.ON APRIL 2ND a contraption being similar to a snow device on the deck of the USS Hornet, an inoperative aircraft-carrier anchored in San Francisco Bay, began generating a moisture of salty aerosol bits. The clinical collaboration behind the project, the Marine Cloud Whitening (MCB) program, led by the University of Washington, is looking into whether such aerosols might help to reduce temperature improvement by means of a kind of "geoengineering". What does this phrase way-- and also why is it so controversial?