The symbolic Doomsday Clock, which indicates how close our planet is to complete annihilation, is now only 100 seconds away from midnight.
The Bulletin of the Atomic Scientists (BAS) said on Thursday that the change was made due to nuclear proliferation, failure to tackle climate change and "cyber-based disinformation".
The clock now stands at its closest to doomsday since it began ticking.
The idea began in 1947 to warn humanity of the dangers of nuclear war.
Last year the clock was set at two minutes to midnight - midnight symbolises the end of the world - the same place it was wound to in 2018.
BAS President Rachel Bronson told reporters in Washington DC on Thursday that the time was now being kept in seconds rather than minutes because the "moment demands attention" and that the threats level is worsening". She said the world was now menaced by powerful leaders who "denigrate and discard the most effective methods for addressing complex threats".
The decision is made by the BAS Science and Security Board, which includes 13 Nobel Laureates. For the first time this year, the board was joined by members of The Elders - a group of international leaders and former officials first founded by Nelson Mandela in 2007.
"We must act and work together," said former UN General Secretary Ban Ki-moon, a member of The Elders. "Not a single country or person can do it alone. We need all hands on deck and we can all work together."
Former California Governor Jerry Brown, another member of the panel, said: "Dangerous rivalry and hostility among the superpowers increases the likelihood of nuclear blunder. Climate change just compounds the crisis. If there's ever a time to wake up, it's now."
Astrophysicist and panel member Robert Rosner said: "The fact that the clock is now a mere 100 seconds from midnight signals really bad news,
"What we said last year is now a disturbing reality in that things are not getting better.
"Past experience has taught us that even in the most dismal periods of the Cold War, we can come together. It is high time we do so again," he added.
The clock was first created by US scientists who worked on the Manhattan Project, which developed the world's first nuclear weapon.
Georgetown University Professor Sharon Squassoni told reporters that the threat from nuclear weapons had increased, in part due to the collapse of the Iran nuclear deal, North Korean nuclear weapons development and continued proliferation from countries such as the US, China and Russia. She called the situation "dangerous" and demanding of an "urgent response".
The committee warned of another threat, particularly ahead of the US presidential election in November: "government-used cyber-enabled disinformation campaigns to sow distrust in institutions and among nations".
Board member Robert Latiff called "untruths, exaggerations and misinterpretations" a problem that could lead to the "wholesale trashing" of scientific evidence. Deepfake videos, he said, "threaten to undermine truth from fiction".
Former Irish President Mary Robinson said "the world needs to wake up", equating her reaction to that of "an angry granny".