.ON APRIL 2ND a gizmo resembling a snow maker on the deck of the USS Hornet, a defunct aircraft-carrier tied in San Francisco Bay, started making a mist of salty spray bits. The scientific partnership behind the task, the Marine Cloud Whitening (MCB) program, led by the Educational institution of Washington, is investigating whether such sprays could help to slow climate improvement via a form of "geoengineering". What does this condition way-- and why is it so controversial?