.ON APRIL 2ND a contraption appearing like a snow maker on the deck of the USS Hornet, an inoperative aircraft-carrier anchored in San Francisco Bay, started making a mist of salty spray bits. The scientific partnership behind the venture, the Marine Cloud Brightening (MCB) programme, led due to the College of Washington, is investigating whether such sprays may help to reduce climate modification by means of a kind of "geoengineering". What does this phrase mean-- and why is it thus questionable?