.ON APRIL 2ND a gizmo looking like a snowfall machine on the deck of the USS Hornet, an invalid aircraft-carrier moored in San Francisco Bay, began creating a mist of salty aerosol bits. The scientific partnership responsible for the job, the Marine Cloud Whitening (MCB) programme, led by the College of Washington, is examining whether such aerosols could help to slow down climate modification by means of a kind of “geoengineering”. What does this condition mean– and why is it so disputable?