.ON APRIL 2ND a contraption resembling a snowfall machine on the deck of the USS Hornet, an inoperative aircraft-carrier tied in San Francisco Bay, began creating a haze of salted spray particles. The medical collaboration behind the project, the Marine Cloud Brightening (MCB) programme, led due to the University of Washington, is actually exploring whether such aerosols may aid to decrease temperature improvement by means of a form of "geoengineering". What performs this condition mean-- and why is it thus controversial?