.ON APRIL 2ND a gizmo looking like a snowfall maker on the deck of the USS Hornet, a defunct aircraft-carrier tied in San Francisco Bay, started generating a mist of salty spray fragments. The clinical collaboration behind the job, the Marine Cloud Whitening (MCB) programme, led by the University of Washington, is checking out whether such aerosols might assist to slow temperature change via a form of "geoengineering". What performs this condition method-- as well as why is it thus controversial?