According to UK and U.S. researchers, it should be possible to fight the global warming effects associated with an increase of dioxide levels by using autonomous cloud-seeding ships to spray salt water into the air. This project would require the deployment of a worldwide fleet of 1,500 unmanned ships to cool the Earth even if the level of carbon dioxide doubled. These 300-tonne ships 'would be powered by the wind, but would not use conventional sails. Instead they would be fitted with a number of 20 m-high, 2.5 m-diameter cylinders known as Flettner rotors. The researchers estimate that such ships would cost between £1m and £2m each. This translates to a US$2.65 to 5.3 billion total cost for the ships only. Even if this project has its merits, who will finance it? The scientists don't answer this question. But read more...
If they're built one day, these cloud-seeding ships will use the concept of the rotor ship developed by German engineer Anton Flettner, which is based "on the Magnus effect where a spinning body in a moving airstream experiences a force perpendicular to the direction of the airstream." (Credit: Wikipedia) Flettner built two sea-going ships. The first one, shown above, initially named Buckau, then renamed Baden-Baden crossed the Atlantic in 1926. Here is a larger version of this vintage image. (Credit: Wikipedia)
You can see above a conceptual Flettner spray vessel. "The wind would be blowing from the reader's right-hand side, the rotor spin would be clockwise seen from above and rotor thrust to the left. Vessels can also report sea and air temperatures, humidity, solar input, the direction and velocities of winds and currents, atmospheric pressure, visibility, cloud cover, plankton count, aerosol count, salinity, radio reception and could even rescue yachtsmen in distress." (Credits: John Latham et al. for the caption, John MacNeill for the picture).
This project has been led by Professor John Latham of the National Center for Atmospheric Research in Boulder, Colorado. Latham worked with colleagues at the University of Edinburgh led by Professor Stephen Salter. You can see another picture of these conceptual cloud-seeding ships on page 3 of a paper presented by Slater in October 2005, "Beyond carbon: consideration of albedo control technologies to mitigate climate change" (PDF format, 6 pages, 653 KB).
Now, let's discover the Latham proposal. It "involves increasing the reflectivity, or “albedo”, of clouds lying about 1 km above the ocean’s surface. The idea relies on the “Twomey effect”, which says that increasing the concentration of water droplets within a cloud raises the overall surface area of the droplets and thereby enhances the cloud’s albedo. By spraying fine droplets of sea water into the air, the small particles of salt within each droplet act as new centres of condensation when they reach the clouds above, leading to a greater concentration of water droplets within each cloud."
And here is another quote from the PhysicsWorld.com about these future cloud-seeding rotor ships. "These rotors would be easier to operate remotely than sails and would also serve as the conduits for the upward spray, with the spray consisting of droplets 0.8 µm in diameter generated by passing sea water through micro nozzles. The power for the spray and the cylinder rotation would be provided by oversized propellers operating as turbines. The immediate effect of seeding clouds in this way would be a local cooling of the sea surface, and as such the technique could be targeted at coral reefs, diminishing polar ice sheets or other vulnerable regions. However, the great thermal heat capacity of the ocean and the currents within it mean that these initial effects would eventually spread across the globe."
This research work has been published in the journal Philosophical Transactions of the Royal Society (PTRS) under the title "Sea-going hardware for the cloud albedo method of reversing global warming" on August 29, 2008. Here is a quote from the abstract. "The present paper describes in outline the rationale and underlying engineering hardware that may bring the strategy from concept to operation. Wind-driven spray vessels will sail back and forth perpendicular to the local prevailing wind and release micron-sized drops of seawater into the turbulent boundary layer beneath marine stratocumulus clouds. The combination of wind and vessel movements will treat a large area of sky. When residues left after drop evaporation reach cloud level they will provide many new cloud condensation nuclei giving more but smaller drops and so will increase the cloud albedo to reflect solar energy back out to space."
Here is a second excerpt describing the ships. "The vessels will drag turbines resembling oversized propellers through the water to provide the means for generating electrical energy. Some will be used for rotor spin, but most will be used to create spray by pumping 30kgs-1 of carefully filtered water through banks of filters and then to micro-nozzles with piezoelectric excitation to vary drop diameter. The rotors offer a convenient housing for spray nozzles with fan assistance to help initial dispersion. The ratio of solar energy reflected by a drop at the top of a cloud to the energy needed to make the surface area of the nucleus on which it has grown is many orders of magnitude and so the spray quantities needed to achieve sufficient global cooling are technically feasible."
Here is a link to the full text of this paper (PDF format, 18 pages, 1.75 MB). This article was included in a special issue of the PTRS journal named "Geoscale engineering to avert dangerous climate change" which carried another article co-authored by Latham, "Global temperature stabilization via controlled albedo enhancement of low-level maritime clouds."
Sources: Edwin Cartlidge, PhysicsWorld.com, September 4, 2008; and various websites
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