.ON APRIL 2ND a device appearing like a snowfall device on the deck of the USS Hornet, a defunct aircraft-carrier anchored in San Francisco Bay, started producing a haze of salted aerosol particles. The scientific cooperation responsible for the venture, the Marine Cloud Whitening (MCB) programme, led due to the University of Washington, is actually exploring whether such sprays may aid to slow down climate modification with a kind of "geoengineering". What does this phrase way-- and why is it thus debatable?