unchecked climate change.
   

Arguments against SRM research include:

• Risks and unknowns: SRM could disrupt weather, nature, and the air we breathe in uneven and potentially harmful ways.
• Incomplete fix: SRM will not stop or reverse ocean acidification, sea-level rise, or polar ice loss.
• Termination shock: Suddenly stopping SRM while greenhouse gas emissions remain high could trigger rapid extreme global warming.
• Governance and geopolitical dangers: Since SRM can be deployed locally but would have global effects, a single country or company deploying it alone could cause global conflict.
• Less motivation to cut emissions: People relying on SRM as a quick fix may focus less on the long-term solution (reducing pollution).
• Fairness and trust issues: Decisions about SRM will affect the globe but are likely to be made by wealthy countries and companies, raising questions about who should make decisions about SRM and whether the public should trust the process.
• Other risks and unknowns: SRM might reduce energy generated by solar panels, interfere with airplanes and satellites, create more pollution, and cause health risks.

Many experts agree that rules and management systems (governance) must be in place before outdoor research and deployment happens.
 

Priorities include:

• Clear legal definitions of SRM research and use (deployment);
• Openness around SRM funding, risks, and monitoring;
• Inclusion of Indigenous leaders and developing nations (Global South) in decisions; and
• International cooperation to establish rules, monitoring, and accountability.
   

Questions for lawmakers:

• Should outdoor SRM research proceed and, if so, under what safeguards?
• Who should control outdoor SRM research and use at local, national, and international levels?
• If something goes wrong and outdoor SRM research or use causes harm, who should be responsible for damage compensation?
• How can we share trusted research and fight misleading information about SRM?
• How can we ensure that SRM will not lead people to forget about the need to reduce greenhouse gas emissions?