.ON APRIL 2ND a device resembling a snowfall machine on the deck of the USS Hornet, an inoperative aircraft-carrier tied in San Francisco Gulf, started generating a haze of salty spray fragments. The clinical cooperation responsible for the job, the Marine Cloud Whitening (MCB) programme, led due to the University of Washington, is looking into whether such sprays might help to slow down weather improvement with a type of "geoengineering". What does this term method-- and why is it therefore disputable?