.ON APRIL 2ND a gizmo resembling a snowfall maker on the deck of the USS Hornet, an obsolete aircraft-carrier tied in San Francisco Gulf, started creating a mist of salty aerosol particles. The scientific cooperation behind the project, the Marine Cloud Brightening (MCB) program, led due to the Educational institution of Washington, is investigating whether such aerosols could aid to slow down weather change through a form of "geoengineering". What does this term mean-- and why is it so debatable?