The Doomsday Clock now stands at 89 seconds to midnight, the closest to catastrophe in its nearly eight-decade history. Here's a look at how — and why — it's moved.

Atomic scientists on Tuesday moved their "Doomsday Clock" closer to midnight than ever before, citing Russian nuclear threats amid its invasion of Ukraine, tensions in other world hot spots, military applications of artificial intelligence and climate change as factors underlying the risks of global catastrophe.