.ON APRIL 2ND a device appearing like a snow equipment on the deck of the USS Hornet, an inoperative aircraft-carrier anchored in San Francisco Bay, started creating a moisture of salted aerosol bits. The clinical partnership behind the project, the Marine Cloud Whitening (MCB) program, led by the University of Washington, is investigating whether such sprays may help to decrease environment adjustment by means of a kind of "geoengineering". What does this term mean-- and why is it thus controversial?