Researchers study using planes to cool the earth amidst global warming

As global temperatures rise, extreme weather is forcing families from their homes.

Floods, hurricanes and melting glaciers are displacing communities across the planet.

Some scientists are researching ways to deal with climate change by manipulating the world’s atmosphere or oceans.

Known as geoengineering, it’s often rejected because of potential side effects, and is usually mentioned not as an alternative to reducing carbon pollution, but in addition to emission cuts.

One idea is to reflect sunlight away from the Earth before it can heat the surface - a process known as stratospheric aerosol injection.

A new study by University College London researchers suggests that this could be done using planes already in service today, rather than developing costly new aircraft to reach the highest parts of the atmosphere.

Stratospheric aerosol injection would work by releasing tiny particles into the atmosphere’s dry, stable upper layer called the stratosphere.

These particles would scatter sunlight back into space, reducing the amount reaching the Earth’s surface and helping to cool the planet.

Previous research focused on injecting aerosols high above the tropics, at altitudes of 20 kilometres or more, which is beyond the reach of most existing planes.

But the new study found that injecting lower down, around 13 kilometres, near the poles, could still have a significant impact.

It could mean aircraft like the Boeing 777, which is already capable of reaching these altitudes, could be adapted for the task.

Alistair Duffey, a PhD researcher at UCL, led the study.

He says: “So our study examined a climate intervention technique called stratospheric aerosol injection, which is an idea to cool down the planet by adding a layer of small reflective particles, aerosols, into the high atmosphere. Those particles would reflect a small amount, perhaps 1% of the incoming sunlight. And there was good evidence that this could be used to cool the planet, and perhaps to reduce some climate impacts on vulnerable people around the world.”

Using the UK’s advanced Earth System Model, researchers simulated injecting sulphur dioxide (a gas that quickly transforms into reflective sulphate aerosols) into the stratosphere over the polar regions during their respective spring and summer seasons.

The study showed that despite the lower altitude, it would still be possible to cool the planet by around 0.6 degrees Celsius.

This is roughly the same as the temporary cooling after the eruption of Mount Pinatubo in 1991, when volcanic gases injected into the atmosphere caused global temperatures to dip.

The researchers examined how the effectiveness of cooling changes depending on where and how high the particles are released, as well as how much sulphur dioxide would be needed.

“What we were interested in is understanding the trade-off between the difficulty, the logistical challenge of doing this and the climate impacts on the ground. So in particular, we wanted to understand how, if you could get to different altitudes in the sky, how the level of impact on the ground would vary depending on how high we could go. In general, it’s harder to do this at high altitudes. So our central finding was that if we were limited to using existing large aircraft and therefore limited to altitudes of up to around 13 kilometres, we found that there was still meaningful climate impacts. We could still cool the planet meaningfully with plausible injection magnitudes of aerosols.”

The cooling effect comes from a chain of chemical reactions.

Once sulphur dioxide is released into the dry stratosphere, it reacts with water vapour and oxygen to form sulphuric acid, which then forms microscopic droplets — sulphate aerosols.

These aerosols remain suspended for months, reflecting sunlight away from Earth.

Eventually, they fall into the lower atmosphere and are washed out by rain — mostly as diluted acid rain.

“We are imagining releasing sulphur dioxide, which is a gas, which would react with water vapour and oxidise into sulfuric acid, which then dissociates and part of that sulfuric acid is the sulphate aerosol, which this kind of small liquid droplet. They tend to produce a size distribution in the stratosphere, which makes them good reflectors of sunlight. Those sulphate aerosols then slowly sediment downwards through the stratosphere and ultimately once they re-enter the troposphere, the part of the atmosphere we live in, most of them rain out so they come out in water and as acid rain essentially.”

While the chemical processes are well understood, the engineering challenges are significant.

Delivering large volumes of sulphur dioxide safely at high altitude would require modifying existing aircraft or building entirely new ones.

Creating new specialist aircraft capable of reaching 20 kilometres would likely take a decade and billions of pounds in development costs.

Instead, the researchers believe adapting existing aircraft could provide a faster and cheaper option.

But even this would require careful redesign to allow for planes to safely store and release a toxic gas at high altitudes without posing risks to crew, passengers or the environment.

“In our case, if you were using existing aircraft, then there would still be a modification programme required. You’d need some way to vent the slipper dioxide and to carry it safely. It’s a toxic gas, right? If you release this at ground level, it could be quite harmful. So there are definitely big engineering challenges, but they will be less intensive than the higher altitude deployment.”

The UCL researchers behind the study stress that stratospheric aerosol injection would not be a substitute for cutting emissions and would carry serious risks if not carefully managed.

However, other researchers, such as Raymond Pierrehumbert, Professor of Physics at University of Oxford, are sceptical about the the risks posed by using geoengineering to limit the most dangerous impacts of climate change.

He says: "Carbon dioxide will continue to affect the climate and give us warming for thousands of years, but the stratospheric aerosols fall out in a matter of a year or so. And so if you get into a situation where you rely on it, where you're relying on stratospheric aerosol injection, you're really locking humanity into doing it without fail for centuries at least. And that's a very perilous situation to be in. And if you do it at a time when we haven't yet reached net zero, then you have to do more of it each and every year. And if you ever stop, you get hit in the face with massive catastrophic warming very quickly."

There are concerns that relying on aerosol injection could trap future generations into a risky, long-term commitment, with dangerous consequences if it’s ever interrupted.

"Among other things, when you deploy stratospheric aerosol injection, you can change atmospheric circulation patterns. And so this can do things like disrupt precipitation patterns, cause droughts in some places, cause excessive flooding in other places," cautions Pierrehumbert.

Groups from the U.S. National Academy of Sciences to the United Nations Environment Programme have looked at the ethics, side effects, legal complications and benefits of geoengineering with various degrees of skepticism and cautious interest.