.ON APRIL 2ND a gizmo looking like a snowfall maker on the deck of the USS Hornet, a defunct aircraft-carrier tied in San Francisco Gulf, started generating a haze of salted aerosol particles. The clinical collaboration responsible for the venture, the Marine Cloud Whitening (MCB) program, led due to the University of Washington, is actually investigating whether such aerosols may help to slow temperature adjustment with a kind of "geoengineering". What does this term mean-- and also why is it therefore debatable?