.ON APRIL 2ND a device appearing like a snowfall device on the deck of the USS Hornet, an invalid aircraft-carrier moored in San Francisco Bay, began generating a haze of salty aerosol bits. The clinical partnership behind the job, the Marine Cloud Whitening (MCB) programme, led by the College of Washington, is actually examining whether such sprays may help to slow down weather change through a type of "geoengineering". What does this condition method-- and also why is it so controversial?