.ON APRIL 2ND a contraption resembling a snow machine on the deck of the USS Hornet, a nonexistent aircraft-carrier anchored in San Francisco Bay, started creating a mist of salty spray particles. The clinical collaboration behind the venture, the Marine Cloud Whitening (MCB) program, led by the College of Washington, is examining whether such sprays could assist to reduce environment modification by means of a kind of "geoengineering". What performs this phrase mean-- and also why is it so disputable?