.ON APRIL 2ND a contraption appearing like a snow machine on the deck of the USS Hornet, an invalid aircraft-carrier moored in San Francisco Gulf, began producing a haze of salty aerosol fragments. The medical cooperation responsible for the project, the Marine Cloud Brightening (MCB) programme, led due to the College of Washington, is investigating whether such aerosols might assist to slow environment improvement through a kind of "geoengineering". What does this phrase method-- and why is it so debatable?