.ON APRIL 2ND a contraption appearing like a snow machine on the deck of the USS Hornet, an inoperative aircraft-carrier anchored in San Francisco Gulf, began creating a haze of salty aerosol bits. The medical partnership behind the project, the Marine Cloud Whitening (MCB) programme, led by the College of Washington, is actually exploring whether such sprays could help to slow temperature change by means of a form of "geoengineering". What does this term mean-- and also why is it so questionable?