.ON APRIL 2ND a device looking like a snowfall maker on the deck of the USS Hornet, an obsolete aircraft-carrier moored in San Francisco Bay, began generating a haze of salty spray particles. The clinical partnership responsible for the task, the Marine Cloud Whitening (MCB) program, led by the University of Washington, is actually investigating whether such aerosols may aid to decrease temperature modification with a type of "geoengineering". What does this condition method-- and also why is it thus controversial?