Since I'm the one who started everybody? off on the airplane idea, here are some thoughts about these analyses from AGU as described by Oliver Morton. First, the Overworld Stratosphere, the part where aerosol transport is controlled by the Brewer-Dobson circulation begins at around 53,000 ft. None of the subsonic heavy lift aircraft or refueling tankers can get that high. Near the poles, the Lowermost Stratosphere (LS) is at its lowest, around 30,000 ft at certain times of the year. Aerosols in this region of the atmosphere are governed by other processes, namely weather fronts and tropospheric folding (which is also associated with weather systems). The article and presumably the presentation blurred the distinction between them making it sound as if they were one and the same. Aerosol lifetimes in the Overworld are around a year and in the LS, a few weeks to months, although that has to be firmed up also. It is also uncertain what the lifetimes of Overworld aerosol would be if created closer to the Poles as the one year lifetime is based on tropical volcanic eruptions.
Also, the KC-135 is based on a different design than the 707, but they do look very similar. I proposed the KC-135 and the F-15C because the planes exist today in significant numbers, are to be retired soon? and could be modified rather quickly for use. I also am on record as stating that they would only be considered a stopgap themselves, until something else could be found or built. I'm not sure what David Keith would consider to be a serious geoengineering scheme. I also didn't know that the millionaire thrill ride plane from Rutan Entertainment was going to be able to carry cargo. It is intended to carry the space plane underneath. It is also unclear when such a plane could or would be built in numbers sufficient to accomplish the mission. But there's always hope. I would predict that Rutan's design would look suspiciously like an F15, however as there are reasons why most of the large supersonic aircraft are very similar in appearance. Finally, White Knight Two has a designed service ceiling of 60,000 ft, about 5000-10,000 ft lower than the F-15C Eagle and has a projected payload lift capacity of about twice the Eagle, but a ceiling on that of 50,000 ft, making it problematic as a lift vehicle for stratospheric precursor gas. In an emergency, break glass, don't look in a catalog. http://en.wikipedia.org/wiki/Scaled_Composites_White_Knight_Two http://blogs.nature.com/news/blog/2008/12/agu_geoengineering_costs.html AGU: Geoengineering costs How much would it cost to dim the sun a little with a dusty layer of aerosol particles in the stratosphere? The service comes for free if you can find an obliging volcano, like Mount Pinatubo, but they can hardly be relied on in the long term. Some schemes for doing it to order, though, could be pretty cheap, according to an analysis by Alan Robock and colleagues at Rutgers. In the 1990s a National Research Council panel in the US estimated the costs of delivering dusty particles ito the stratosphere from big guns like those on old battleships. The panel came up with a figure of $30 billion a year: a lot cheaper than most proposals for carbon cutting, but still a fair chunk of change. Robock looked at the costs of getting into the stratosphere by the more orthodox means of aircraft. Near the poles, where the bottom of the stratosphere comes closest to the ground, big aircraft like Air Force tankers can get high enough to inject aerosols. Robock calculated that getting a billion tonnes of sulphur up to the stratosphere would take just three flights a day by each plane in a nine or fifteen plane squadron (nine if you use KC-10s, which are basically LD11 TriStars, fifteen if you use KC-135s, derived form the original design of the Boeing 707; plane spotting interlude ends here). That represents a purchase price of a billion or so and operational costs of well under $100 million. If you want to take the sulphur higher, Robock says, think about F15-C Eagles (now we're talking...). With the smaller planes you need something more like a whole wing than a squadron -- 167 planes doing three flights a day. That's a purchase cost of about $6 billion, and an ops cost more like a billion a year. As David Keith of the University of Calgary points out, though, no serious geoengineering scheme would really do this. Among the many hurdles such a scheme might face, designing planes optimised to its needs rather than buying them off the peg (or at the Air Force surplus store) is a no-brainer. For an example of the sort of thing you might go for, Keith points to the White Knight Two, a jet which will be put to use hauling Virgin Galactic's SpaceShip 2 up into the stratosphere before disengaging so that the spacecraft's rockets can take it up into space. White Knight Two is a lot less noisy and environmentally obnoxious than an Eagle, gets higher into the stratosphere and carries more cargo. If you want a sulphur deliverer, start from something like that and ask designer's like White Knight 2's Burt Rutan to optimise it for the task at hand. Not all geoengineering requires aircraft. John Latham of NCAR has long touted a scheme for making the clouds over the ocean thicker and more reflective by kicking up particles of sea salt that will cause more droplets to form in them. He and his colleagues think that if you could build a specialist sea-salt-kicking-up ship every week, for a cost of a maybe three million dollars, that would be enough to offset that week's carbon emissions. So you need a fleet that grows at a rate of about 50 ships a year. But Daniel Rosenfeld of the Hebrew University, Jersualem, who has devoted his life to studying various forms, thinks that might be overkill. According to his analysis, if you pick the right places to seed the clouds (he didn't call them tipping points, and neither will I, but the idea is not dissimilar) you could create enough added reflection to counterbalance two degrees of global warming with just 50 specially designed fast hydrofoils. That would be cheaper than the cheapest aircraft option, for a far more powerful effect. None of this, as Robock in particular is keen to point out, means that a schemes that made use of such techniques would work as advertised and be a good idea. There may well be all sorts of other reasons why such schemes are a bad idea. But there do seem to be some pretty low cost options around. Posted by Oliver Morton on December 19, 2008 08:04 PM | Permalink --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "geoengineering" group. 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