It looks like something out of a Dan Dare comic book, and it might just help to save the world. A scientist at the University of Edinburgh has devised a new weapon in the fight against global warming: a fleet of 1,500 unmanned sailing ships creating wakes that whiten clouds to reflect the heat of the Sun better.

The concept involves vessels powered by a radical rotary-sail technology that could patrol selected areas of ocean, spraying tiny droplets of seawater into existing clouds. The droplets increase the surface area and so whiten the cloud, bouncing more radiation back into space and offsetting the warming caused by burning fossil fuels.

“The beauty of the system is that it runs on wind and seawater,” said Stephen Salter, author of a paper published today in the Royal Society's Philosophical Transactions. “You can apply the effect locally, to cool down the Arctic or the seas around coral reefs. It would give us complete control. We could even take ourselves back to a little ice age. The effects can be turned up or down, or shut off completely if something unexpected happens.”

The cloud ships will be propelled by the wind, using a rotational aerodynamic force not used in ships for 80 years. The “Magnus Effect” was first observed by Sir Isaac Newton while watching tennis players use spin to change the trajectory of their shots. In 1926 a rotor-ship designed by Anton Flettner crossed the Atlantic, but the technology petered out in the Great Depression. Modern materials and the high cost of oil have sparked a revival: earlier this month Enercon, a German energy company, launched the first rotor-powered cargo vessel.

“The main reason for us to use these rotors is that they are computer-friendly,” said Dr Salter. “Traditional sailing ships have evolved to be sailed by humans. It's much easier to sail a Flettner system. All you need to do is steer and adjust the rotor speed. Reverse the spin and you go backwards.”

The spinning sails deliver surprising power. The cloud ships will cruise at gentle speeds of eight knots while spraying, but when moving location or running from bad weather, the vessels are theoretically capable of up to 24 knots - fast even for a racing yacht. A back-up diesel engine can also help to bring the ships, costing £1million to £2million each, safely back to port.

Propeller-like turbines in the water beneath the ship power both the spinning rotors and the droplet-generator. Seawater is filtered before being forced through a 6in diameter disc perforated with more than a billion holes to produce a mist of droplets less than one micron wide. These seeds - or cloud condensation nuclei - are then blown into the skies via a fan mounted inside the rotor cylinders.

The 300-tonne cloud ships will be guided from a central traffic control-room. “Suitable sites for spraying have lots of sunlight to give something to reflect, have reliable but not extreme winds and a low density of shipping and icebergs,” Dr Salter said. Dirt, dust or pollution in the air act as nuclei, and for the ships to make a difference they need to operate away from such conditions. The seas off California, Namibia and Peru show year-round promise, while the Southern Ocean will be a key area in the Antarctic summer.

A companion paper published in the same Royal Society issue shows that the change in the brightness of marine clouds could cool the planet enough to compensate for the doubling in man-made carbon dioxide since the industrial revolution. A reduction of only 3.7 watts per square metre - less than 1.1 per cent of the 340 watts of heat per square metre that the Sun on average provides - would keep global temperatures stable until at least 2050.

Dr Salter estimates that £20 million is needed to move the technology and the science to a point where production of the vessels can begin. Once the ships are in the water, they will do double duty as science labs, collecting meteorological data on the actions of aerosols and information on ocean salinity, plankton counts and acidity.

“The boats will also be equipped with blankets and drinking water,” says Dr Salter. By linking into the maritime emergency services, the cloud ships could then come to the rescue of stricken sailors, not just the planet.

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