.ON APRIL 2ND a gizmo appearing like a snow machine on the deck of the USS Hornet, an invalid aircraft-carrier moored in San Francisco Bay, began creating a haze of salty spray fragments. The clinical cooperation behind the task, the Marine Cloud Whitening (MCB) programme, led due to the College of Washington, is actually checking out whether such sprays may aid to slow temperature change by means of a type of "geoengineering". What does this phrase method-- and why is it therefore disputable?