.ON APRIL 2ND a device looking like a snow machine on the deck of the USS Hornet, an invalid aircraft-carrier anchored in San Francisco Bay, started making a haze of salted aerosol bits. The medical collaboration responsible for the job, the Marine Cloud Brightening (MCB) programme, led due to the University of Washington, is looking into whether such aerosols could aid to slow down climate adjustment with a kind of "geoengineering". What does this term method-- and also why is it so controversial?