.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 Bay, started creating a moisture of salty spray bits. The medical collaboration behind the task, the Marine Cloud Brightening (MCB) program, led by the College of Washington, is exploring whether such aerosols could aid to slow temperature adjustment via a kind of "geoengineering". What does this phrase mean-- as well as why is it so questionable?