.ON APRIL 2ND a gizmo looking like a snow maker on the deck of the USS Hornet, an obsolete aircraft-carrier tied in San Francisco Gulf, started making a haze of salty spray bits. The medical partnership behind the task, the Marine Cloud Whitening (MCB) programme, led by the University of Washington, is actually looking into whether such sprays might assist to slow down weather improvement with a form of "geoengineering". What does this phrase method-- and why is it thus disputable?