In 1991, after lying dormant for 600 years, Mount Pinatubo in the Philippines suddenly woke up. A series of small eruptions over a two-month period culminated in a major one in mid-June, considered the second-largest volcanic eruption of the twentieth century. Some 200,000 people were forced to leave their homes and more than 700 died in the Philippine archipelago. The plume of smoke and volcanic ash from the eruption rose to heights of up to 40 kilometers (km), even reaching the Earth’s stratosphere, the second of the five atmospheric layers that surround the planet. A blanket of mostly micrometric suspended particles was created, disrupting air traffic, burning plants and crops, and causing other local damage.
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Despite causing major damage and loss of human life in the Philippines, the Pinatubo eruption is today remembered in scientific circles for having had a surprising consequence on the global climate: the Earth’s average temperature dropped by around 0.5 degrees Celsius (°C) in the two years following the volcanic event. The air circulation system in the stratosphere carried the enormous quantity of suspended particles (known as aerosols) released by the volcano around the planet, where they served as a kind of sunscreen. Some of the Sun’s rays that would normally reach the Earth’s surface were reflected when they hit the aerosol particles. The result was a temporary cooling of the planet.
Aerosols also cool the Earth when they are in the troposphere, the lowest layer of the atmosphere, but to a lesser extent than they do in the stratosphere. Over the past 20 years, the Pinatubo effect has inspired a controversial line of research marked by scientific uncertainties and environmental and geopolitical risks: solar geoengineering, or solar radiation modification (SRM). The field has slowly grown at universities in the USA and Europe as global warming has become more conspicuous. The central idea of the approach is to deliberately increase the Earth’s albedo, especially in the stratosphere, so that more radiation is reflected back into space, making the planet slightly cooler.
The albedo is the fraction of sunlight reflected by a body or surface in relation to the amount absorbed. The higher the albedo (like that of a light or white surface, for example), the less heat is absorbed. Injecting aerosols into the atmosphere is one way to try to increase Earth’s albedo. Some calculations suggest that a 1% to 2% reduction in the amount of solar radiation reaching the Earth would be enough to lower its average temperature by 1 °C.
The idea of reducing the amount of solar radiation on Earth was first considered in the 1960s, but it was always seen as a dangerous eccentricity—almost a pipe dream. Scientists began to take the topic more seriously after the Pinatubo eruption, and more recently with the emergence of the climate crisis, caused by a significant increase in global temperature due to the emission of greenhouse gases. Even so, real-world experiments—which would involve releasing kilograms of aerosols into the stratosphere to observe the effects on a local level (not globally, as occurred after the enormous eruption of the volcano in the Philippines)—have struggled to progress due to opposition from parts of the scientific community and environmental groups.
“So far, there have been very few climate modeling studies involving solar geoengineering techniques,” says Paulo Artaxo of the Physics Institute at the University of São Paulo (IF-USP), who specializes in the study of atmospheric aerosols. “No significant field experiments have been carried out.” Discussions on the topic are dominated by two main approaches to modifying solar radiation (see tables on pages 14 and 16). The first is stratospheric aerosol injection (SAI) at an altitude of 15 to 20 km, with the aim of artificially reproducing the natural effects of major volcanic eruptions.
The second, seen as having a more localized impact, is marine cloud brightening (MCB), which also involves releasing aerosols (in this case, sea salt particles). The particles act as cloud condensation nuclei, also known as cloud seeds. They are released at much lower altitudes, at a maximum of 2 km, which is within the troposphere. When there are more aerosols, cloud droplets become smaller, reflecting more solar radiation back into space and thus cooling the Earth’s surface. Other approaches are also being considered, such as increasing the albedo of the planet’s large white surfaces like the Arctic, but SAI and MCB dominate the debate.
Artaxo is working on computational modeling with a group from Harvard University in efforts to understand whether aerosols behave similarly in the stratosphere as they do in the troposphere. “We need more research on this topic before we even think about implementing any such intervention,” says the USP physicist, one of the coordinators of the FAPESP Research Program on Global Climate Change. “We cannot guarantee that injecting more aerosols will not, for example, reduce monsoon rains in Southeast Asia and put billions of people at risk. If that were to occur, who would decide whether the aerosol injection stops or continues? That type of decision cannot be left in the hands of a small group of countries or a billionaire who funds the experiment.”
There are also indications that extra aerosols in the stratosphere could damage the ozone layer, which protects life on Earth from ultraviolet solar radiation. Not to mention that these suspended particles are a form of air pollution. They naturally settle, descending from the stratosphere to the troposphere, where they can cause or exacerbate health issues, especially respiratory problems. For now, there are no satisfactory answers to these questions or many others.
The position of the USP physicist is shared by many of his colleagues. “Modifying solar radiation is a sensitive issue and the IPCC [UN Intergovernmental Panel on Climate Change] recognizes that much remains unknown about the potential effects,” says mathematician Thelma Krug, who was vice president of the IPCC between 2015 and 2023 and has represented Brazil in international climate negotiations for the last decade. “Personally, I am in favor of research in this area. But the experiments need to be carried out transparently, step by step, and under strict governance.”
