.ON APRIL 2ND a contraption looking like a snowfall machine on the deck of the USS Hornet, an invalid aircraft-carrier anchored in San Francisco Gulf, started making a mist of salty spray fragments. The medical partnership behind the venture, the Marine Cloud Whitening (MCB) programme, led due to the University of Washington, is actually exploring whether such aerosols could assist to slow environment adjustment by means of a type of "geoengineering". What performs this condition mean-- and also why is it therefore debatable?