.ON APRIL 2ND a gizmo looking like a snowfall equipment on the deck of the USS Hornet, a defunct aircraft-carrier moored in San Francisco Bay, started producing a haze of salty spray fragments. The scientific partnership responsible for the job, the Marine Cloud Brightening (MCB) programme, led by the Educational institution of Washington, is actually looking into whether such aerosols might help to reduce temperature change through a type of “geoengineering”. What does this term method– and why is it therefore debatable?