.ON APRIL 2ND a device appearing like a snow machine on the deck of the USS Hornet, a nonexistent aircraft-carrier tied in San Francisco Bay, began generating a haze of salty aerosol particles. The scientific partnership behind the job, the Marine Cloud Whitening (MCB) program, led by the University of Washington, is examining whether such sprays may assist to slow down temperature modification with a kind of “geoengineering”. What does this phrase mean– and why is it so disputable?