.ON APRIL 2ND a gizmo resembling a snow maker on the deck of the USS Hornet, an invalid aircraft-carrier tied in San Francisco Bay, began making a haze of salted spray particles. The clinical cooperation behind the venture, the Marine Cloud Brightening (MCB) program, led due to the College of Washington, is actually exploring whether such sprays could assist to slow down climate modification via a form of "geoengineering". What does this phrase mean-- as well as why is it therefore controversial?