The 1991 eruption of the Pinatubo volcano is considered the second-largest eruption of the last centuryArlan Naeg / AFP via Getty images
The topic is so controversial that some scientists have spoken out against solar geoengineering research. One problem is that it in no way helps to reduce greenhouse gas emissions, which is what is causing the Earth’s temperature to rise. Even if they prove to be relatively safe and efficient at temporarily cooling the planet—which is still only hypothetical for now—techniques such as SAI would be palliative at best. Ultimately, critics of the approach argue that studies would divert resources and time that could be better spent pursuing alternatives that serve to reduce the emission of gases such as carbon dioxide (CO2) and methane (CH4). “Studies on solar geoengineering could be used as the perfect excuse for major producers of greenhouse gases not to reduce their emissions,” points out Carlos Nobre, a climatologist from USP’s Institute of Advanced Studies (IEA).
In addition to being seen as a distraction from the target of achieving zero greenhouse gas emissions over the coming decades, the adoption of SRM could result in the planet being held hostage to this type of climate intervention for a long and indefinite period of time—decades or even centuries. This would create another problem: the risk of something known as termination shock. When SRM is eventually stopped, temperatures could rise again much faster than the current rate of global warming, a situation that would be almost impossible to manage. Any significant fluctuation in temperature in a short period of time, whether a rise or a fall, represents an adaptive challenge.
Some climate models have suggested worrying scenarios when simulating the potential impacts of solar geoengineering. These studies usually investigate the other possible (side) effects of climate intervention techniques, beyond a temporary reduction in the Earth’s temperature. One problem is that most of these studies focus on potential consequences in the Northern Hemisphere, where the richest countries are located and where most climate researchers live and work.
However, research that focuses on other parts of the planet is now beginning to emerge. A study published in the journal Environmental Research Climate in June this year suggests that adopting SAI for the rest of this century would alter the likely impacts of global warming on the formation of extratropical cyclones in the Southern Hemisphere, such as those that regularly form in the South of Brazil. It is predicted that by the end of this century, the increase in global temperatures will reduce the number of cyclones that form in this part of the world, but it will increase the intensity of the phenomena when they do occur. The result: fewer but stronger cyclones.
Different levels of aerosols injected into the stratosphere were simulated in three international climate models until the year 2100. The results showed an increase in the frequency of cyclones, but a reduction in their strength compared to projections from global warming scenarios in which SAI was not adopted. “We are neither for nor against solar geoengineering,” says study leader Michelle Reboita of the Federal University of Itajubá (UNIFEI) in Minas Gerais. “We need to study it. It could have positive results in one part of the world and negative effects in another.”
There are also models that attempt to predict the possible impacts of SAI on biodiversity. “Our goal is to understand how SAI can affect terrestrial vertebrate species in the context of climate change,” says Brazilian biologist Andreas Schwarz Meyer, who is leading a research project on the subject as part of a postdoctoral fellowship at the University of Cape Town, South Africa. “In other words, we want to know which species would be the ‘winners’ and ‘losers’ if these techniques were used to reduce the planet’s temperature.”
In the ongoing project, Meyer is adopting an approach called horizontal biodiversity profiling, which uses historical climate data to estimate the thermal range (the minimum and maximum temperatures) and humidity levels in which species live. The technique is typically used to forecast the impact on species of various global warming scenarios predicted by the IPCC for the rest of this century.
“This gives us an idea of how many species will be exposed to these changes, and when and how quickly this could occur,” explains the biologist. In 2022, he published an article in the scientific journal Philosophical Transactions of the Royal Society B describing the effects of a particular scenario on more than 30,000 species of marine and terrestrial vertebrates over the rest of this century. The study simulated global warming of more than 2 °C followed by an artificial reduction in the Earth’s temperature due to the direct removal of carbon dioxide from the atmosphere. Scientists are currently testing the removal of this greenhouse gas using a set of techniques that, for now, are very expensive and inefficient.
Cloud trails created at sea by the emission of dust particles from shipsJesse Allen / LAADS / NASA
The general conclusion of the study was that a rise and subsequent forced fall in the Earth’s temperature could threaten the survival of many species, causing damage to wildlife decades after a hypothetical stabilization of the planet’s temperature. Meyer is now working on another similar study, but using SAI instead of direct carbon removal.
The work being done by Reboita and Meyer is part of an international initiative called DEGREES (DEveloping country Governance REsearch and Evaluation for SRM), which aims to encourage studies and train specialists in techniques that could be used to modify solar radiation in Africa, Latin America, and South Asia. The DEGREES initiative was created over a decade ago by the World Academy of Sciences (TWAS) and was later taken over by the eponymous British nongovernmental organization Degrees. It has invested in almost 40 research projects. In addition to the UNIFEI meteorologist’s research, the initiative has been funding two lines of research in Brazil by professors from the Federal University of Santa Catarina (UFSC) since last July.
