.ON APRIL 2ND a device looking like a snow maker on the deck of the USS Hornet, an inoperative aircraft-carrier anchored in San Francisco Bay, started making a mist of salty aerosol particles. The scientific partnership behind the venture, the Marine Cloud Brightening (MCB) programme, led by the University of Washington, is actually checking out whether such sprays could aid to slow down climate adjustment through a form of "geoengineering". What performs this term mean-- and why is it thus disputable?