.ON APRIL 2ND a gizmo appearing like a snowfall device on the deck of the USS Hornet, an inoperative aircraft-carrier anchored in San Francisco Bay, started generating a mist of salted aerosol fragments. The medical cooperation responsible for the venture, the Marine Cloud Brightening (MCB) program, led by the College of Washington, is actually exploring whether such sprays might aid to reduce environment modification by means of a form of "geoengineering". What does this term way-- and why is it so disputable?