.ON APRIL 2ND a gizmo appearing like a snow maker on the deck of the USS Hornet, a defunct aircraft-carrier anchored in San Francisco Gulf, started generating a haze of salty aerosol particles. The clinical partnership behind the venture, the Marine Cloud Brightening (MCB) program, led by the University of Washington, is checking out whether such aerosols could assist to slow down environment modification with a kind of “geoengineering”. What does this term method– and why is it so disputable?