Cooling the planet: The new race to reflect the sun

Introduction

Over the past months, I’ve often written about the finance and policy side of climate action, carbon markets, Article 6, and removals. But Climate Playbook was also meant to explore something broader: how technology and innovation are reshaping the way we respond to the climate crisis. This week, I looked into one of the most provocative frontiers of climate tech: the idea of cooling the planet itself.

Mount Pinatubo (island of Luzon in the Philippines)

Keeping the 1.5°C target within reach is becoming harder every year. The IEA’s 2025 World Energy Outlook (published this week) still sees a narrow path, but one that depends on an extraordinary acceleration of clean energy deployment and carbon removals by 2035. That uncertainty is pushing scientists, policymakers, and now entrepreneurs to look beyond traditional mitigation to explore technologies that could, in principle, cool the planet itself.

I’m not an expert in these technologies, but after reading some fascinating research, I decided to dig deeper. This article offers a snapshot of where things stand and how scientists and the private sector are beginning to approach the topic. My focus is Solar Radiation Modification (SRM), also called solar geoengineering, the most studied and the most controversial.

For decades, the climate conversation revolved around three pillars: mitigation, adaptation, and removal. Cut what we emit, adjust to what we can’t, and capture what’s already in the air. But as the physics of delay becomes clearer, CO₂ lasting centuries, warming locked in even after emissions peak, a new frontier is opening: planet-cooling technologies.

Among these ideas, Solar Radiation Modification (SRM) dominates. The principle is simple but profound: reflect a small fraction of sunlight back into space to lower global temperature. Nature has done it before, when Mount Pinatubo erupted in 1991, 15 million tonnes of sulfur dioxide cooled the planet by about half a degree for more than a year. Because the effect lasts only as long as the particles remain aloft, SRM would commit future generations to maintaining a continuous atmospheric intervention. Stop, and temperatures could rebound rapidly, a phenomenon known as termination shock.

Evidence from natural analogues supports the premise: aerosols can measurably cool Earth’s surface. Climate models confirm that carefully designed interventions could offset some near-term warming, yet they all agree on one point: SRM cannot replace emissions cuts. It might serve as a temporary stabiliser, but it carries major uncertainties, from potential rainfall shifts to ozone-layer effects. The scientific community increasingly views it as a last-resort measure that could buy time if the world fails to decarbonise fast enough, but never as a substitute.

Models also show that SRM could cool some regions while intensifying heat or drought in others. Even small imbalances could alter monsoons, storm tracks, or the polar jet stream, raising ethical and geopolitical questions: who bears responsibility if cooling in one region triggers flooding or crop loss elsewhere?

Crater of Mount Pinatubo today

Two weeks ago, an American–Israeli startup called Stardust announced a $60 million round to develop proprietary particles for stratospheric aerosol injection, the largest private investment ever seen in solar geo engineering. For some, it signals innovation catching up with urgency; for others, it’s exactly the risk they feared.

In a recent MIT Technology Review essay, David Keith (University of Chicago) and Daniele Visioni (Cornell) warned that SRM research must remain public, transparent, and peer-reviewed, not driven by patents or profit. The problem is not the science but the incentives: startups must promise results, while SRM’s true value lies in knowing if, when, and how it should ever be used.

Stardust claims its particles are “chemically inert.” Atmospheric scientists disagree. Even diamond dust, among the most inert materials known, can react with radicals in the upper atmosphere, potentially affecting ozone chemistry. History is full of “safe” innovations that proved anything but: DDT, CFCs, PFAS. The lesson is simple: in complex systems, it’s not what you release that matters, but what it becomes once it’s out there.

Perhaps the biggest risk is not scientific but political. SRM’s apparent affordability, roughly $30 billion a year, according to one Yale study, makes it dangerously tempting. A single government, or a coalition, could decide to “dim the sun” without global consent. The question is not whether it can be done, but who gets to decide.

Despite its growing visibility, SRM remains largely unregulated. There is currently no dedicated international legal framework. Experts argue that without a shared multilateral framework, unilateral deployment of SRM could spark major geopolitical tension. In May 2025, the UNEP and WMO will hold a two-day joint workshop in Geneva to assess the state of SRM science and explore international governance options, a modest but crucial step toward collective oversight.

Cooling the planet is not a software problem that can be patched if it fails; it’s a collective-risk decision that demands legitimacy. If the public perceives this research as happening behind closed doors, support will vanish long before the science is complete.

The world needs open, publicly funded research, multilateral oversight, and global dialogue. Large-scale planetary risk requires political consensus; once the rules are agreed, however, economic principles can help: well-designed markets allocate capital efficiently, reward performance, and create competition within a defined set of rules.

We have seen early signs of this in climate policy: the Paris Agreement created the foundation, and Article 6 provides a framework for the carbon market, enabling international cooperation through transparency, accounting integrity, and shared methodologies. We don’t yet know how effectively it will work at full scale; implementation has only just begun, but it offers a clear example of how we should proceed: first, a global rule, then let the market drive efficiency.

I deeply believe in the power of innovation to accelerate climate solutions. What we’ve seen with artificial intelligence is extraordinary, and in the climate space, I see the same energy in the carbon-removal community, where entrepreneurs and researchers are working to scale technologies that permanently remove CO₂ from the atmosphere.

A month ago at Carbon Unbound London, I saw how the carbon-removal ecosystem is pushing innovation at an incredible pace. What was once a slow and technical field is now full of brilliant founders committed to building scalable, cost-efficient, high-performing solutions, promising investors both real climate impact and solid returns.

I am a strong supporter of market-based instruments and the efficiency they can unlock. But I also recognise that the atmosphere is a global commons, and we cannot treat it as a space for unregulated experimentation. We cannot privatize the atmosphere or allow new particles to be tested in the stratosphere without collective oversight. With a clear international governance framework, however, we can harness the power of innovation responsibly and ensure that competition accelerates solutions rather than amplifies risks.

Planetary cooling is both fascinating and unsettling. It carries real risks, real unknowns, and real ethical dilemmas. But with 1.5°C slipping away, we cannot afford to ignore any tool that might buy time. SRM should be studied, openly, cautiously, and under global governance. If it proves safe and effective, it could become a bridge to deeper decarbonisation and permanent removals. The real question now is not whether we explore it, but how we do so together, transparently, and before anyone acts alone.

I’ll keep following this topic closely, but I’m curious to hear from you: is SRM a temporary bridge worth exploring, or a line we should never cross?

This article is also published on Substack. illuminem Voices is a democratic space presenting the thoughts and opinions of leading Sustainability & Energy writers, their opinions do not necessarily represent those of illuminem.

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