.ON APRIL 2ND a contraption resembling a snowfall machine on the deck of the USS Hornet, an inoperative aircraft-carrier moored in San Francisco Bay, started producing a mist of salted aerosol bits. The scientific cooperation behind the project, the Marine Cloud Whitening (MCB) program, led due to the College of Washington, is actually exploring whether such sprays may aid to slow environment modification with a form of "geoengineering". What performs this term method-- and why is it thus controversial?