.ON APRIL 2ND a device looking like a snowfall equipment on the deck of the USS Hornet, a nonexistent aircraft-carrier moored in San Francisco Gulf, began creating a haze of salty spray bits. The medical collaboration responsible for the task, the Marine Cloud Brightening (MCB) programme, led by the Educational institution of Washington, is exploring whether such sprays could assist to slow down temperature change through a kind of "geoengineering". What does this term method-- and also why is it thus disputable?