Solar Geoengineering
The methods, potential impacts, and organizations involved in solar geoengineering
If someone has a heart attack, they may have to undergo open heart surgery, which involves breaking ribs and a long recovery period. However, in certain situations, the surgery has to be done in order to save the person’s life. Similarly, climate change may cause global emergencies, such as widespread crop failure, extreme heatwaves, or freshwater access disruptions, that require emergency break solutions. These solutions include immediately halting deforestation, methane emissions from fossil fuels, black carbon from cookstoves, and biomass burning. Solar geoengineering is another potential emergency break solution.
What is solar geoengineering?
While carbon geoengineering, which I described in my previous post, removes CO2 from the atmosphere, solar geoengineering, or solar radiation management (SRM), reflects sunlight back into space to decrease the amount of solar radiation in the atmosphere. While carbon geoengineering addresses the root problem of too much CO2 in the atmosphere, solar geoengineering temporarily reduces temperature caused by the increased concentration of CO2. It is estimated that using SRM to block 1-2% of solar radiation would offset the warming effects from the increased CO2. Unlike carbon geoengineering, solar geoengineering can take effect within months to a few years and is at least hundreds of times cheaper than carbon geoengineering.
Types of solar geoengineering
There are six main solar geoengineering technologies, four of which are detailed in this graphic.
1. Stratospheric Aerosol Injection (SAI)
When a large volcano erupts, it releases an ash cloud, and the sulphur dioxide in the cloud combines with water to form sulfuric acid aerosols, which reflect sunlight and therefore decrease temperature. For example, after the Pinatubo volcano in the Philippines erupted in 1991, injecting sulfur dioxide into the stratosphere, the global temperature decreased by an average of 0.5C over the next 18 months. SAI mimics the effects of volcanic eruptions by injecting sulfate aerosols into the stratosphere to scatter a fraction of sunlight back into space.
Sulfur dioxide is very effective at cooling the planet compared to the heat from CO2 – 1 gram of SO2 can offset the heat of 1 ton of CO2 per year.
SAI could be deployed using planes or high-altitude balloons. Its deployment has to be studied carefully to limit risks, which include negative impacts on precipitation, acid rain, storms, the ozone layer, high-altitude tropospheric clouds, and temperature. For example, releasing aerosols in only one hemisphere could disrupt tropical precipitation. Also, due to political issues or attacks, SAI is vulnerable to termination shock, where the release of aerosols is suddenly stopped. This could lead global temperatures to quickly rise and pose catastrophic risk to wildlife.
SAI is the most promising form of SRM because its impact would be more uniform than local initiatives, and the technology needed to deploy SAI is not far out.
2. Marine cloud brightening
Marine cloud brightening involves ships spraying salt water into the clouds. The salt particles would act as cloud condensation nuclei, allowing water vapor to condense around them and form cloud droplets. The process would brighten and enlarge clouds, which would then reflect more sunlight. Marine cloud brightening can be deployed in specific areas, including over coral reefs, ice cover, and hurricane areas. Unlike SAI, which would have a small even effect over the planet, marine cloud brightening would have a large impact on local areas and lead to regional variation.
3. Cirrus cloud thinning
Cirrus clouds are thin and wispy clouds made of ice crystals that are found at high altitudes. Though they reflect sunlight, they absorb long-wave radiation, and their net effect results in greater warming than the CO2 released by humans. Cirrus cloud thinning involves injecting aerosol particles into cirrus clouds with drones to dissipate them. However, the process could produce the opposite effect or impact other environmental systems.
4. Space sunshades
Space sunshades involve sending mirrors into orbit to reflect sunlight from Earth. The size and number of mirrors would have to be adjusted to offset the changing CO2 levels in Earth’s atmosphere. Deploying space sunshades requires huge technological innovation and therefore extremely high costs compared to other SRM methods but would result in the least environmental impact.
5. Surface Albedo Modification (SAM)
Albedo is the fraction of light that a surface reflects. SAM involves increasing the albedo of surfaces, including through making roofs brighter by painting them white, making plants waxier by introducing relevant genes, and covering deserts with reflective materials. Although not as effective as other SRM methods, SAM can be effective in reducing extreme temperatures and heat waves, and SAM does not pose termination shock or cross-boundary conflict risks.
