.ON APRIL 2ND a contraption appearing like a snow equipment on the deck of the USS Hornet, an invalid aircraft-carrier anchored in San Francisco Gulf, began making a haze of salty aerosol particles. The scientific cooperation behind the project, the Marine Cloud Whitening (MCB) programme, led due to the Educational institution of Washington, is investigating whether such aerosols could assist to reduce environment change by means of a kind of "geoengineering". What does this phrase mean-- and why is it therefore questionable?