(CNN) The Doomsday Clock was reset Thursday to just 100 seconds before midnight -- the closest we have ever been to the complete and total annihilation of the earth (well, at least metaphorically).

Midnight on the Clock symbolizes the end of the world, and each year, the Bulletin of the Atomic Scientists decides what time it is.

"It is 100 seconds to midnight. We are now expressing how close the world is to catastrophe in seconds -- not hours, or even minutes," the Bulletin's President Rachel Bronson said in a statement. "We now face a true emergency -- an absolutely unacceptable state of world affairs that has eliminated any margin for error or further delay."

So what factors determine how close we are to midnight? Mainly, the threat of nuclear weapons and climate change, Bronson said.

When the Clock was created in 1947, the greatest threat to humanity was nuclear war as the US and Soviet Union were headed into a nuclear arms race.