Experts assessing the dangers posed to civilisation have added climate change to the prospect of nuclear annihilation as the greatest threats to humankind.
As a result, the group has moved the minute hand on its famous "Doomsday Clock" two minutes closer to midnight.
The concept timepiece, devised by the Bulletin of the Atomic Scientists, now stands at five minutes to the hour.
The clock was first featured by the magazine 60 years ago, shortly after the US dropped its A-bombs on Japan.
Not since the darkest days of the Cold War has the Bulletin, which covers global security issues, felt the need to place the minute hand so close to midnight.
The decision to move it came after BAS directors and affiliated scientists held discussions to reassess the idea of doomsday and what posed the most grievous threats to civilisation.
Growing global nuclear instability has led humanity to the brink of a "Second Nuclear Age," the group concluded, and the threat posed by climate change is second only to that posed by nuclear weapons.
"When we think about what technologies besides nuclear weapons could produce such devastation to the planet, we quickly came to carbon-emitting technologies," said Kennette Benedict, executive director of the Chicago-based BAS.
CHICAGO CLOCK OF DOOM
Symbolic Doomsday Clock first established in 1947
Chicago BAS offices keep a representation of the timepiece
First positioned at 7 mins to the hour; 18 changes since then
Originally reflected concern about nuclear annihilation
Bulletin now considers other threats to global security also
The announcement was made at simultaneous events held by the magazine in London and in Washington DC that included remarks from the English Astronomer Royal, Sir Martin Rees, and physicist Stephen Hawking.
"Humankind's collective impacts on the biosphere, climate and oceans are unprecedented," said Sir Martin.
"These environmentally driven threats - 'threats without enemies' - should loom as large in the political perspective as did the East/West political divide during the Cold War era."
A number of alarming nuclear trends led to a statement by the Bulletin that "the world has not faced such perilous choices" since the atomic bombs dropped on Hiroshima and Nagasaki.
The worries include Iran's nuclear ambitions, North Korea's detonation of an atomic bomb, the presence of 26,000 launch-ready weapons by America and Russia, and the inability to secure and halt the international trafficking of nuclear materials such as highly enriched uranium and plutonium.
The Bulletin of the Atomic Scientists, founded by former Manhattan Project physicists, has campaigned for nuclear disarmament since 1947.
Its board periodically reviews issues of global security and challenges to humanity, not solely those posed by nuclear technology, although most have had a technological component.
This is the first time it has included climate change as an explicit threat to the future of civilisation.
A less immediate threat, but included in the assessment, is the one posed by emerging life science technologies, such as synthetic biology and genetic modification.
While the harm done to the planet by carbon-emitting manufacturing technologies and automobiles was more gradual than a nuclear explosion, nonetheless, it could also be catastrophic to life as we know it and "irremediable", the board said.
It cited in support the conclusions of the UN's Intergovernmental Panel on Climate Change (IPCC). Its broad assessment is that the warming over the last few decades is attributable to human activities, and that its consequences are observable in such events as the melting of Arctic ice.
In the years ahead, rising sea levels, heat waves, desertification, along with new disease outbreaks and wars over arable land and water, would mean climate change could bring widespread destruction, the board said.
It also warned against the use of nuclear power as an alternative to fossil fuels.
While the technology had the potential to alleviate the climate warming effects of burning coal, its development raised the spectre that nuclear materials would be available for nefarious ends as well, the board argued.
Some scientists - even climate scientists - may not support the comparison of global warming to the catastrophe that would follow a nuclear engagement.
"Whether it's a threat of the same magnitude or slightly less or greater is beside the point," said Michael Oppenheimer, a geoscientist from Princeton University, US.
"The important point is that this organisation, which for 60 years has been monitoring and warning us about the nuclear threat, now recognises climate change as a threat that deserves the same level of attention," he said.
Both the nuclear menace and a runaway greenhouse effect were the result of technology whose control had slipped from humans' grasp, the BAS directors said. But it was also within our power to pull them back under control, they added.
"We haven't figured out how to do that yet, but the potential is within our institutions and our imaginations," said Dr Benedict.
Dr Oppenheimer agrees that people should not despair. After all, he said, for a long time the world took the nuclear threat seriously and reduced the numbers of weapons.
"I'm optimistic that we can address climate change," he said. "We've dealt with such problems before, and we can do it again."
Over the past 60 years, the Doomsday clock has now moved backwards and forwards 18 times. It advanced to two minutes before midnight - its closest proximity to doom - in 1953 after the United States and the Soviet Union detonated hydrogen bombs.
Its keepers last moved the clock's hand in 2002 after the United States withdrew from the Anti-Ballistic Missile Treaty and amid alarm about the acquisition of nuclear weapons and materials by terrorists.