.ON APRIL 2ND a device resembling a snow maker on the deck of the USS Hornet, an inoperative aircraft-carrier anchored in San Francisco Gulf, began creating a haze of salty aerosol bits. The medical cooperation responsible for the project, the Marine Cloud Brightening (MCB) programme, led by the Educational institution of Washington, is actually exploring whether such aerosols might assist to slow temperature change via a form of "geoengineering". What does this condition mean-- and why is it so disputable?