6. Ocean mirror
The ocean mirror method involves ships churning microbubbles on the ocean surface. The seafoam can be ten times more reflective than seawater. However, the microbubbles could reduce the sunlight below the ocean’s surface and harm ecosystems, including marine plants. Also, creating and maintaining the microbubbles would require large amounts of energy.
Key Impacts
The impacts of SRM include governance risks, climate and weather risks, moral hazard, and indirect impacts.
Governance Risks
For some SRM methods, deployment would impact several or all countries, so it would require a governance system. As mentioned above, termination shock could be catastrophic, so mechanisms to prevent one are essential. Also, a governance system would require buy-in from developing countries, who would likely be the most impacted. To protect them from the risks of SRM, developing countries could be given access to parametric insurance on key risk metrics.
Although a centralized world government to regulate SRM would be the ideal governance system, it is unlikely to emerge. In the absence of one, a country could deploy SRM technology without agreement from other countries or consideration for broader impacts, which would lead to geopolitical tensions.
International bodies like the UN or the Paris Agreement could develop the frameworks for responsible SRM research and deployment.
Climate and Weather Risks
SRM could restore rainfall patterns to pre-industrial levels, but it could also weaken summer monsoons, cause droughts, and change storm frequency and intensity. For the SRM methods that involve injecting particles into the atmosphere, the falling particles could have negative effects since the particles would stay in the atmosphere for only one to three years.
Moral hazard
An argument against SRM is the moral hazard effect, where companies and governments would stop or slow down their decarbonization efforts due to SRM-induced temperature decreases.
However, SRM investment could signal the gravity of the climate crisis and accelerate decarbonization efforts. Also, the moral hazard argument is weak because governments and companies are already failing to take sufficient action on decarbonization. Complacency concerns would be more valid if the world without SRM were pushing emissions down at the required scale, which we are not. Furthermore, global temperatures will continue to rise for 100 years after peak emissions, and we have not reached peak emissions yet.
More importantly, instead of as a substitute for decarbonization, SRM should be viewed as a potential supplement.
Indirect Impacts
Because SRM does not remove CO2 from the atmosphere, it would not reduce ocean acidification, but it could reduce sea surface temperature, lower the risk of coral bleaching, sea ice loss, and glacier melting.
SRM could also indirectly lower atmospheric CO2 through its effects on permafrost, energy demand, and carbon cycles.
Permafrost is frozen soil or underwater sediment, and it contains large amounts of carbon. When it thaws, CO2 and CH4 are released. By reducing global temperatures, SRM could slow or prevent permafrost thaw.
Lower global temperatures caused by SRM could reduce demand for cooling and the emissions associated with it. They could also stabilize carbon cycle feedback in natural carbon sinks, such as forests, oceans, and soils.
Why SRM may be necessary?
We face a risk-risk situation. SRM is risky but so is climate change without SRM, so the question is which one is less risky?
In the early days of the COVID-19 pandemic, people in countries without reported cases largely carried on with their daily routines. It was only once it reached people’s communities that they paused to acknowledge the threat and look for solutions. Similarly to COVID-19, climate change could cause a scenario that forces everyone to act immediately, in which case SRM could be a tool. Climate change has already affected many communities around the world, but many do not pay much attention to it because its effects are not strong enough in their lives, and although people may think decarbonization is important, willpower is hard.
Also, new technologies have always posed risks. When humans started building agricultural communities, millions could die from a single drought, while before, since hunter-gatherer communities were more fragmented, no single natural disruption could impact as many humans.
Another example is that cities used to be filled with carts pulled by horses, which meant streets were covered in horse manure and urine that created sanitation issues. When cars were introduced, the sanitation issues were solved, but car-related deaths doubled the number of deaths in cities.
In both situations, humans learned to innovate and implement measures to make crops more resilient and cars and roads safer. We can leverage the advanced modeling and research capabilities today to understand the risks and benefits of SRM and to deploy it only if necessary and with a focus on limiting these risks.
