.ON APRIL 2ND a contraption looking like a snow machine on the deck of the USS Hornet, an inoperative aircraft-carrier tied in San Francisco Bay, started making a moisture of salty aerosol bits. The scientific partnership behind the venture, the Marine Cloud Brightening (MCB) program, led due to the University of Washington, is actually investigating whether such sprays might assist to slow down climate adjustment with a kind of "geoengineering". What performs this term mean-- and also why is it so controversial?