.ON APRIL 2ND a contraption appearing like a snowfall maker on the deck of the USS Hornet, an obsolete aircraft-carrier moored in San Francisco Gulf, started producing a mist of salted spray particles. The scientific collaboration behind the project, the Marine Cloud Brightening (MCB) programme, led by the University of Washington, is actually examining whether such aerosols could assist to slow down temperature improvement via a form of "geoengineering". What does this phrase way-- as well as why is it therefore debatable?