Climate ‘geoengineering’ technologies, where particles are sprayed into the stratosphere to deflect more sunlight away from a heating Earth, have a height problem.
Recent studies have suggested spraying aerosols into the atmosphere at huge heights – 15 miles up – to deflect incoming sunlight.
But a new report warns that this will increase costs and make the endeavour less practical, as even high-flying spy planes only fly at 12 miles above ground.
Wake Smith, the lead author of the study, says: "This conclusion should alter how climate intervention models are run globally and shows that practical limits need to be weighed against radiative efficacy in designing solar geoengineering programs."
"There is a ceiling in the sky above which traditional aircraft cannot operate, and 25 km is above it."
The study was published in Environmental Research Communications.
Several recent studies showed that that deployment of stratospheric aerosols at an altitude of 25 km (15 miles) would be more effective than at 20 km (12.4 miles).
Normal planes and military jets routinely cruise around six miles up, whereas 12 miles is the realm of high-flying spy planes and drones.
Planning to fly hundreds of thousands of annual solar geoengineering deployment flights to altitudes inaccessible even to elite spy planes would substantially increase costs.
It would also pose unacceptable safety risks for flight crews, aircraft, and the uninvolved public on the ground.
The new report responds directly to a question posed by the US National Academy of Science, Engineering, and Medicine in a landmark study in March 2021, which recognised the need for additional research on the viability of depositing aerosols well above 12 miles.
The idea of ‘solar geoengineering’ or solar radiation management (SRM) is controversial, mimicking the world-chilling effects of huge volcanic eruptions.
Some scientists have suggested that such technology could be used as a ‘stop gap’ to reduce temperatures while measures to limit CO2 emissions are put in place.
But others have suggested that when the SRM was withdrawn, it could lead to rapid global warming in a phenomenon known as ‘termination shock’.
One project investigating the idea involves billionaire Microsoft founder Bill Gates and top scientists from Harvard.
The Stratospheric Controlled Perturbation Experiment (SCoPEx) will see carbonate dust released into the atmosphere.
The researchers suggest that jets would complete more than 60,000 missions in 15 years, starting with a fleet of eight and moving up to 100 planes.
At present, there are no aircraft capable of doing this, so they would need to be developed.
The researchers previously wrote: "Dozens of countries would have both the expertise and the money to launch such a program.
"Around 50 countries have military budgets greater than $3 billion, with 30 greater than $6 billion."
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