.ON APRIL 2ND a device looking like a snow maker on the deck of the USS Hornet, an inoperative aircraft-carrier anchored in San Francisco Bay, started producing a haze of salty spray fragments. The clinical collaboration behind the venture, the Marine Cloud Whitening (MCB) programme, led due to the Educational institution of Washington, is checking out whether such aerosols could aid to reduce temperature improvement with a form of "geoengineering". What performs this phrase mean-- and also why is it so questionable?