Spraying aerosols to stop global warming would be too costly

A programme to reduce Earth's heat capture by injecting aerosols into the atmosphere from high-altitude aircraft is possible, but unreasonably costly with current technology, scientists say.

A study, published in the journal Environmental Research Letters, looked at the capabilities and costs of various methods of delivering sulphates into the lower stratosphere, known as stratospheric aerosol injection (SAI).

They examined the costs and practicalities of a large scale, hypothetical 'solar geoengineering' project beginning 15 years from now. Its aim would be to halve the increase in anthropogenic radiative forcing, by deploying material to altitudes of around 20 kilometres.

"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," said Gernot Wagner, from Harvard University in the US.

"It would also be remarkably inexpensive, at an average of around USD 2 to 2.5 billion per year over the first 15 years," Wagner said.

The researchers confirm earlier studies that discuss the low direct costs of potential stratospheric aerosol geoengineering intervention, but they arrive at those numbers with the help of direct input from aerospace engineering companies in specifying the 'SAI Lofter (SAIL)'.

"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," said Wake Smith, a lecturer at Yale College in the US.

"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," Smith said.

"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," he said.

The team estimated the total development costs at less than USD 2 billion for the airframe, and a further USD 350 million for modifying existing low-bypass 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.

However, this should not reinforce the often-invoked fear that a rogue country or operator might launch a clandestine SAI program upon an unsuspecting world.

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

"Even our hypothesised Year one deployment program entails 4,000 flights at unusually high altitudes by airliner-sized aircraft in multiple flight corridors in both hemispheres," he said.

"This is far too much aviation activity to remain undetected, and once detected, such a programme could be deterred," he added.