.ON APRIL 2ND a gizmo resembling a snowfall machine on the deck of the USS Hornet, an obsolete aircraft-carrier moored in San Francisco Bay, started making a mist of salty spray fragments. The clinical cooperation responsible for the venture, the Marine Cloud Brightening (MCB) program, led due to the University of Washington, is actually investigating whether such aerosols could assist to decrease environment improvement with a type of "geoengineering". What performs this condition mean-- and why is it therefore controversial?