Is playing with the sun to fight climate change worth the risk? 

Technologies for modifying the sun's rays have long been taboo, but they are now attracting growing interest in the fight against climate change. Jon G. Fuller / Vwpics / Science Photo Library

Reflecting the sun’s rays back to space to prevent climate change, known as solar geoengineering, is as controversial as it is intriguing. SWI swissinfo.ch explains all there is to know about this “Plan B” for the climate that is dividing the scientific community. 

Last year was the warmest year on record globally with the impact of climate change increasingly evident: glaciers are melting, sea levels are rising, and extreme events such as droughts and floods are becoming more frequent.  

After the announcement of the Trump administration to withdrawal the United States from the Paris Agreement, the goal of limiting climate warming to 1.5°C appears increasingly out of reach. 

This has put solar radiation modification (SRM) technologies – long considered taboo as they raise ethical questions without addressing the origins of climate change – back at the forefront of research. Private foundations and countries such as the US, Britain and China have invested tens of millions of dollars in research projects to reflect the sun’s rays back to space. 

Companies such as US start-up Make Sunsets have already started outdoor experiments, and new projects are planned for this year. 

Cape Town, South Africa, will host the largest conference to date on SRMExternal link on May 12-16. Researchers, NGO representatives and policy makers will discuss the scientific, social and political aspects of so-called solar geoengineering. 

“SRM has always been a question of when [it will be deployed], not if,” Gernot Wagner, climate economist, author and co-founder of Harvard University’s solar geoengineering research programme, tells SWI swissinfo.ch in an email. 

“Sadly, what has been happening since noon on January 20 has only made eventual deployment more likely,” Wagner says, referring to Donald Trump’s second term as president. 

What are solar radiation modification technologies? 

SRM technologies are deliberate, large-scale climate interventions that aim to reduce global warming. There are essentially two ways to change the effect the sun’s rays have on the planet: increase the reflection of the sunlight back to space or reduce the Earth’s absorption of solar radiation. 

Those in favour of SRM technologies argue that they represent the only sustainable option for rapidly returning the planet’s temperature to near pre-industrial levels, at relatively low cost. The Earth reflects about 30% of the solar energy it receives. Increasing this reflectivity by even 1% could reduce the globe’s average temperature by about 1°CExternal link, according to one study. 

The scientific world agrees that urgent and drastic reductions in CO2 emissions are imperative to counter climate change. SRM supporters say solar geoengineering would buy time while waiting to reach net-zero emissions. It would also mitigate some of the impacts of global warming, such as rising sea levels. 

How is it possible to reflect sunlight back to space? 

The most researched method is the injection of aerosols into the atmosphere (stratospheric aerosol injection or SAI). Aerosols are microscopic particles that can reflect solar radiation back to space, reducing the temperature on the ground. 

Planes or balloons would release billions of sulphur dioxide molecules into the atmosphere. This would lead to the formation of aerosols and create a kind of thin reflective layer around the Earth. 

The same phenomenon occurs naturally during major volcanic eruptions. In 1991, the ash and sulphur dioxide released from the Pinatubo eruption in the Philippines caused the planet to cool by 0.5°C for two years. 

According to a recent Swiss study on the release of aerosols into the atmosphereExternal link, the use of solid particles would be more efficient than sulphur dioxide. Calcite or diamond dust would have a greater ability to reflect sunlight and a lower environmental impact, according to the study. 

The other most advanced SRM technology is marine cloud brightening (MCB). Special cannon mounted on ships spray a mixture of seawater into the atmosphere. The salt crystals facilitate the formation of brighter clouds over the oceans. These clouds reflect more sunlight, protecting coral reefs from high temperatures. 

>> See how to brighten sea clouds in the short animation: 

Painting streets and rooftops white also increases the portion of sunlight reflected back to space. Bright surfaces can make cities more liveable during heat waves. 

