Combating the effects of climate change by spraying aerosols into the atmosphere is a contentious topic, with many rightfully pointing out the risk of unintended side effects of such large-scale climatic intervention. But with the heads of so many governments around the world still firmly stuck in the sand on the realities of climate change, the prospect of increasingly desperate times calling for increasingly desperate measures becomes more and more conceivable. But putting aside the issues of should we or shouldn't we, or the prospects for success or failure, would such an approach even be practically achievable? A new study says yes.

Of the two main categories of geoengineering techniques, solar radiation management is the most contentious (the other being the removal of greenhouse gases from the atmosphere). Solar radiation management would basically involve increasing the reflectivity of the planet, likely by injecting sulfates into the stratosphere, to reduce the amount of solar radiation the Earth absorbs. Known as stratospheric aerosol injection (SAI), this technique is designed to mimic the temporary climatic effects of volcanic eruptions, such as the Mount Pinatubo eruption in 1991 that caused a drop on global temperatures of around 0.5° C (0.9° F). It was the practicality of SAI that Wake Smith from Yale and Gernot Wagner for Harvard examined in their study.

The pair examined the practical requirements and costs of deploying an SAI project 15 years in the future to halve the increase in human-induced radiative forcing, which is the difference between the amount of solar energy absorbed by the Earth and the amount reflected back into space. The hypothetical project would involve delivering sulphates into the atmosphere at an altitude of around 20 km (12 mi). Taking input from aerospace engineering companies, they found such a project would be technically and economically feasible.

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

The researchers point out that these calculations only include development and direct operating costs, not indirect costs associated with any monitoring or measuring of the impacts of such a program. The development costs also include the design and manufacture or an entirely new aircraft, despite previous studies claiming that, with some modifications, existing aircraft could do the job.

"Turns out that is not so," says Smith. "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."

The proposed aircraft would be equivalent in weight to a large narrow body passenger aircraft, but would need roughly double the wing area, double the thrust, and four engines instead of two, to achieve sustained flight at 20 km, as opposed to the roughly 10 km most airliners generally fly at. To carry a heavy, dense mass of molten sulphur matter instead of the large volume of space and air required for passenger aircraft, Smith says the fuselage of the hypothetical SAI plane would also need to be stubby and narrow.

The study estimates total development costs of such an aircraft would be less than $2 billion for the airframe, with an additional $350 million required to modify existing low-bypass engines. The program would start with a fleet of eight aircraft in the first year, increasing to just under 100 within 15 years. In the first year, just over 4,000 missions would be flown, rising at a rate of 4,000 a year to just over 60,000 annually by the 15th year.

Despite showing such a program is technically possible using today's technologies, and at a cost that would be within the reach of numerous countries, Wagner and Smith say there is little chance that a rogue state could secretly carry out such a project.

"No global SAI program of the scale and nature discussed here could reasonably expect to maintain secrecy," says Smith. "Even our hypothesized year one deployment program entails 4,000 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."

So, with the study apparently showing the economic and engineering feasibility of SAI, the question is not could we, but should we, and would it actually work?

The team's paper is published in Environmental Research Letters.

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