Nov 182010
 
Clouds above Pacific. The picture also shows a...
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Here’s an idea to cool Earth: make marine clouds into better reflectors of sunlight. After all, clouds already reflect more of the sun’s radiation back into space than the amount trapped by human emissions of carbon dioxide. So why not make them even more effective?

Enter “marine cloud brightening,” a geoengineering scheme that would increase cloud reflectivity over the ocean by spraying them with an ultrafine saltwater mist from ships. The clouds, containing more particles, would cast enough sunlight back into space to at least partially offset the warming effects of all that CO2 from burning fossil fuels.

Climate scientists have mixed reactions to the idea, which was first proposed in 1999 by climatologist Jonathan Latham of the National Center for Atmospheric Research in Boulder, Colo. For instance, Andy Jones and colleagues at the Hadley Center for Climate Prediction and Research, Met Office in the U.K. think that if marine cloud brightening were deployed in the Atlantic Ocean, it might help turn the Amazon rainforest into a desert by cooling the South Atlantic, which would lead to less evaporation from the ocean, thus reducing rainfall over the forest. A new study however, has found that marine cloud brightening would enhance rainfall over land, counters Latham, who did not want to identify the authors because the paper has yet to be published.

Nevertheless, a growing number of engineers have already begun to think about how marine cloud brightening could actually be accomplished. A handful of obstacles to the realization of Latham’s vision have been identified, chief among them the problem of creating hundreds of gallons per minute of sea spray, but the engineers involved are optimistic.

How “albedo yachts” could work

Stephen Salter, an emeritus professor of engineering design at the University of Edinburgh is the lead and was for many years the only engineer working on a proposal to accomplish marine cloud brightening by populating the world’s oceans with up to 1,500 ships of a somewhat exotic design—sometimes known as “albedo yachts”. Each vessel would be remote-controlled, wind-powered, and capable of generating (via turbines dragged through the water) the electricity required to create a mist of seawater and loft it 1,000 meters into the atmosphere.

Instead of using sails, Salter’s ships would transform wind energy into thrust via the use of Flettner rotors. First used on a ship in 1926, Flettner rotors consist of spinning tubes resembling oversize masts on a conventional sailing vessel. The tubes interact with wind in much the same way the wing of an airplane does, generating “lift” perpendicular to the axis of their rotation. Flettner rotors are a proved, if rarely used, technology because they were not invented until the twilight of wind-powered shipping, when propellers and fossil fuels became the propulsion mechanism of choice. Salter chose them, however, because they can be powered by a renewable resource, they’re easy to control with an onboard computer, they’re better in hurricane winds than conventional sails—very necessary for an unmanned craft wandering the seas for years—and they provide a natural place to house the vessel’s seawater spraying system.

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