Bulletin of Atomic Scientists President and CEO Rachel Bronson said the move reflected the opinion of the scientists that the world had entered "a two-minute warning" for its survival.
"We must act and work together", Ki-Moon said during the press conference.
The hands on the clock have been moved on 24 occasions over the decades and were set at two minutes to midnight for the past two years.
The safest the world has ever been, according to the clock, was 1991.
The greatest threats to humanity, as outlined by the Bulletin, are "nuclear war and climate change", which are "compounded by a threat multiplier - cyber-enabled information warfare - that undercuts society's ability to respond".
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After the atomic bombings of Hiroshima and Nagasaki in 1945, the Bulletin was founded by University of Chicago scientists who had been involved with the Manhattan Project. The United Nations put out a report just last month noting carbon emissions need to fall almost 80 per cent this decade to avert the worst impacts of climate change yet this week atmospheric levels reached a new height. He added that governments across the globe have normalized a risky world in terms of the risks of nuclear warfare and climate change.
"It is the closest to Doomsday we have ever been in the history of the Doomsday Clock". These days, we've got a lot more than that to worry about than nuclear weapons, which is why the clock has moved so perilously close to midnight.
Bulletin also names the dire erosion of global security and political infrastructure as motives for their decision to give the human race just 100 seconds on the clock.
The Doomsday Clock first hit the two-minute mark in 1953, when the US and Soviet Union began testing the hydrogen bomb.
The Science and Security Board said it is compelled to declare a state of emergency that requires the immediate, focused and unrelenting attention of the entire world. The Bulletin declared that "the need for emergency action is urgent".
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'This shows that what's happening is persistent, not a fluke due to some weather phenomenon: We know that the long-term trends are being driven by the increasing levels of greenhouse gases in the atmosphere, ' a NASA official said of the findings. On the nuclear front, global leaders were urged to reinstate treaties, cut countries' arsenals, limit modernization programs, and "start talks on cyber warfare, missile defenses, the militarization of space, hypersonic technology, and the elimination of battlefield nuclear weapons".
It returned to 11:58 in 2018 and stayed there until Thursday's update.
Former California Gov. Jerry Brown, the Bulletin's executive chairman, warned that "dangerous rivalry and hostility among the superpowers increases the likelihood of nuclear blunder".
The scientists said the world is closer now to global catastrophe than ever before and this needs to be a wake-up call for the world.
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