Key Organizations
Research on SRM is being carried out by several types of organizations, including startups, research institutions, and governments.
Startups
Stardust Initiative is an Israeli-US startup inventing a new particle for SAI with the goal of derisking the technology instead of deployment.
Make Sunsets is a US startup currently selling cooling credits.
One of the most important aspects of SRM is that the R&D is open and transparent. Because startups can withhold research findings and have profit incentives, they are not the best candidates for SRM research.
Research Institutions
There have been various SRM research programs at universities and other institutions.
Harvard University launched the multidisciplinary Solar Geoengineering Research Program in 2017 and raised around $7.5 million from Bill Gates, the Hewlett Foundation, and other private sources. Many SRM research programs rely on private funding, making it especially important to share findings with the public, welcome input, and establish oversight.
Most studies have been based on computer simulations, which leave high uncertainty around the risks and benefits of SRM. The Stratospheric Controlled Perturbation Experiment (SCoPEx) was led by Harvard Professors David Keith and Frank Keutsch and aimed to advance understanding of stratospheric aerosols in order to enable the necessary impact estimates. To test types of aerosols that could reduce or eliminate ozone loss and stratospheric heating for example, the researchers planned to propel high-altitude balloons to inject aerosols above Tucson, Arizona and the Esrange Space Center in Sweden. However, SCoPEx faced opposition from environmental and indigenous Saami leaders in Sweden in 2024, which forced the cancellation of the experiment. Future SRM projects will have to prioritize dialogue across researchers and local communities.
The Alliance for Just Deliberation on Solar Geoengineering launched in 2023 and aims to engage policymakers in SRM research and governance.
The Carnegie Climate Governance Initiative studied technologies that alter climate, including carbon and solar geoengineering, and their governance approaches. It closed in 2023.
The Solar Radiation Management Governance Initiative, a partnership between the Royal Society, the Academy of Sciences for the Developing World, and Environmental Defense Fund, has evolved into the Degrees Initiative and aims to advance SRM outreach, research, and governance. They launched the Socio-Political Fund and the Degrees Modeling Fund, the first SRM modeling funds in the Global South.
The Marine Cloud Brightening Project, led by researchers at the University of Washington and Silicon Valley, aims to study the interaction between aerosols and clouds to better understand the feasibility and impacts of increasing marine cloud reflectivity. They are developing a nozzle that sprays salt particles into clouds, and they plan to conduct trials on the US Pacific coast. They release open datasets and peer-reviewed publications and collaborate with US federal agencies.
With the bleaching of the Great Barrier Reef, the Sydney Institute of Marine Science, University of Sydney School of Geosciences, and Southern Cross University are exploring seeding and brightening low-altitude clouds on the northeast coast of Australia.
Governments
Biden signed a 2022 federal appropriations act directing his Office of Science and Technology Policy to develop a group, including NASA, NOAA, and the Department of Energy, to prepare an SRM research and governance blueprint.
China’s Ministry of Science and Technology raised $3 million to study SRM impacts on China through governance and computer modeling. For example, they studied how injecting aerosols could slow the melting of Himalayan glaciers, which China relies on for water.
The UK launched the Stratospheric Particle Injection for Climate Engineering (SPICE) experiment to study the impact of spraying particles into the stratosphere, but it was cancelled due to patent issues.
Lately, there has been a push for the EU to impose a moratorium on SRM.
What’s next?
Paul Crutzen, who won the Nobel Prize in Chemistry in 1995 for his research on the ozone hole, argued in 2006 for more serious research on solar geoengineering because reducing GHG emissions at the necessary scale was unlikely. Given the severe threats of climate change, SRM may serve as a supplement to decarbonization and mitigate the most extreme effects of global warming. SRM should therefore receive the required attention, funding, and global collaboration to research potential impacts and develop potential deployment strategies.
Hooked me with the analogy to the human patient. Clever to give urgency to our climate emergency. And love the solar shades in space. Would that cause dusk to come earlier and what consequences could that have, including to our animal earth co-inhabitants, including birds.