.ON APRIL 2ND a device appearing like a snow equipment on the deck of the USS Hornet, an invalid aircraft-carrier moored in San Francisco Gulf, started producing a mist of salted spray fragments. The clinical partnership behind the job, the Marine Cloud Brightening (MCB) programme, led due to the University of Washington, is checking out whether such aerosols may aid to slow environment modification with a kind of “geoengineering”. What performs this condition method– and why is it therefore controversial?