.ON APRIL 2ND a contraption resembling a snow device on the deck of the USS Hornet, an inoperative aircraft-carrier anchored in San Francisco Gulf, began making a mist of salted spray bits. The scientific collaboration responsible for the venture, the Marine Cloud Whitening (MCB) programme, led due to the University of Washington, is actually looking into whether such sprays might assist to slow environment change with a type of “geoengineering”. What does this phrase mean– and also why is it therefore questionable?