.ON APRIL 2ND a gizmo resembling a snowfall maker on the deck of the USS Hornet, an inoperative aircraft-carrier tied in San Francisco Gulf, began generating a mist of salted spray fragments. The medical collaboration behind the job, the Marine Cloud Whitening (MCB) programme, led due to the University of Washington, is actually exploring whether such aerosols may assist to decrease temperature adjustment with a kind of "geoengineering". What does this phrase mean-- and also why is it therefore debatable?