.ON APRIL 2ND a gizmo being similar to a snow machine on the deck of the USS Hornet, a defunct aircraft-carrier tied in San Francisco Gulf, started generating a mist of salty aerosol bits. The clinical partnership behind the task, the Marine Cloud Brightening (MCB) programme, led by the University of Washington, is actually checking out whether such sprays might help to slow weather improvement with a type of “geoengineering”. What does this term method– and why is it therefore debatable?