Rising threats from climate change and nuclear arsenals prompted the scientists who maintain the Doomsday Clock, a symbolic countdown to global catastrophe, to move it two minutes closer to midnight - its first shift in three years.
The Doomsday Clock, devised by the Chicago-based Bulletin of the Atomic Scientists, now stands at three minutes to midnight, or doomsday.
It has been adjusted 18 times since its creation in 1947. It was set as close as two minutes to midnight, in 1953 when the United States tested a hydrogen bomb, and as far as 17 minutes from midnight, in 1991 as the Cold War expired.
It was last adjusted in January 2012, when it was moved one minute closer to midnight.
"Today, unchecked climate change and a nuclear arms race resulting from modernisation of huge arsenals pose extraordinary and undeniable threats to the continued existence of humanity," Bulletin Executive Director Kennette Benedict said.
The change was announced less than a week after US scientists announced that 2014 was Earth's hottest on record, fresh evidence that people are disrupting the climate by burning fossil fuels that release greenhouse gases into the air.
Mr Benedict said world leaders had failed to act with the speed or on the scale necessary to protect against potential catastrophe, and the adjustment in the clock was intended to inspire action.
"These failures of leadership endanger every person on Earth," Mr Benedict said.
The Bulletin was a periodical founded in 1945 by University of Chicago scientists who helped develop the first atomic weapons in the Manhattan Project.
The Bulletin's science and security board, which adjusted the clock, said steps to address climate change did not match expectations of even five years ago, let alone the actual need, while investments continued to pour into fossil fuel infrastructure.
Sweeping nuclear weapons modernization programs and a disarmament machinery that had ground to a halt has dampened cautious optimism about the possibility of keeping the nuclear arms race in check, the scientists said.
Doomsday Clock is now at 3 minutes to midnight. Was at 3 minutes in 1984 & 1949. Watch: http://t.co/xvFhhcXsGT— BulletinOfTheAtomic (@BulletinAtomic) January 22, 2015
The board recommended action to cap greenhouse gas emissions, cut spending on nuclear weapons modernisation programs and a renewed drive toward nuclear disarmament.
"We are not saying it is too late to take action, but the window for action is closing rapidly," Mr Benedict said.
"The world needs to be awakened from its lethargy and start making changes."