.ON APRIL 2ND a gizmo looking like a snow maker on the deck of the USS Hornet, a nonexistent aircraft-carrier moored in San Francisco Gulf, started making a haze of salty spray fragments. The clinical partnership behind the venture, the Marine Cloud Brightening (MCB) programme, led due to the College of Washington, is exploring whether such aerosols might assist to slow down environment change via a kind of "geoengineering". What does this phrase method-- and also why is it therefore disputable?