.ON APRIL 2ND a device appearing like a snowfall machine on the deck of the USS Hornet, a nonexistent aircraft-carrier moored in San Francisco Bay, began making a moisture of salty aerosol particles. The clinical partnership behind the task, the Marine Cloud Whitening (MCB) programme, led due to the University of Washington, is exploring whether such aerosols could help to slow down temperature modification by means of a form of "geoengineering". What does this phrase way-- and why is it thus disputable?