.ON APRIL 2ND a gizmo being similar to a snowfall equipment on the deck of the USS Hornet, an obsolete aircraft-carrier moored in San Francisco Gulf, started producing a haze of salted spray bits. The scientific partnership behind the venture, the Marine Cloud Brightening (MCB) programme, led by the University of Washington, is examining whether such sprays may assist to slow down environment improvement via a type of "geoengineering". What does this condition method-- and why is it therefore debatable?