Another option, which is still theoretical, is to place giant mirrors in space to reflect solar radiation before it reaches Earth. 

Finally, there is also be the possibility of dispersing microparticles into cirrus clouds to make them thinner. Cirrus are high-altitude clouds that can trap heat radiated from Earth’s surface. 

Kai Reusser / SWI swissinfo.ch

At what stage is the development of SRM technologies? 

Solar geoengineering remains mostly a theoretical approach for now. However, research programmes on SRM are increasingExternal link, and several projects are already conducting outdoor experiments. 

Make Sunsets says it has already sent 147 balloons filled with sulphur dioxide molecules into the atmosphere. In Australia, Southern Cross University has been studying the effects of artificial cloud brightening on the Great Barrier Reef since 2020. 

If humanity as a whole or a group of powerful and sufficiently rich countries agrees that stratospheric aerosol injection on a global scale is a necessity, it can be rolled out in a few years, says Claudia Wieners, assistant professor of climate physics at Utrecht University in the Netherlands. “Building the atomic bomb was more difficult,” she says. 

Why is solar geoengineering controversial? 

SRM can lower temperatures, but it doesn’t reduce the amount of greenhouse gases we emit into the atmosphere. It therefore addresses the symptoms and not the causes of the climate crisis. Those who oppose it see solar geoengineering as a loophole that distracts from the real goal of cutting emissions. 

“SRM will project false hopes that we can ‘tech’ our way out of the crisis,” Xiao-Shan Yap, a policy adviser at the Space Center of the Swiss Federal Institute of Technology Lausanne, told the Swiss magazine HorizonsExternal link. The risk, she adds, is that politicians will backpedal on their commitment to cut greenhouse-gas emissions. 

The injection of aerosols into the atmosphere could also have unexpected impacts on climate, precipitation patterns and the environmentExternal link. Sulphur dioxide particles, for example, are harmful to health and cause acid rain. 

This Plan B for climate change also raises fundamental questions. Who should decide when and where to deploy SRM? Who would be responsible in case of harmful side effects? And what would happen in case of premature termination of a project, for example due to war? 

There are no globally shared rules. More than 500 researchers in Switzerland and around the world have joined an international call for a ban on solar geoengineeringExternal link. They want no public funds to be invested in SRM and all outdoor experiments to be banned. 

In contrast, Sandro Vattioni, a climate researcher at federal technology institute ETH Zurich, calls for more research and more experiments – “purely scientific and on a small-scale”. 

Vattioni is author of the diamond dust study mentioned earlier and will be one of the speakers at the SRM conference in Cape Town. “We need to study these technologies, including the potential negative effects, should we decide we need them,” he tells SWI swissinfo.ch. 

A moratorium on SRM that allows for research into the science and politics of such projects is the most reasonable solution, according to Wieners. “A unilateral deployment of SRM would be highly undesirable, because it could lead to conflicts between countries,” she says. Wieners is co-signer of an open letter calling for balanced researchExternal link. 

The white roofs of Santorini, Greece, help disperse the heat that builds up in summer. Martin Child

What is Switzerland’s position on SRM? 

Switzerland wants more research on solar geoengineering. In 2024, it submitted a resolution to the United Nations Environment Assembly (UNEA) calling for the creation of a group of experts to assess its risks and benefits. 

However, Switzerland withdrew the resolution because the countries did not reach a consensus. The African Group of States, Pacific Island States and some countries including Mexico would have opposed any proposal that paved the way for SRM. 

Switzerland is not actively pursuing SRM as part of its national climate policy, the Federal Office for the Environment (FOEN) told SWI swissinfo.ch. However, the country is making international efforts to ensure that states have access to existing information, particularly on possible transboundary risks and impacts, it added. 

The next UNEA assembly is scheduled for December. Switzerland has not yet decided whether it will resubmit the SRM resolution, FOEN said. 

Edited by Gabe Bullard/vm/ts