The world needs to lower its dependence on fossil fuels. Countries have done so in fits and starts: issues like war, poverty, disease, and inflation have often left climate mitigation on the back burner. Today, greenhouse gas emissions are increasing worldwide.

In this situation, some researchers have proposed the use of technologies to directly cool the planet rather than bank on reducing emissions alone. Stratospheric aerosol injection (SAI) is one such technology — and a controversial one. In SAI, aerosols are injected into the earth’s stratosphere to reduce the amount of sunlight reaching the surface.

A study recently published in the journal Earth’s Future offered an innovative approach to this technique that could reduce its costs but also bring it closer to fruition despite the opposition to it.

A volcano-inspired tool

SAI is a “proposed method of cooling the planet and reducing the impacts of climate change by adding a layer of tiny reflective particles to the high atmosphere,” Alistair Duffey, a PhD student at the Department of Earth Sciences at the University College London and the study’s lead author, said.

The method was inspired by volcanic eruptions, which have been known to have a cooling effect on the planet by spewing aerosols into the air. By reflecting more sunlight away from the earth, SAI aims to create a cooling effect that could help combat rising surface temperatures.

How well SAI works depends on the type of material injected, the timing of the injection, and the location. Technical challenges are also more pronounced at higher altitudes. Most studies of SAI’s efficacy have focused on implementing it at 20 km or more, particularly over areas closer to the equator. Doing so demands specially designed aircraft capable of operating at such elevations.

A contrasting approach

The study’s authors have explored an alternative approach to undertake SAI using existing aircraft. “We were interested in understanding how the effectiveness of stratospheric aerosol injection varies with the injection altitude,” Duffey said, adding that “low altitude injection strategies are necessarily” meant for the polar regions.

At the equator and regions close to the equator, the stratosphere is higher — 18 km and above — where existing aircraft can’t fly. In polar and extratropical regions, the boundary between the troposphere (the lowermost layer of the atmosphere) and stratosphere, called the tropopause, is at a lower altitude than over the equator or subtropics. This means existing jets can reach the stratosphere at these close-to-polar regions.

“Higher altitude injection is generally more effective because the particles stay in the stratosphere for longer,” anywhere between months or years, Duffey said. In contrast, particles released at lower altitudes are more likely to be caught in clouds and washed out by rain.

Despite this, researchers are exploring low-altitude SAI because spraying particles at lower heights is technically less challenging and doesn’t require specially designed high-altitude aircraft, also making the approach potentially more accessible and cost-effective.

Even when using existing aircraft for this mission, various modifications are necessary, according to Duffey. An August 2024 study said aircraft like the Boeing 777F would have to be modified to install insulated double-walled pressurised tanks to ensure the safe transport of aerosols and maintain the desired temperature during flight.

Time-, cost-effective

The new study’s researchers simulated various particle-injection strategies. Using the UK’s Earth System Model 1 (UKESM1), a computer model of the climate, they simulated the “spraying” of sulphur dioxide at different altitudes, latitudes, and seasons.

The team found that injecting 12 million tonnes of sulphur dioxide every year at an altitude of 13 km in the local spring and summer seasons of each hemisphere could cool the planet by approximately 0.6º C. The spray quantity is comparable to the amount added to the atmosphere by the Mount Pinatubo volcano in 1991.

For cooling by 1º C, their models suggested spraying 21 million tonnes of sulphur dioxide a year. If the particles were injected at an even higher altitude in the subtropics, only 7.6 million tonnes would be required annually to achieve the same effect.

An added advantage is that this technique could begin sooner than conventional higher altitude methods because designing and building specialised aircraft meant for flying 20 km and above requires almost a decade and several billion dollars in capital expenses. Modifying existing aircraft can be faster and cheaper.

Is it worth the risk?

But while there are some benefits to this method, using three times the usual amount of aerosols carries greater risk. “There are lots of important risks and side-effects related to SAI, including social and geopolitical risks, as well as direct side-effects such as delayed recovery of the ozone hole and acid rain,” Duffey said.

The cooling effect will also be more pronounced in polar regions rather than in the tropics, where warming is more severe.

Duffey also stressed that the cooling effect wouldn’t reverse climate change. The cooling may also have some other ecological effects but it would also introduce new challenges. As The Hindu reported recently, the cooling could mask warming on the ground and make countries complacent about curtailing emissions.

SAI is also controversial because its effects are global: if one country injects aerosols into the stratosphere, all countries will be affected and not always in a good way. In 2021, the US National Academies of Sciences, Engineering, and Medicine recommended the US government fund solar geoengineering research with a focus on transparency. But a year later, an international coalition of scholars called for a moratorium on solar geoengineering R&D because the technology is “ungovernable in a fair, democratic and effective manner”.

Duffey also said the team’s results were limited by the number of simulations they conducted and that they’re working on a better follow-up study.

Shreejaya Karantha is a freelance science writer.