The group of scientists who orchestrate the Doomsday Clock, a symbolic instrument informing the public when the earth is facing imminent disaster, on Thursday moved its minute hand to 2½ minutes before the final hour.

It was the closest the clock had been to midnight since 1953, the year after the United States and the Soviet Union conducted competing tests of the hydrogen bomb.

Though scientists maintain the clock’s status, it is not a scientific instrument, or even a physical one. The movement of its symbolic hands is decided upon by the Science and Security Board of the Bulletin of the Atomic Scientists. The organization introduced the clock on the cover of its June 1947 edition, placing it at seven minutes to midnight. Since then, it has moved closer to midnight and farther away, depending on the board’s conclusions.

Thursday’s announcement was made by Rachel Bronson, the executive director and publisher of the bulletin. She was assisted by theoretical physicist Lawrence Krauss, climate scientist and meteorologist David Titley, and former US ambassador Thomas Pickering.

In an op-ed for The New York Times, Titley and Krauss elaborated on their concerns, citing the increasing threats of nuclear weapons and climate change, as well as President Donald Trump’s pledges to impede what they see as progress on both fronts, as reasons for moving the clock closer to midnight.

“Never before has the Bulletin decided to advance the clock largely because of the statements of a single person,’’ they wrote. “But when that person is the new president of the United States, his words matter.’’

In 1990, at the end of the Cold War, the clock was at 10 minutes to midnight. The next year, it was a full 17 minutes away, at 11:43.

But over the next two decades the clock slowly ticked back. By 2015, the scientists were back in a state of unmitigated concern, with the clock at three minutes to midnight, the closest it had been since 1984.

“Unchecked climate change, global nuclear weapons modernizations, and outsized nuclear weapons arsenals pose extraordinary and undeniable threats to the continued existence of humanity,’’ the bulletin said. “World leaders have failed to act with the speed or on the scale required to protect citizens from potential catastrophe.’’

“These failures of political leadership endanger every person on Earth,’’ it added.

New York Times