.ON APRIL 2ND a device appearing like a snowfall machine on the deck of the USS Hornet, an invalid aircraft-carrier anchored in San Francisco Gulf, started making a haze of salty aerosol bits. The scientific partnership responsible for the job, the Marine Cloud Brightening (MCB) programme, led by the College of Washington, is examining whether such sprays might help to slow down temperature modification by means of a form of "geoengineering". What performs this term mean-- and also why is it thus controversial?