.ON APRIL 2ND a device looking like a snow maker on the deck of the USS Hornet, an obsolete aircraft-carrier tied in San Francisco Gulf, began producing a haze of salty spray particles. The medical collaboration responsible for the venture, the Marine Cloud Brightening (MCB) programme, led by the University of Washington, is actually exploring whether such sprays might help to reduce climate improvement via a form of "geoengineering". What does this term method-- and why is it so debatable?