.ON APRIL 2ND a device looking like a snow machine on the deck of the USS Hornet, a nonexistent aircraft-carrier anchored in San Francisco Gulf, started creating a haze of salted spray fragments. The clinical partnership responsible for the venture, the Marine Cloud Whitening (MCB) program, led due to the Educational institution of Washington, is exploring whether such aerosols may aid to slow down weather change through a kind of "geoengineering". What does this term method-- and why is it thus controversial?