.ON APRIL 2ND a device being similar to a snow machine on the deck of the USS Hornet, an invalid aircraft-carrier moored in San Francisco Bay, started creating a mist of salty spray fragments. The scientific partnership behind the venture, the Marine Cloud Whitening (MCB) programme, led due to the Educational institution of Washington, is investigating whether such sprays may help to reduce weather improvement via a kind of "geoengineering". What does this phrase mean-- and also why is it so debatable?