Developers of solar geoengineering, also known as solar radiation management, propose to combat warming by limiting the amount of sunlight that reaches the Earth and is converted to heat through the greenhouse gas effect.
Many approaches to solar geoengineering have been proposed, ranging from small-scale reflective ‘cool roofs’ to space-based sun shields. But two are garnering serious investigation by researchers, according to a report released earlier this year by the National Academies of Sciences, Engineering, and Medicine. One, ‘stratospheric aerosol injection’, would release small reflective particles, such as calcium carbonate or sulfate, into the stratosphere to scatter the sun’s rays. The second, ‘marine cloud brightening’, would add aerosols—most often proposed are sea salts—to low-level clouds over ocean waters to increase their reflectivity.
The report also said that neither of these techniques is without risks. Large uncertainties loom in how solar geoengineering interacts with atmospheric chemistry, regional and local climate or the potential for ‘termination shock’ if interventions are suddenly stopped. Barely studied are solar geoengineering’s likely domino effects on critical ecosystem functions, such as photosynthesis, said ecologists in a 2021 report in the Proceedings of the National Academies of Science.
Despite the potential downsides to solar geoengineering, both reports acknowledge the increasing urgency to conduct research now, before any measures are hastily put into effect out of climate-driven desperation in the future. Still, scientists stress that solar geoengineering is only being considered as a last resort to avoid crossing disastrous tipping points and prevent irreparable damage to Earth’s inhabitants.
Climate justice researchers and advocates, meanwhile, argue against investing in solar geoengineering research because these strategies neither address the root cause of climate change—greenhouse gas emissions, primarily from fossil fuels—nor stop related changes such as ocean acidification. They also worry that adopting these technologies might exacerbate socioeconomic inequities and weaken commitments by business and governments to cut emissions.
Solar geoengineering isn’t only a scientific pursuit, but also a political and social decision, said Sikina Jinnah, an associate professor of Environmental Studies at the University of California, Santa Cruz. The current governance structures for regulating this technology, she said, are “sorely inadequate”.
“The social and justice issues, in my view, are just as if not more important than some of the technical questions,” she said. “How do you make decisions about how solar geoengineering might be deployed? Who controls the thermostat?”