.ON APRIL 2ND a contraption appearing like a snowfall machine on the deck of the USS Hornet, an obsolete aircraft-carrier tied in San Francisco Bay, started producing a mist of salty aerosol particles. The scientific cooperation behind the task, the Marine Cloud Whitening (MCB) program, led by the College of Washington, is looking into whether such aerosols could assist to slow temperature improvement via a kind of "geoengineering". What does this term way-- and also why is it so controversial?