.ON APRIL 2ND a contraption appearing like a snow equipment on the deck of the USS Hornet, an inoperative aircraft-carrier moored in San Francisco Gulf, began creating a mist of salted spray particles. The medical partnership responsible for the project, the Marine Cloud Brightening (MCB) program, led due to the Educational institution of Washington, is actually looking into whether such aerosols might help to slow temperature improvement through a form of "geoengineering". What does this term mean-- and why is it so disputable?