Bill Gates' "StratoShield" would later be rebranded as David Keith's SCoPeX "StratoCruiser."
Called a Stratospheric Shield, or StratoShield for short, the system would deliver sulfur dioxide to an altitude of 30 kilometers in liquid form, through a very long hose supported by large, long-duration balloons. At the top of the hose, a series of atomizers would disperse the liquid into a fine mist of aerosol particles, each about 100 nanometers in diameter.
What is the StratoShield?
The StratoShield is one possible way to respond to a climate emergency in which greenhouse warming becomes intolerable. The StratoShield would reverse greenhouse warming by slightly reduc¬ing the amount of solar radiation that hits the Earth. The shield does this by increasing the amount of sulfur aerosols injected into the atmosphere by about 1%, a process that happens naturally whenever volcanoes erupt. The aerosols reflect incoming sunlight back into space. Although the change in sunlight would be imperceptible to human eyes—and probably beneficial for plants—it would have a substantial cooling effect for the part of the Earth under the shield.
How much aerosol would the StratoShield put into the stratosphere?
The reference system we’re studying would inject 100,000 metric tons of sulfur dioxide a year into the stratosphere, which at a constant flow rate works out to only about 34 gallons (130 liters) a per minute. About 100 million tons of sulfur dioxide already rises into the stratosphere each year, about half from manmade sources (such as power plants) and half from natural processes (such as volcanoes) . One StratoShield installation would thus increase annual aerosol input to the atmosphere by about one part in 1,000. Scientific studies so far have concluded that a worldwide system (which would require a dozen or more StratoShield installations) would probably have to spread several million metric tons a year of sulfur dioxide throughout the stratosphere to reduce solar radiation hitting the entire planet by about 1.8% (4 W/m²) globally. Climatologists believe that small reduction in sunlight would be adequate (if it occurred equally around the globe) to counter all of the warming caused by a doubling of CO₂ over preindustrial levels. A StratoShield placing 100,000 metric tons of aerosol a year into the upper atmosphere would be expected to reduce incoming solar radiation by less than half a watt per square meter, averaged over the globe. More research is needed to confirm these estimates.
What is the aerosol made of?
The aerosol would likely be made of sulfur dioxide (SO₂), a natural component of volcanic ash that is present in the air we all breathe every day. Another possibility is to use SO3 instead. Engineered aerosols, not found naturally in the atmosphere, could be more efficient at reflecting certain parts of the solar spectrum, but their benefits over SO₂ might not be worth the cost of development and production—or the uncertainties about their environmental effects. Science has produced a good understanding of both the global sulfur cycle (which includes volcanic ash) and the safety of sulfur dioxide at the very low concentrations required for geoengineering. A good deal more research would be required to establish the safety and environmental life cycle of customized aerosol particles.
Why not just use airplanes to disperse the aerosols?
Others have proposed this approach; we also gave it serious consideration. We concluded that airplanes may not be the best solution, for a number of reasons. Some existing military aircraft do fly high enough to reach the stratosphere, and in principle could be re-tasked to deliver sulfur-bearing aerosols in the event of a climate emergency—which would after all constitute a threat to international security. Calculations so far suggest the operating costs to use aircraft could be quite high, however, and if the required altitude for aerosol injection is beyond the bottom of the stratosphere (due to stratospheric wind patterns), the cost would go up dramatically.
A second concern with using military aircraft as delivery vehicles is the emissions of carbon dioxide and other greenhouse gases that they would produce, exacerbating the very problem they were deployed to solve. If fighter jets were used, 167 jets would each have to make three flights a day, 250 days a year to deliver the amount of aerosol required, according to one recent study [Robock et al. 2009].
A related, more promising idea is to adjust the fuel mixture in commercial airplanes to generate the needed aerosols in their ex¬haust (rather than flying a cargo hold full of aerosols). Unfortunately, this option would reduce their fuel efficiency and is not likely to be accepted by stakeholders in commercial airplane operations.
Aren’t there other ways of achieving the same effect?
There are many other ways of enhancing Earth’s albedo to reduce average global insolation. I.V. has been collaborating with Professors John Latham and Stephen Salter on one very promising idea of theirs to increase marine cloud cover by spraying salty sea water into the air. The small droplets would serve to nucleate more clouds, which increases the albedo of that area. U.S. Secretary of Energy Steven Chu has advocated painting roofs white to increase their reflectivity. Our inventors have begun exploring ways to brighten ground cover such as asphalt by, for example, incorporating crushed glass into the mix.
Many of these ideas will no doubt prove ineffective or impractical for one reason or another when they are fully studied, but there does seem to be a wide array of options still to explore. It is an area ripe for invention.
What is the lifetime of the aerosols in the stratosphere?
The eruption of Mt. Pinatubo in 1991 gave us an opportunity to learn many things about using sulfur-based aerosols to cool the Earth. The aerosols it spewed into the stratosphere remained there for an average of 1-2 years before falling down through the troposphere.
Why are you building this now?
We are not building or even planning to build the StratoShield. Intellectual Ventures is simply urging that research on geoengineering options, including stratospheric aerosol enhancement, begin in earnest now. We share with many others a concern that the massive scale of technological development, deployment, investment, and lifestyle changes required to bring greenhouse gas levels down to sustainable levels will take more time to implement than we have before the climate starts changing in intolerable ways.
If that happens, geoengineering options could buy humanity additional time to complete the shift to a cleaner energy system. The solution to the problem of climate change is new energy systems, not geoengineering. But we may find that we need geoengineering technologies as stop-gap responses if the transition to these cleaner energy systems takes too long, or if abrupt changes in climate occur unexpectedly.
Why did you choose this idea to study?
If the world decided that it had to use geoengineering as a stop-gap solution, the goal would be to deploy it quickly but also to phase it out relatively quickly. That leads us to prefer geoengineering approaches that are less expensive and that require little or no new technology, so are easier to deploy quickly. It also leads us to prefer approaches whose cooling effects are well understood and readily controlled, and which dissipate quickly once the system is turned down or turned off.
The StratoShield is an example of a geoengineering system that draws on existing technology and has deployment and annual operation costs amounting to millions of dollars, rather than billions. Although we have explored the general principles of how a system like this would operate, many technical details would have to be worked out. The detailed R&D is not something that IV currently contemplates doing, although if a responsible research program on geoengineering is launched, we may participate and collaborate with others in inventing and refining a variety of technical options.
In concert with technical development, a great deal of environ¬mental science must be done to identify possible side effects. There may be work-arounds to avoid some side effects, but others could be show-stoppers. Much more intellectual effort needs to be applied to this area so that a body of scientific and engineering knowledge exists, should it ever be needed to address a climate emergency. 
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