.ON APRIL 2ND a contraption being similar to a snowfall machine on the deck of the USS Hornet, an invalid aircraft-carrier anchored in San Francisco Bay, began creating a haze of salty aerosol fragments. The scientific partnership responsible for the project, the Marine Cloud Brightening (MCB) programme, led due to the Educational institution of Washington, is examining whether such aerosols may aid to slow down environment modification via a form of "geoengineering". What does this phrase mean-- and why is it thus disputable?