.ON APRIL 2ND a contraption resembling a snowfall maker on the deck of the USS Hornet, a defunct aircraft-carrier tied in San Francisco Bay, started producing a haze of salty aerosol particles. The scientific cooperation behind the job, the Marine Cloud Whitening (MCB) programme, led due to the Educational institution of Washington, is exploring whether such sprays might help to decrease climate change via a type of “geoengineering”. What does this term mean– and why is it thus controversial?