.ON APRIL 2ND a device looking like a snowfall device on the deck of the USS Hornet, a nonexistent aircraft-carrier moored in San Francisco Bay, began making a haze of salted spray bits. The scientific cooperation responsible for the task, the Marine Cloud Brightening (MCB) programme, led due to the Educational institution of Washington, is exploring whether such aerosols could help to slow climate improvement by means of a kind of "geoengineering". What does this term method-- as well as why is it therefore controversial?