.ON APRIL 2ND a gizmo looking like a snow device on the deck of the USS Hornet, a nonexistent aircraft-carrier moored in San Francisco Gulf, started making a mist of salty spray particles. The scientific collaboration responsible for the job, the Marine Cloud Brightening (MCB) programme, led due to the Educational institution of Washington, is looking into whether such aerosols may assist to slow climate change by means of a type of "geoengineering". What does this term mean-- and also why is it so disputable?