Reusable rockets designed to deliver sun-reflecting aerosols into the upper stratosphere could help cool the planet. However, this fleet of climate-modifying rockets presents its own challenges.

The increase in global temperatures has led to a surge in research on solar geoengineering, a controversial method aimed at cooling the Earth by reflecting sunlight. The most recognized technique, known as stratospheric aerosol injection (SAI), entails continuously releasing reflective particles, such as sulfur dioxide, into the stratosphere.

Typically, researchers believe that aerosols will be released from cargo aircraft flying at around 20 km high. Climate models suggest that this could offset warming caused by rising greenhouse gas levels. Nonetheless, it also presents numerous other climate risks, some of which are unknown.

One significant concern is that the aerosols absorb sunlight, warming the stratosphere itself, even while surface temperatures cool. Because of wind patterns, aerosols tend to cluster in the tropical stratosphere, resulting in more warming than in other areas of the atmosphere. This can disrupt jet streams and circulation patterns that influence global weather.

Aerosols may also hasten ozone depletion caused by chlorine, potentially postponing the recovery of ozone holes over Antarctica by as much as 50 years.

Pengfei Yu from Jinan University in China and his team explored whether injecting aerosols at much greater heights—50 kilometers in the upper stratosphere—would change these dynamics. They discovered that high-altitude injections could yield greater cooling benefits than lower altitude methods, as aerosols persist longer, particularly at the poles. This additional height also prevents the aerosols from warming the lower stratosphere until they reach the poles, avoiding the harmful heating in the tropical stratosphere.

Finally, researchers found that these aerosols interact with another chemical that depletes ozone, which results in only a five-year delay in ozone recovery. “We weren’t aware that [injecting at] 50 kilometers offered such a different perspective,” says Yu.

Since planes cannot reach nearly 50 km, researchers propose using rockets. They estimate that deploying 80 reusable, hydrogen-powered rockets every other day could inject between 3 million and 8 million tonnes of aerosols annually, which they say falls within current technological capabilities.

While this scenario may be theoretically feasible, it is likely to be significantly more complex than traditional SAI approaches, according to Douglas McMartin at Cornell University in New York. Some advantages, such as preventing warming in the tropical stratosphere, can be more easily achieved by focusing on higher latitudes instead of high altitudes.

“It may rise higher in the atmosphere for increased efficiency, but the costs are astronomical in comparison,” he states.

Moreover, the high-altitude method does not fully mitigate many risks associated with solar geoengineering, including the rapid temperature increase that could follow if injection ceases. “What happens if the rockets fail on the ground?” Yu questions. “That’s a legitimate concern.”