We can safely experiment with reflecting sunlight away from Earth. Here’s how | Dakota Gruener and Daniele Visioni

THe world is warming fast – and our opportunities to avoid catastrophic harm narrow. 2024 was the first full year was more than 1.5°C hotter than the 19th century average. Emissions are still rising and fossil fuel use is expected to reach a new high in 2025. Permanent carbon removal technologies, often cited as a solution to the problem, remove only tens of thousands of tons annually, almost nothing compared to the 5-10 billion tons needed. Reducing emissions and scaling up carbon removal remain important. But they may not be enough.

As suffering increases and ecosystems collapse, more people are asking: Can we do anything to prevent this harm? The idea of ​​reflecting a small portion of incoming sunlight to reduce warming is not new. In 1965, Lyndon B. Johnson's scientific advisers proposed him as only way to cool the planet. The Earth already reflects about 30% of incoming sunlight; A small increase in this proportion – say to 31% – could strengthen the planet's natural heat shield. But how?

In 1991, Mount Pinatubo erupted. sent about 15 million tons of sulfur dioxide enters the stratosphere, cooling the planet by about 0.5°C. This eruption became a natural experiment and inspired the idea of ​​stratospheric aerosol ejection (SAI). Models suggest that SAI could offset 1 degree of warming with about 12 million tonnes of SO₂ per year – far less than we currently unintentionally emit from industrial processes, but with a much greater cooling effect.

Let's be clear: SAI is not a substitute for reducing emissions. If it were to unfold and then suddenly stop, the planet would warm rapidly. Poorly planned or uncoordinated measures can lead to catastrophic changes in precipitation patterns. But that's why research is needed—not to green-light deployment, but to understand whether SAI can ever be used safely, effectively, and in the public interest.

Some argue that the risk of misuse means it shouldn't even be studied. We disagree. A rigorous open-label study could clarify whether a well-managed approach can reduce harm, especially for the most vulnerable populations. It can also help identify risks and failure modes early on, making reckless proposals less likely to gain traction. In this sense, research acts as a guardrail rather than a slippery slope.

But how can we know if something is safe or too risky? We don't need to reinvent the wheel. Medicine solved the “too risky to test” dilemma 60 years ago by systematizing staged clinical trials. A similarly structured step-by-step program for SAIs can safely provide the evidence that policymakers will ultimately need.

We are now stuck in the “preclinical” or phase zero: laboratory work and computer models. These are great tools – they have helped to correctly forecast the risks of rising emissions – but we cannot trust their forecasts without checking that they correctly reflect the key processes for the SAI. How do aerosols form, evolve and disperse in the stratosphere? How do they interact with the environment? These are key factors for any reliable assessment. What would similar clinical trial phases look like for SAI?

The first stage will involve releasing a small amount of SO₂—about 10 tons of SO₂ (a fraction of what many coal-fired power plants emit per day)—at the right altitudes and carefully measuring its evolution using a suite of instruments: aircraft, ground-based, and satellite. This amount would be too small to affect climate, but would allow researchers to study how aerosols form and behave, which remains one of the biggest scientific uncertainties in the field. Comparing these observations with model predictions could offer an early test of key assumptions and help determine where current predictions are reliable and where they need refinement.

A potential phase two experiment could be 10 or 100 times larger, but still several orders of magnitude smaller than a “small” volcanic eruption such as Mount Ruang, which injected about 300,000 tons simultaneously in 2024 and still had no measurable impact on global climate. This will allow researchers to study how aerosols mix and distribute. How fast do particles travel? How do they interact with the stratospheric circulation? Are our models capturing this correctly? If not, what are we missing? Are we seeing something completely unexpected? The surveillance capabilities required for these tests will also be critical to detecting unauthorized deployment.

Once researchers around the world have had a chance to look at the data and draw their own conclusions, the findings can be put to the test: Are governments interested in pushing ahead with something that's starting to look like a rollout? If so, the studies will move into phase three (akin to phase four of post-licensing clinical trials) with a small, deliberate cooling of perhaps about 0.1°C for five years under constant monitoring and strict supervision. This slow (and reversible) deployment, when combined with a strong governance structure, will be the opposite of unauthorized or reckless deployment.

The world may never need to reflect sunlight. But if it does, the only way to make a responsible decision about its use in the future is to create real evidence in a transparent manner. to crisis forces us to act. This means creating tools, rules and oversight mechanisms now, not later.

We see the British Agency for Advanced Research and Invention (Aria) program as a strong first step in this direction. In a laboratory led by one of us, a new project funded by Aria is developing a theoretical framework for determining minimum The extent to which an outdoor experiment can significantly reduce key uncertainties is an important basis for the safe and transparent conduct of any future research. And at Reflective, an organization led by one of us, we work to support open science, careful coordination, and strong public accountability throughout the field.

Outdoor exploration is not a slippery slope to deployment. This ensures that any future decision—whether to move forward, reject an idea entirely, or improve upon it—is based on facts and not on fear or wishful thinking. If done well, small-scale experiments can reduce both scientific uncertainty and political risks. The real danger is not in asking questions. It's too long to wait to find out the answer.

• Dakota Gruner is the CEO of Reflective, a nonprofit climate initiative that accelerates the rate at which sunlight is reflected, and Dr. Daniele Visiony is an assistant professor of earth and atmospheric sciences at Cornell University.