.ON APRIL 2ND a contraption resembling a snowfall device on the deck of the USS Hornet, an inoperative aircraft-carrier tied in San Francisco Bay, began generating a mist of salted spray bits. The medical collaboration responsible for the project, the Marine Cloud Whitening (MCB) programme, led due to the College of Washington, is examining whether such sprays could help to slow down climate modification via a kind of "geoengineering". What does this condition mean-- and why is it therefore controversial?