Washington (CNN) -- The symbolic "Doomsday Clock" scientists use to portray what they feel is the risk from nuclear holocaust has moved a minute closer to midnight.
The Bulletin of Atomic Scientists made the adjustment in reaction to an increased risk of weapons proliferation, the failure of a nuclear power plant in Japan, and terrorist threats to use "dirty bombs" with stolen atomic material.
The clock was moved to five minutes before midnight after a symposium where experts presented their reasoning and risk assessment. It had been at six minutes before midnight since January 2010, when scientists moved the hand back, reflecting a decreased risk of nuclear proliferation.
"Two years ago, it appeared that world leaders might address the truly global threats we face," said Allison Macfarlane, who chairs the group's science and security board. "In many cases, this trend has not continued or has been reversed. For that reason, the Bulletin of Atomic Scientists is moving the clock one minute closer to midnight, back to it's time in 2007."
The group, in announcing the decision Tuesday at a news conference, said in a news release that one factor was the question of nuclear weapons management "in a world of increasing economic, political and environmental volatility."
Nuclear weapons are one of several issues that determine the setting of the Doomsday Clock. Atomic energy also contributed to the higher risk represented by moving the clock ahead one minute. But the group declined to take a position on the future viability of nuclear energy, just its existing threat to human survival, including the diversion of atomic material for weapons production.
Kennette Benedict, the group's executive director, told reporters that even after more than 60 years of the scientists assessing nuclear risk, "it remains to be seen whether the safety and the proliferation risks can be addressed in a way that really is acceptable to people."
"Management issues, rather than technical issues, seemed to have been the problem in Fukushima," said Lawrence Krauss, who co-chairs an executive board with the Bulletin of Atomic Scientists. He said the disaster in Japan represented a broader political lack of will to ensure safe global energy.
"The need to act is urgent," he said, saying action also is needed against the factors scientists believe cause global warming.
"Climate change is happening," Krauss said, listing it among the risk factors in the Doomsday Clock announcement. "It's happening now, it's measurable, and it's clearly related to human activity, and the need to react becomes more urgent."
The Doomsday Clock was established in 1947 by scientists who helped develop the first atomic weapons in the Manhattan Project. The countdown to catastrophe has grown to reflect the risk to humanity and the planet not just from nuclear weapons, but also from a broad range of possible sources.