.ON APRIL 2ND a gizmo looking like a snow equipment on the deck of the USS Hornet, an inoperative aircraft-carrier tied in San Francisco Bay, began making a haze of salted aerosol bits. The medical cooperation behind the venture, the Marine Cloud Whitening (MCB) program, led by the University of Washington, is looking into whether such aerosols may aid to reduce environment improvement with a type of "geoengineering". What performs this condition method-- and why is it thus controversial?