Carbon credits can be traded in the EU's emissions trading scheme (ETS), but unlike other commodity markets, it's not clear that carbon credits are tied to something that will have value tomorrow, or next year. Can the credits be owned, like a piece of property, or can they just disappear into thin air? And disappear they may. The entire EU trading system was shut down recently, with credits worth €28m missing following a series of highly effective cyber attacks, reports the London Guardian.