Stratospheric aerosol injection is a form of geoengineering that attempts to combat the effects of increased greenhouse gas concentration in the atmosphere, by spraying certain gases such as sulfur dioxide into the stratosphere, at an altitude of ~20km, using a variety of means. These gases would then reflect some of the incoming solar radiation, decreasing global temperatures, to counter the increase due to climate change. Such a proposal has met significant opposition, both over the potential health impacts of the aerosols, and due to the fear that this will allow governments and businesses to continue as normal, and cause significant environmental damage under the pretext that climate change has been solved.
Most proposed aerosols (sulfur dioxide, dimethyl sulfide, hydrogen sulfide, etc.) contain sulfur. This is because, when the gases are released into the stratosphere, they react with water vapour to produce sulfuric acid. The sulfuric acid then condenses on dust, forming larger particles in a process called coagulation. These particles are closest to the right size for scattering light (0.3μm). This reduces the percentage of light which reaches the Earth's surface, decreasing global temperatures.
The impacts of large-scale stratospheric aerosol injection can be modelled using data gathered from climate after large volcano eruptions, such as the 1991 eruption of Mount Pinatubo in the Philippines, which injected roughly 15 million tonnes of sulfur dioxide into the stratosphere, very similarly to proposed plans for aerosol injection. As a result, the average global temperature dropped by 0.6°C over the next 15 months. Imaging taken by NASA's ERBS satellite shows that while the initial plume of sulfur dioxide was localised, over the next 15 months the effect spread across almost the entire Earth, showing that wind patterns should be enough to distribute stratospheric aerosol if released in the right place with favourable conditions.
Modified aeroplanes are a contender for bringing the aerosol to the stratosphere; planes are trusted technology and are therefore much more reliable than other methods. Commercial aircraft such as the Boeing 747-400 and Boeing C-17 are readily available to purchase for as low as $12 million dollars (relative to the economic cost of global warming). Unfortunately, these planes are optimised for lower altitude flight, and their efficiency drops significantly the higher they fly. Supersonic bombers have also been suggested, as they would require relatively little modification, already being able to reach the required altitude and hold and drop the payload. However, these planes are far more expensive and hard to acquire than commercial aeroplanes, and for that reason, they are probably not ever going to be used for this purpose.
Airships have also been suggested as a method of stratospheric aerosol injection, and after modification have the potential to deliver aerosols to the stratosphere for a lower cost. A key innovation has been the use of ballonets, or internal bladders, which inflate or deflate with air, making the airship change altitude without dumping helium, a costly and wasteful process. However, this would be after significant modification with associated high R&D costs. One of the major problems is higher wind velocity at high altitude posing a greater problem for airships than planes, due to the airship's much greater surface area, leading to a much greater force applied on the airship, and potential problems with steering and even accidental crashing.
Rocket-powered gliders were also briefly considered, but were quickly realised to be so much more costly than other methods that they are no longer in the running. Rocket-powered gliders involved using rocket engines to get a glider with payload to altitude, release the payload, and then safely fly the glider back to the ground for the next run. Unfortunately, the cost of the rocket engines is too high to compete with planes and other methods.
Another method considered was repurposing military-grade guns found on battleships to fire the aerosol high into the stratosphere. Unfortunately, they were found to be one to two orders of magnitude more expensive than other methods. More experimental designs of gun, such as electromagnetic or hydrogen-gas powered guns, were also found to be more expensive than other methods, with the added complication that access to these guns is limited for non-military uses.
A more futuristic method, currently out of the reach of modern technology, is the construction of a "stratospheric tether", modelled on a space tether, but shorter. This would consist of a pipe that stretched up to at least 20 km above the ground, with a lighter-than-air platform at the top to keep the tether in the air. Sulfur dioxide could then be pumped through the pipe to the stratosphere. Even if we had the technology to achieve this, a leak in the tether could prove catastrophic, with potentially millions of tonnes of sulfur dioxide being released into the troposphere, causing acid rain with severe consequences for both wildlife and human structures.
According to a cost analysis published on the Institute of Physics online journal (see sources), modified aeroplanes would be able to deliver 1 million tonnes to an altitude of 20-30km for $1-3bn (since an operation like this and of this scale has not been attempted, estimates are very vague). Costs decrease if larger quantities are used, at roughly $5-8bn for 5 million tonnes. While airships might cost roughly $0.5bn less, the technology for them to be used for aerosol injection is not yet present, so they are not a feasible solution. Overall, modified planes seem to be the way to go for stratospheric aerosol injection.
Another concern with this method of geoengineering is that the sulfuric acid might slowly diffuse into the troposphere, and end up in the water cycle, causing acid rain and ocean acidification. If this happened, it would have serious adverse effects on wildlife, potentially killing trees, disrupting fish spawning by killing fish eggs, damaging coral and other shelled marine creatures, and soil acidification, harming crops, and causing food shortages. Some acid rain is expected as part of this solution, but it is hoped that by injecting the aerosols high enough into the atmosphere, as few particles as possible will make their way back to the ground.
Aerosol injection could also lead to decreases in precipitation worldwide, with summer monsoons being hit most severely. In addition, ozone depletion would increase, as the increased surface area of the particulate matter would lead to more areas where ozone-depleting reactions could take place, leading to a larger ozone hole and greater rates of UV light reaching Earth.
The greatest concern in general about geoengineering is that it will allow governments and companies to not do anything about environmental crises, under the pretext that they have been solved. Stratospheric aerosol injection will not prevent deforestation or overfishing, or many other current ecological catastrophes. In addition, carbon dioxide will still cause ocean acidification, leading to the death of marine life, even if the greenhouse effect has been countered by stratospheric aerosol injection, and carbon dioxide restrictions are likely to be relaxed in the event of a large geoengineering project.
Overall, due to current uncertainties over the efficiency and side-effects of stratospheric aerosol injection, it should not be attempted currently. However, it is important to note that the cost of stratospheric aerosol injection has been estimated to be less than 1% of the cost of monetary losses due to climate change, and for that reason, stratospheric aerosol injection should not be ruled out.
Sources:
Comments