.ON APRIL 2ND a device resembling a snowfall machine on the deck of the USS Hornet, an obsolete aircraft-carrier tied in San Francisco Bay, began creating a mist of salted aerosol particles. The clinical collaboration responsible for the job, the Marine Cloud Whitening (MCB) programme, led by the University of Washington, is checking out whether such aerosols might aid to slow climate change by means of a kind of "geoengineering". What performs this term way-- and also why is it therefore disputable?