.ON APRIL 2ND a contraption looking like a snowfall maker on the deck of the USS Hornet, an inoperative aircraft-carrier tied in San Francisco Gulf, started generating a haze of salty spray particles. The clinical cooperation responsible for the task, the Marine Cloud Whitening (MCB) program, led by the University of Washington, is actually investigating whether such aerosols could help to reduce environment modification via a form of "geoengineering". What performs this phrase mean-- and why is it so debatable?