.ON APRIL 2ND a gizmo looking like a snow device on the deck of the USS Hornet, a nonexistent aircraft-carrier tied in San Francisco Bay, started making a haze of salty aerosol particles. The scientific cooperation responsible for the venture, the Marine Cloud Whitening (MCB) programme, led by the University of Washington, is investigating whether such sprays could help to reduce climate change through a type of "geoengineering". What does this phrase mean-- as well as why is it thus questionable?