.ON APRIL 2ND a device being similar to a snow device on the deck of the USS Hornet, a defunct aircraft-carrier tied in San Francisco Bay, started producing a mist of salty aerosol particles. The scientific cooperation behind the venture, the Marine Cloud Brightening (MCB) programme, led by the Educational institution of Washington, is examining whether such aerosols could aid to reduce climate adjustment with a type of "geoengineering". What does this term method-- as well as why is it therefore controversial?