LONDON — The world is nudging closer to nuclear or environmental apocalypse, a group of prominent scientists warned Wednesday as it pushed the hand of its symbolic Doomsday Clock closer to midnight.

The clock, which was set two minutes forward to 11:55, represents the likelihood of a global cataclysm. Its ticks have given the clock’s keepers a chance to speak out on the dangers they see threatening Earth.

It was the fourth time since the Soviet collapse in 1991 that the clock ticked forward amid fears over what the scientists describe as “a second nuclear age” prompted largely by standoffs with Iran and North Korea. But urgent warnings of climate change also played a role.

The Bulletin of the Atomic Scientists, which sets the clock, was founded in 1945 as a newsletter distributed among nuclear physicists concerned about nuclear war, and midnight originally symbolized a widespread nuclear conflict.

The bulletin has grown into an organization focused more generally on manmade threats to human civilization.

“The dangers posed by climate change are nearly as dire as those posed by nuclear weapons,” said Kennette Benedict, director of the bulletin.

Stephen W. Hawking, the renowned cosmologist and mathematician, told The Associated Press that global warming has eclipsed other threats to the planet, such as terrorism.

“Terror only kills hundreds or thousands of people,” Hawking said. “Global warming could kill millions. We should have a war on global warming rather than the war on terror.”

This is the first time the bulletin has explicitly addressed the threat from climate change.

“We are transforming, even ravaging the entire biosphere. These environmentally driven threats – threats without enemies – should loom as large as did the East-West divide during the Cold War era,” said Martin Rees, president of the Royal Society, Britain’s academy of science.

“Unless they rise higher on international agendas, remedial action may come too late,” he added.

The clock has moved 18 times since it started in 1947.