When Harvard researchers unveiled a speculative technology blocking sunlight by dispersing reflective particles, they chose Esrange Space Center, located in Sweden’s far north, for a pioneering test. But public protests in 2021 forced them to cancel the experiment.

Solar geoengineering, also known as solar radiation modification (SRM), aims to cool the planet by reflecting sunlight into space. It proposes techniques such as stratospheric aerosol injection, cloud brightening, and space mirrors. While the US ramps up research, the EU’s scientific advisors are pressing the pause button, calling for a global agreement to halt deployment. A key unknown variable is the incoming Trump administration’s approach.

The split over solar geoengineering underlines a deep transatlantic difference towards how to deal with scientific risk. It echoes previous debates over pathbreaking technologies ranging from genetic engineering to artificial intelligence. The EU wants global governance to mitigate danger. The US prefers to go it alone.

Boosted by private sector funding, the US leads on solar engineering research. Silicon Valley executives are investing in the hope of providing a quick, cheap remedy to climate change. Microsoft founder Bill Gates and Facebook-funded Open Philanthropy, among others, offered financial support to the Harvard Solar Geoengineering Research Program, which managed the geoengineering experiment in Sweden. The University of Chicago, Colorado State, Cornell, and Princeton have all launched research projects.

Small-scale solar engineering deployment is also taking off. California-based startup Make Sunsets, founded in 2022, raised $1m in venture capital and now launches balloons filled with sulfur dioxide to reflect sunlight. The company sells “cooling credits”: for $9.95, it promises to send “at least one gram of our ‘clouds’ into the stratosphere.” Initial field tests in Mexico sparked widespread criticism and angered the Mexican government, which banned the practice. Make Sunsets currently operate in the US where the balloon launches are allowed to proceed.

Across the Atlantic, the EU is pressing on the brakes. This month, the EU’s scientific advisors released a new skeptical assessment, arguing that solar geoengineering does not represent a viable solution to combating climate change. Instead, the scientists say that solar geoengineering brings deep uncertainty, making the technologies inconsistent with Europe’s precautionary principle. Risks include changes in rainfall, ecosystem disruptions, and ‘termination shock’: rapid warming that could occur if solar geoengineering deployment were to suddenly halt.

A similar split is visible with other controversial new technologies. Americans have been eating genetically modified foods for several decades, while Europe continues to fight over deploying gene editing. American companies are rushing ahead with artificial intelligence, while Europe has pushed back with a new AI Act designed to mitigate potential catastrophic risks.

Europe continues to believe in international governance of potentially dangerous new technologies. Because solar engineering will impact the entire planet, a “strong global governance framework would be needed,” the EU advisors argue. But “no such framework exists, and it is not clear how one could be created.”

In its absence, the European scientists advise policymakers to establish an EU-wide moratorium on solar geoengineering and push for a global non-deployment agreement. This advice could put “oil on the wheels” to get a UN governance process started, according to Janos Pasztor, former UN Assistant Secretary-General on climate change and a long-time proponent of solar engineering governance. It’s not an easy task.

The US disagrees, preferring to exempt itself from international regulations to maintain freedom of maneuver. It is the only country that has failed to ratify the Convention on Biological Diversity, which in 2010 de facto banned any solar engineering technologies that threaten biodiversity. The US has also repeatedly blocked efforts to open a global conversation on solar geoengineering. At a UN Environment Assembly in February 2024, the US expressed doubts about a Swiss proposal to establish a UN expert group to study the technologies. When negotiations failed to reach a consensus, Switzerland withdrew the proposal.

Many US scientists share the European concerns. More than 500 scholars – among them almost 100 US academics – have signed an open letter, launched in 2022, calling for a global non-use agreement on solar geoengineering.

Under a second Trump administration, support for solar geoengineering regulation looks implausible. “It is likely that SRM will remain unregulated, paving the way for private-sector actors and venture capitalists to dominate SRM research with little to no transparency,” says Dr. Shuchi Talati, founder of the Alliance for Just Deliberation on Solar Geoengineering. Influential tech billionaires Elon Musk and Peter Thiel, among others, could push the administration in a pro-solar engineering direction, she believes.

But much remains uncertain about the new administration’s stance. As Talati puts it,“we also see a chaotic counterweight in the elevation of RFK Jr., a chemtrails conspiracy theorist.” Donald Trump’s health secretary-nominee has lent support to the idea that governments are releasing chemicals in the atmosphere to control populations. In an unlikely convergence with the EU’s scientific advisors, adherents of the conspiracy theory support a ban. For instance, Tennessee lawmakers recently voted to prohibit geoengineering due to worries about chemtrails.

Without international governance, the likelihood of unilateral deployment of solar geoengineering increases. The 1977 Environmental Modification Convention bans weaponizing climate technologies – yet the concern looms large. Many recall the US Operation Popeye which sought to disrupt rainfall during the Vietnam War. The need to fight against rogue solar geoengineering deployment generates transatlantic alignment: both the US and the EU’s scientific advisors support the development of solar geoengineering detection systems.

Solar geoengineering could slow – and perhaps reverse – climate change. Time is short: 2024 set to become the warmest year on record. But political agreement on the speculative technologies remains elusive. Without it, solar geoengineering is likely to add, rather than subtract, to transatlantic disagreements and the world’s growing climate woes.

Oona Lagercrantz is a Project Assistant with the Digital Innovation Initiative at the Center for European Policy Analysis in Brussels. Oona received a first-class bachelor’s degree and a master’s degree with distinction from the University of Cambridge, specializing in the politics of emerging technologies.

Bandwidth is CEPA’s online journal dedicated to advancing transatlantic cooperation on tech policy. All opinions are those of the author and do not necessarily represent the position or views of the institutions they represent or the Center for European Policy Analysis.

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