.ON APRIL 2ND a contraption being similar to a snowfall maker on the deck of the USS Hornet, an invalid aircraft-carrier anchored in San Francisco Gulf, began making a haze of salted aerosol fragments. The clinical partnership responsible for the job, the Marine Cloud Whitening (MCB) programme, led due to the College of Washington, is checking out whether such aerosols might aid to decrease climate improvement via a form of "geoengineering". What does this term way-- and why is it therefore disputable?