.ON APRIL 2ND a contraption resembling a snow maker on the deck of the USS Hornet, a defunct aircraft-carrier tied in San Francisco Gulf, started producing a mist of salted spray fragments. The clinical collaboration behind the venture, the Marine Cloud Whitening (MCB) program, led by the University of Washington, is exploring whether such aerosols might assist to slow temperature improvement by means of a form of "geoengineering". What does this term mean-- as well as why is it so questionable?