A team led by engineer Mauricio Uriona of the Department of Production and Systems Engineering at UFSC, including partners from abroad, plans to study the perception of the potential risks of SRM in the production sector, the government, and the scientific community in three countries (Brazil, India, and South Africa). “In the past, we have studied the energy transition from a socioeconomic perspective and now we saw a good opportunity to carry out a similar study on solar geoengineering,” says Uriona.
Julia S. Guivant, an environmental sociologist from the Institute for Research in Risks and Sustainability (IRIS) at UFSC, intends to study how several key stakeholders in the country—such as the scientific community, political regulators, farmers, and representatives of nongovernmental organizations—plan to deal with the governance challenges of solar geoengineering. “We have not taken a stance on whether SRM should be used or how it should be governed if it is used. We are in favor of research and democratic debate on the topic, given how difficult it has been to achieve the goals of mitigating and adapting to climate change,” says the sociologist. Researchers from USP and the Federal University of São Paulo (UNIFESP) will also contribute to Guivant’s study.
There are concerns that solar geoengineering could affect rainfall patterns during India’s monsoonsAmarjeet Kumar Singh / Anadolu Agency via Getty images
Because SRM is so controversial and is not yet subject to any international regulations, even research groups from renowned institutions face great difficulties carrying out small field experiments. Studies on this scale do not influence the global climate—at best they provide an opportunity to understand the processes involved and the impact of local changes. Even so, the practical obstacles are almost insurmountable.
In March this year, the Stratospheric Controlled Perturbation Experiment (SCoPEx), conceived last decade by a group led by Harvard University chemist Frank Keutsch, was abandoned. The idea of the experiment was to use a high-altitude balloon to inject 2 kilograms of aerosols (in this case, calcium carbonate) at an altitude of approximately 20 km. “This is a very small amount of particles. It is equivalent to the pollution expelled by a commercial jet in just one minute of flight,” Keutsch said in an interview with Pesquisa FAPESP in 2021 (see Pesquisa FAPESP issue n° 303). The SCoPEx balloon was originally slated to be launched over the USA in 2018, but it never happened. The team later planned to release it in Sweden, but again without success. Due to protests from environmentalists and Indigenous groups, the project never got off the ground.
Marine cloud brightening, a less ambitious approach than SAI, has been tested a number of times, but the experiments almost always face opposition and criticism from various sectors of society. In April this year, a group from the University of Washington, USA, used a type of fan to spread sea salt particles onto the runway of a decommissioned aircraft carrier anchored off the coast of Alameda in California. The aim was simply to see if the particles represented a health risk. Two months later, the Californian municipality banned this type of experiment within its territory.
In the Townsville region of Australia, scientists from Southern Cross University and local organizations have been running a pilot project since 2020 to determine whether MCB could help reduce coral bleaching. The purpose of the experiment is to see if the method decreases localized ocean temperatures in the center of the Great Barrier Reef. Warming sea waters are the main cause of coral bleaching.
Altering the Arctic’s ability to reflect sunlight could theoretically reduce global warmingSodar99 via Getty images
The distrust of practical experiments stems partly from the occasional occurrence of initiatives that lack transparency, sometimes managed by obscure private companies. In 2022, an American startup called Make Sunsets released two balloons of aerosols into the stratosphere above northern Mexico without authorization. Shortly afterwards, the Mexican government banned such experiments in the country. Now, the company has announced that it is carrying out a similar study in the US, but the results, for now, remain unknown.
American physicist David Keith of the University of Chicago believes there is a growing interest in solar geoengineering research, despite the scientific uncertainties. “This can be seen in major international reports, such as those released by the UN’s Environment Program and World Climate Research Program, as well as major environmental groups such as Environmental Defense,” Keith said in an email interview with Pesquisa FAPESP. “There is no doubt that opposition to investigating the approach has weakened, but it is difficult to say why. Maybe it’s because of rising temperatures or because [I believe] the world is now making substantial efforts to reduce greenhouse gas emissions.”
Keith was a member of Harvard’s solar geoengineering program for 12 years. He now supports the idea of an international moratorium on real-world experiments until the science on the subject is better established and some form of international governance is implemented. If that happens, he says humanity should consider conducting a test in which it injects about 10% of the amount of aerosols needed to lower global temperatures by 1°C into the stratosphere for a decade. This would allow scientists to clearly monitor the effects of the approach without taking too many risks.
The task would involve a fleet of 15 jets capable of flying at high altitudes spraying some 100,000 tons of sulfur into the stratosphere per year—equivalent to 0.3% of the amount of sulfur pollution that reaches the atmosphere annually. It would cost approximately US$500 million per year, and the idea is considered controversial. For some, the only viable part of the suggestion would be the adoption of a moratorium on this type of experiment.
The story above was published with the title “Controlling the Sun” in issue 343 of September/2024.
Scientific articles
REBOITA, M. S. et al. Response of the Southern Hemisphere extratropical cyclone climatology to climate intervention with stratospheric aerosol injection. Environmental Research: Climate. June 20, 2024.
MEYER. A. L. S. et al. Risks to biodiversity from temperature overshoot pathways. Philosophical Transactions of the Royal Society B. June 27, 2022.
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