The Doomsday Clock has been set at 90 seconds to midnight - representing a time of "unprecedented danger" - largely because of Russia's invasion of Ukraine.
It puts it 10 seconds closer to midnight, the closest to global catastrophe it has ever been. It had been at 100 seconds to midnight since 2020.
The countdown is a metaphor for global collapse agreed on by world-renowned experts at the Bulletin of the Atomic Scientists.
They said Russia's "thinly veiled threats to use nuclear weapons remind the world that escalation of the conflict - by accident, intention or miscalculation - is a terrible risk.
"The possibility that the conflict could spin out of anyone's control remains high."
They also pointed out that the last remaining nuclear weapons treaty between Russia and the United States is "in jeopardy".
They said: "Unless the two parties resume negotiations and find a basis for further reductions, the treaty will expire in February 2026.
"This would eliminate mutual inspections, deepen mistrust, spur a nuclear arms race, and heighten the possibility of a nuclear exchange."
They also pointed towards China's considerable expansion of its nuclear capabilities as being "particularly troubling", North Korea's enhancement of intermediate and longer-range missile testing, Iran's increasing capacity for the enrichment of uranium, and India's development of its arsenal.
Other influences on the decision to move the clock forward include the climate crisis and carbon dioxide emissions, the increasing number of bio-threats like COVID-19, and disinformation and disruptive technology.
'An alarm for the whole of humanity'
Mary Robinson, the first female president of Ireland and former UN High Commissioner for Human Rights, said: "The Doomsday Clock is sounding an alarm for the whole of humanity.
"We are on the brink of a precipice. But our leaders are not acting at sufficient speed or scale to secure a peaceful and liveable planet.
"From cutting carbon emissions to strengthening arms control treaties and investing in pandemic preparedness, we know what needs to be done.
"The science is clear, but the political will is lacking. This must change in 2023 if we are to avert catastrophe.
"We are facing multiple, existential crises. Leaders need a crisis mindset."
The closest the clock has ever been to midnight
Rachel Bronson, president and CEO of the Bulletin of the Atomic Scientists, said: "We are living in a time of unprecedented danger, and the Doomsday Clock time reflects that reality.
"90 seconds to midnight is the closest the clock has ever been set to midnight, and it's a decision our experts do not take lightly.
"The US government, its NATO allies and Ukraine have a multitude of channels for dialogue; we urge leaders to explore all of them to their fullest ability to turn back the clock."
History of the clock
The Doomsday Clock was created in 1947 by experts at The Bulletin of the Atomic Scientists who had worked on the Manhattan Project to design and build the first atomic bomb.
They set up the clock to provide a simple way of showing the danger to the Earth and humanity posed by nuclear Armageddon.
It is an independent non-profit organisation run by some of the world's most prominent scientists.