"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."

http://physicsworld.com/cws/article/news/35693