.ON APRIL 2ND a contraption appearing like a snow device on the deck of the USS Hornet, an inoperative aircraft-carrier anchored in San Francisco Bay, started generating a mist of salty aerosol particles. The medical cooperation behind the venture, the Marine Cloud Whitening (MCB) program, led by the College of Washington, is investigating whether such sprays may assist to slow temperature modification by means of a kind of "geoengineering". What performs this condition way-- as well as why is it so controversial?