.ON APRIL 2ND a contraption looking like a snow maker on the deck of the USS Hornet, an inoperative aircraft-carrier moored in San Francisco Bay, started making a mist of salty aerosol bits. The scientific collaboration responsible for the job, the Marine Cloud Whitening (MCB) program, led by the University of Washington, is actually exploring whether such aerosols could aid to slow weather improvement with a kind of “geoengineering”. What performs this term way– and also why is it so controversial?