.ON APRIL 2ND a device appearing like a snow equipment on the deck of the USS Hornet, a defunct aircraft-carrier tied in San Francisco Gulf, began creating a mist of salty spray particles. The medical cooperation behind the job, the Marine Cloud Whitening (MCB) programme, led by the College of Washington, is looking into whether such sprays might aid to slow down weather change with a form of "geoengineering". What does this term mean-- and why is it therefore questionable?