.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, began producing a mist of salty spray fragments. The clinical collaboration behind the task, the Marine Cloud Brightening (MCB) programme, led due to the Educational institution of Washington, is exploring whether such aerosols may assist to decrease temperature improvement by means of a form of "geoengineering". What performs this phrase method-- and also why is it so debatable?