.ON APRIL 2ND a contraption appearing like a snowfall device on the deck of the USS Hornet, an invalid aircraft-carrier moored in San Francisco Gulf, started generating a mist of salted spray bits. The medical collaboration behind the task, the Marine Cloud Brightening (MCB) programme, led by the College of Washington, is examining whether such sprays could assist to slow down environment adjustment with a kind of "geoengineering". What does this condition way-- and also why is it therefore controversial?