Injecting aerosols into the stratosphere at 25 km altitude to mitigate global warming is not as cost efficient as injection at 20 km. That is the conclusion of scientists in the US, who have looked at five possible ways that the aerosols could be delivered to the stratosphere – including modifying a supersonic reconnaissance aircraft and firing “mortar shells” of material from a modified passenger jet aircraft.
Solar geoengineering offers a way of mitigating the effects of global warming by reflecting incoming sunlight back into space – thereby cooling the Earth. One way of doing this is to inject aerosols into the atmosphere. We know that this works because of the historical cooling that occurred after major volcanic eruptions, which injected vast amounts of material into the atmosphere.
The lower portion of the atmosphere (the troposphere) is turbulent with air moving up and down, so scientists believe that aerosols should be injected into the much calmer stratosphere at about 20 km or above. There, aerosols are expected to persist for a much longer time than they would in the troposphere.
In 2018 Wake Smith at Yale University and Gernot Wagner at Harvard University proposed the SAIL-1 system, which would use special large-winged aircraft to disperse aerosols at 20 km. To maintain a 1 °C cooling effect, this would cost about $43bn per year to operate.
Longer lasting
Studies have suggested that aerosols injected at about 25 km would persist for significantly longer – and therefore this could be a better strategy for solar geoengineering. This altitude, however, is right at the limit of advanced reconnaissance aircraft so getting the material up there in a safe and cost-effective way would be a major technological challenge.
Now, a team led by Smith has looked at five different ways of injecting aerosols at 25 km to see if the increased costs could be offset by higher performance. The first option is a rocket assisted version of SAIL-1; the second is a supersonic ballistic climber derived from the F-15C fighter jet; and the third is a two-stage mothership and rocket-powered drone that are modelled on Virgin Galactic technology. The fourth option is based on the Lockheed SR-71 Blackbird supersonic reconnaissance aircraft and the fifth involves using a Boeing 747–400 cruising at 12 km as a platform to hurl aerosol-containing mortar shells to 25 km.
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For all five of these options, Smith and colleagues concluded that the cost of lofting a tonne of material to 25 km would be least three times that of SAIL-1 operating at 20 km. After considering the extra cooling that would be gained by reaching 25 km, the team found that none of the options is more cost effective than SAIL-1 at 20 km. The rocket-assisted SAIL-1 option was the cheapest, costing about $71bn per year to achieve a 1 °C cooling effect. The Boeing 474 mortar platform proved to be the most expensive option, costing nearly 20 times more than SAIL-1 at 20 km.
All the options would require fleets of aircraft numbering in the thousands, and each aircraft would fly one or more sorties per day. Given that the aircraft would be flying under extreme conditions, the safety of pilots and people on the ground is another important factor that was investigated by the researchers. They found that the two-stage mothership and rocket-powered drone had the lowest operational safety concerns of the 25 km options. They also point out that working prototypes of both the stages are already operational. As a result, the team describes this option as the “winning concept”, but only marginally.
The research is described in Environmental Research Communications.
