.ON APRIL 2ND a gizmo looking like a snowfall machine on the deck of the USS Hornet, an inoperative aircraft-carrier moored in San Francisco Bay, started generating a haze of salted aerosol particles. The medical partnership behind the task, the Marine Cloud Whitening (MCB) programme, led by the University of Washington, is checking out whether such sprays might assist to slow temperature change by means of a kind of "geoengineering". What does this phrase method-- and why is it thus controversial?