The Doomsday Clock has moved closer to midnight, from two-and-a-half minutes to two minutes. In announcing the decision, the Bulletin of the Atomic Scientists cited “the failure of President Trump and other world leaders to deal with looming threats of nuclear war and climate change ”.

But what is the Doomsday Clock, and how accurate is it?

The Doomsday Clock was created by the board of the Bulletin of the Atomic Scientists in 1947 as a response to nuclear threats. The concept is simple ? the closer the minute hand is to midnight, the closer the board believes the world is to disaster.

The clock was originally conceived by a group of atomic scientists who had been involved with the Manhattan Project, the scheme responsible for the first nuclear weapons. The scientists regularly produced a bulletin detailing progress and updates in nuclear weaponry and the clock was first designed as an illustration for the cover of the first edition.

Since then, the clock has moved backwards and forwards ? from seventeen minutes to midnight in 1991 to two minutes to midnight in 1953.

The Doomsday Clock has faced criticism over the years, with some sceptics doubting the 'back and forth' nature of the clock. Writing in The Conversation, Anders Sandberg suggested humanity's biggest threats are manmade ? meaning in this case, "the normal forms of probability estimate are not just inadequate, they are actively misleading."

The 1.4 per cent probability per year of nuclear war sounds very exact, yet the estimate is based on a list of potentially suspect assumptions," Sandberg wrote . "The chance of at least one of them being wrong is high. It may be better to explicitly acknowledge the uncertainty".

It's important to remember, however, that the clock isn't aiming for total accuracy. Instead, it's a symbol of global threats, and a way to "inform the public about threats to the survival and development of humanity" ? so no need to take it literally.

The Bulletin takes several factors into consideration when calculating the time on the clock:

• Nuclear threats
• Climate change
• Biosecurity
• Bioterrorism
• Miscellaneous threats including cyber warfare and AI

Here are some of the most significant swings in the history of the Doomsday Clock.

1947 - 7 minutes to midnight : In its first appearance, the clock's hand sat at seven minutes to midnight to highlight the "urgency of nuclear dangers"

1949 - 3 minutes to midnight : The clock moved ever closer to midnight as the Soviet Union tested its first nuclear device

1953 - 2 minutes to midnight : The US created the hydrogen bomb

1963 - 12 minutes to midnight : Atmospheric nuclear testing ended

1984 - 3 minutes to midnight US-Soviet relations reached their frostiest level in years

1991 - 17 minutes to midnight : The Cold War ended and the clock jumped back

2015 - 3 minutes to midnight : 2015's Doomsday Clock was stuck at three minutes to midnight due to "unchecked climate change, global nuclear weapons modernisations and outsized nuclear weapons arsenals", all of which posed "an extraordinary and undeniable threat to the continued existence of humanity".

In 2017, the clock moved from three minutes to midnight to two-and-a-half minutes to midnight. The Bulletin cites the rise of nationalism, Donald Trump's comments over nuclear weapons and the threat of an arms race between the US and Russia. The year was also the first time a fraction was used in the time.

This article was originally published by WIRED UK