Dimming the Sun: The Radical Science of Solar Geoengineering
Image credit: Selvan B on Unsplash
2024 marked the first year where global average temperatures were above the 1.5°C threshold outlined in the Paris Agreement. From here, we have entered into a crucial period where the world edges closer to the agreement being breached, increasing the risk of devastating and permanent consequences for our global climate.
Solar geoengineering mitigates the temperature increase by using multiple stratospheric aerosol injections (SAI) near the equator. They would be distributed around the globe in a matter of weeks by the rotation of the earth and atmospheric wind circulation. The particles injected would either directly reflect sunlight or form reflective droplets with water vapour in the atmosphere. Conceptually, this has proven to be effective by the 1991 Mt Pinatubo eruption that injected 20 million tons of sulfur dioxide into the stratosphere, uniformly lowering global temperatures by 0.5-1°C to pre-industrial levels for around 2 years.
The idea of solar engineering has been shrouded in taboo, with a large consensus that even research into the field is too risky. As of March 2025, over 500 scientists and scholars have signed an open letter to governments and the United Nations to sign an international non-use agreement prohibiting the funding, patenting, experimentation and deployment of Solar Radiation Modification (SRM) technology. The opposition movement towards solar geoengineering is responding to the transition of solar geoengineering from the fringe to the mainstream of potential climate mitigation strategies. The risks associated with solar geoengineering are broad, with three main concerns outlined by the international non-use agreement.
Firstly, the risks of solar geoengineering are not and cannot be fully known. The effects could vary significantly between different regions, these uncertainties could wreak havoc on weather patterns, wildlife and agriculture. This risk was proven after the Mt Pinatubo eruption, by it taking 25 years for the detrimental effects of less direct sunlight on agriculture to be documented. Regional variations could also lead to disparities between climates in different countries where some countries benefit while others suffer, potentially creating geopolitical conflicts.
The second risk is that the mere availability of solar geoengineering technologies will disenfranchise governments, individuals and businesses from working towards net zero. Carbon dioxide emissions have a direct correlation to economic activity, solar geoengineering technologies would be of great advantage to petrochemical states and businesses that are looking to delay fossil fuel cuts for as long as possible to prevent financial losses.
The third risk is that there will be no global governance system that will be able to uniformly agree on how to control solar geoengineering technology on a planetary scale. The expected low cost also means it is not just governments with the power, non-state billionaire actors and individual countries can decide to deploy solar geoengineering technologies once they become available.
A world with solar geoengineering has the potential to become reliant on it, alongside the societal and environmental issues outlined by the international non-use agreement, termination shock is also a significant concern. The cost of suddenly stopping SAI could result in the planet rapidly warming back up again, leaving the globe less time to adapt to climate change. Many factors could influence the sudden stopping of SAI including natural disasters, government interventions, economic collapse or terrorist attacks.
Gernot Wagner, the founding executive director of Harvard’s solar geoengineering research program predicts that solar geoengineering technology would most likely take the form of specialised aircraft that would cool the earth by 0.1°C per year over the span of 15 years. Wagner estimates a cost of $2-2.5 billion per year, for context the USA alone spent $239 billion investing in green energy in 2023.
The consensus among advocates is that solar geoengineering should not be the sole line of defence against global warming because it primarily works by masking the effects of carbon dioxide emissions. With the potential low cost and ability to work on a global scale, Wagner argues that it could be a powerful tool when used alongside emission cuts, adaptation and carbon removal. In particular, the cuts towards carbon dioxide emissions would not get rid of pre-existing carbon in our atmosphere that persists on the planet for thousands of years, contributing to global warming. Despite carbon dioxide removal geoengineering being regarded as safer, it is more expensive and does not currently have the same large-scale capabilities that solar geoengineering is anticipated to have.
Research efforts into solar geoengineering are growing, the 2020s saw huge financial investment from powerful individual philanthropists such as Bill Gates and businesses within the tech industry. In 2022, a startup called Make Sunsets conducted the first documented field tests of balloons filled with sulfate aerosols. However, the CEO admitted that the venture was to advocate for research into solar geoengineering and profit from being a business early on the scene. As of March 2025, the UK is giving $58 million from the Advanced Research and Invention Agency (aria) for solar geoengineering research. The research entails having small field trials; however, as the international non-use agreement states, the greatest risk could be developing a promising technology without the infrastructure in place first to prevent misuse – if this infrastructure even exists on a global scale.
As more people will experience the frontline effects of climate change, discussions surrounding solar engineering will only become louder. A 2019 study conducted on 723 climate police negotiators found that respondents who expected to have severe impacts of climate change in their home country were more likely to support solar geoengineering. This may create a global scenario where the world is forced to balance the risks of inaction against the uncertainties of intervention.
Words by Isabel Swan