.ON APRIL 2ND a device appearing like a snow maker on the deck of the USS Hornet, an inoperative aircraft-carrier anchored in San Francisco Gulf, started making a haze of salted spray bits. The clinical partnership behind the task, the Marine Cloud Whitening (MCB) program, led due to the University of Washington, is looking into whether such sprays could help to slow temperature modification with a kind of "geoengineering". What does this term mean-- and also why is it so disputable?