.ON APRIL 2ND a contraption appearing like a snow maker on the deck of the USS Hornet, a defunct aircraft-carrier moored in San Francisco Gulf, started producing a moisture of salted aerosol particles. The clinical cooperation responsible for the venture, the Marine Cloud Brightening (MCB) program, led by the University of Washington, is looking into whether such sprays could help to reduce temperature modification via a form of “geoengineering”. What does this phrase method– and why is it thus controversial?