.ON APRIL 2ND a contraption resembling a snowfall equipment on the deck of the USS Hornet, an invalid aircraft-carrier anchored in San Francisco Bay, began making a haze of salted aerosol particles. The clinical collaboration behind the task, the Marine Cloud Whitening (MCB) program, led by the Educational institution of Washington, is checking out whether such aerosols may aid to slow environment modification with a kind of "geoengineering". What does this condition mean-- and why is it therefore controversial?