.ON APRIL 2ND a device being similar to a snowfall machine on the deck of the USS Hornet, an inoperative aircraft-carrier tied in San Francisco Bay, began making a haze of salted spray particles. The medical cooperation behind the venture, the Marine Cloud Brightening (MCB) programme, led due to the Educational institution of Washington, is actually investigating whether such sprays might assist to decrease temperature change by means of a kind of "geoengineering". What does this condition method-- and why is it so debatable?