.ON APRIL 2ND a contraption resembling a snowfall machine on the deck of the USS Hornet, an invalid aircraft-carrier tied in San Francisco Gulf, started generating a mist of salty spray fragments. The clinical collaboration behind the task, the Marine Cloud Whitening (MCB) programme, led due to the College of Washington, is actually checking out whether such aerosols could aid to slow temperature improvement via a kind of "geoengineering". What does this term mean-- as well as why is it so disputable?