.ON APRIL 2ND a contraption appearing like a snowfall device on the deck of the USS Hornet, a defunct aircraft-carrier anchored in San Francisco Gulf, began making a moisture of salty spray particles. The clinical collaboration behind the task, the Marine Cloud Whitening (MCB) program, led due to the College of Washington, is investigating whether such sprays might aid to slow climate change with a kind of "geoengineering". What performs this phrase mean-- and why is it so questionable?