.ON APRIL 2ND a gizmo being similar to a snow device on the deck of the USS Hornet, a defunct aircraft-carrier tied in San Francisco Bay, began creating a mist of salty spray fragments. The scientific collaboration behind the venture, the Marine Cloud Brightening (MCB) programme, led by the University of Washington, is looking into whether such sprays might aid to reduce climate modification through a form of "geoengineering". What performs this condition mean-- and also why is it so controversial?