.ON APRIL 2ND a gizmo resembling a snow machine on the deck of the USS Hornet, an invalid aircraft-carrier tied in San Francisco Bay, began making a mist of salted aerosol particles. The clinical partnership behind the job, the Marine Cloud Whitening (MCB) program, led by the Educational institution of Washington, is investigating whether such sprays could help to reduce temperature modification by means of a kind of "geoengineering". What performs this phrase way-- and why is it thus debatable?