Anti-global warming atmospheric spraying programme ‘possible’, say engineers

Solar geoengineering 'remarkably inexpensive', but rogue states would be detected if programme was launched, scientists say

Purpose-built high-altitude aeroplanes would be required to deliver reflective sulphites to the stratosphere 20km above the surface of the Earth (Getty)

Re-engineering Earth’s atmosphere to reverse the trend of rising global temperatures has long been touted as a potential solution to catastrophic global climate change.

The idea involves spraying reflective sulphites 20km above the surface of the Earth, into the stratosphere, where the particles reflect sunlight back into space, preventing solar energy from warming our planet further.

If effective, the technology, known as stratospheric aerosol injection (SAI), could be used to offset the impacts of our continuing greenhouse gas emissions.

A new study examining only the process of delivering the sulphites to the stratosphere indicates an effective global effort is possible, and would be relatively inexpensive if purpose-built high-altitude aircraft were manufactured.

The study, published today in Environmental Research Letters, focuses on the hypothetical practicalities of a large scale project beginning 15 years from now with the aim of halving the projected increase in man-made temperature rises – also known as anthropogenic radiative forcing.

Dr Gernot Wagner, from Harvard University‘s School of Engineering and Applied Sciences, is a co-author of the study. He said: “Solar geoengineering is often described as ‘fast, cheap, and imperfect’.

“While we don’t make any judgement about the desirability of SAI, we do show that a hypothetical deployment program starting 15 years from now, while both highly uncertain and ambitious, would be technically possible strictly from an engineering perspective.

“It would also be remarkably inexpensive, at an average of around $2bn – $2.5bn (£1.5bn – £1.9bn) per year over the first 15 years.”

The team confirmed earlier studies indicating the low direct costs of potential stratospheric aerosol geoengineering intervention, but they said they arrived at those numbers with the help of aerospace engineering companies in specifying what the paper calls the ‘SAI Lofter (SAIL)’ – the means of delivering the sulphite payload to the stratosphere.

Co-author Wake Smith, a lecturer at Yale College and who held previous positions as chief executive of Pemco World Air Services (an aircraft modification company), chief operating officer of Atlas Air Worldwide Holdings (a global cargo airline), and president of the flight training division of Boeing, said new aircraft would be needed for such a project.

He said: “I became intrigued by the engineering questions around SAI and the many studies that purport to show that modified existing planes could do the job. Turns out that is not so.

“It would indeed take an entirely new plane design to do SAI under reasonable albeit entirely hypothetical parameters. No existing aircraft has the combination of altitude and payload capabilities required.”

He added: “We developed the specifications for SAIL with direct input from several aerospace and engine companies. It’s equivalent in weight to a large narrow body passenger aircraft. But to sustain level flight at 20 kms, it needs roughly double the wing area of an equivalently sized airliner, and double the thrust, with four engines instead of two.

“At the same time, its fuselage would be stubby and narrow, sized to accommodate a heavy but dense mass of molten sulphur rather than the large volume of space and air required for passengers.”

The team estimated the total development costs at less than $2bn for the airframe, and a further $350m for modifying existing engines.

The new planes would comprise a fleet of eight in the first year, rising to a fleet of just under 100 within 15 years. The fleet would fly just over 4,000 missions a year in year one, rising to just over 60,000 per year by year 15.

Dr Wagner said: “Given the potential benefits of halving average projected increases in radiative forcing from a particular date onward, these numbers invoke the ‘incredible economics’ of solar geoengineering. Dozens of countries could fund such a program, and the required technology is not particularly exotic.”

But despite the low costs, the authors say this should not reinforce the fear a rogue country or operator could feasibly launch a clandestine SAI program upon an unsuspecting world.

Mr Smith said: “No global SAI program of the scale and nature discussed here could reasonably expect to maintain secrecy.

“Even our hypothesised year one deployment program entails 4000 flights at unusually high altitudes by airliner-sized aircraft in multiple flight corridors in both hemispheres. This is far too much aviation activity to remain undetected, and once detected, such a program could be deterred.”

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Doubts have previously been raised about the impact of shooting millions of tonnes of sulphites into the atmosphere every year.

Concerns include the impact on the ozone layer, whether the reflective particles would appear to turn the sky white, and whether sulphites could cause localised climatic issues in certain parts of the globe, disrupting rainfall patterns.