Some options involve nothing more complicated than painting roofs white. Others entail constructing fleets of high-altitude aircraft for spraying reflective aerosols into the stratosphere or even space-based mirrors. The optimal solution may be one that deploys modification technologies alongside other climate responses such as emissions reduction and CO₂ removal.² Overall, there is growing scientific consensus that this approach would work in a technical sense, with some limitations.^{3 4}

However, all SRM approaches raise challenging questions such as who should control the technologies, if and when they should be deployed and what should happen if a deployment goes wrong or fails entirely. Another concern is “moral hazard”: the possibility that investing in these SRM approaches would reduce the impetus to cut greenhouse gas emissions.

Given the increase in global mean temperatures, decisions about whether to research and deploy SRM are becoming increasingly urgent. However, there is a pressing need to develop international governance frameworks for deciding whether or not to conduct SRM field experiments and, if that decision is positive, how they should be conducted — and to prepare for making decisions whether or not to deploy at some point in the future.

Some governments and funding bodies are rising to the challenge, with the European Commission⁵ and White House Office of Science and Technology Policy⁶ both promoting further investigation and the UK starting a £10.5 million research programme.⁷ However, in March 2024, the field suffered a setback when the UN Environment Assembly failed to agree on the establishment of a scientific expert group to advise on SRM. The failure reflected concern in developing countries about the nature of the research and how the results would be used.⁸

KEY TAKEAWAYS

Reducing the amount of solar radiation incident upon Earth’s surface could help mitigate global warming, and strategies for reducing this radiation flux are under investigation. Stratospheric aerosol injection, which involves injecting chemicals into the lower stratosphere to reflect back some incoming sunlight, is promising, but raises a number of concerns. Cloud engineering schemes also show some promise but could lead to localised cooling and regional changes in precipitation pattern. Terrestrial solar radiation modification schemes to increase reflection of solar radiation back into space, such as painting more of Earth’s surface white, or artificially regrowing Arctic ice, are also under consideration. Further from possible implementation, but perhaps more impactful in the long term, are the Space-based solar radiation modification strategies such as radiation-reflecting satellites that carry panels to reflect solar radiation or shade portions of the Earth from the sun’s rays. All of these possible pathways require intense scrutiny and international cooperation if unintended adverse consequences are to be avoided.