The Bulletin of Atomic Scientists announced Thursday that the Doomsday Clock now stands at two-and-a-half minutes to midnight, suggesting that existential threats now pose a greater danger to humanity than they have at any time since the height of the Cold War.

The Doomsday Clock is a symbolic warning about how close the world stands to “midnight,” that is, nuclear or existential catastrophe. Since 1947, the Bulletin’s scientists and security experts have updated it annually. Many of the world’s most acclaimed scientists—including Stephen Hawking, Susan Solomon, Lisa Randall, and Freeman Dyson—sponsor, oversee, or consult with the Bulletin.

“This is the closest to midnight the Doomsday Clock has ever been in the lifetime of almost everyone in this room. It’s been 64 years since it was closer,” said Lawrence Krauss, a theoretical physicist at Arizona State University and the chair of the Bulletin’s board of sponsors.

The clock has edged closer to midnight only once before: In 1953, it was moved to two minutes to midnight after the United States and the Soviet Union both tested hydrogen bombs, kicking off the mid-century nuclear-arms race. It remained at two minutes to midnight for another seven years.

At a press conference two blocks from the White House on Thursday, the scientists of the Bulletin specified that they were taking the action out of specific concern for the words of two men: Donald Trump and Vladimir Putin. Not in the 70-year history of the clock had rhetoric from so few individuals so affected the movement of the clock, they said.

“Nuclear rhetoric is now loose and destabilizing. We are more than ever impressed that words matter, words count,” Thomas Pickering, a longtime American diplomat who served as George H. W. Bush’s ambassador to the United Nations and Bill Clinton’s ambassador to Russia, said at the press conference.

Yet the Doomsday Clock no longer warns only of the dangers of nuclear apocalypse. Since 2007, it has countenanced other existential threats—catastrophes that could wipe out all of humanity or, at least, devastate modern civilization. In the Bulletin’s view, the most important of these is climate change.

“Climate change should not be a partisan issue. The Trump administration needs to clearly and unequivocally state that climate change, caused by human activity, is a reality,” said David Titley, a former rear admiral of the U.S. Navy and a current professor of meteorology at Pennsylvania State University. The White House also has to publicly recognize the value of taking even “moderate steps” to combat catastrophic global warming, he said.

Other existential risks factored into this year’s judgement as well. The Russian cyber-attack on the U.S. political system illustrated that cyber warfare could destabilize the basic infrastructure of global decision-making, the Bulletin said. The gene-editing tool CRISPR would make some kinds of biological weapons easier to produce, especially for countries or syndicates who could not afford university-quality scientific labs. Krauss also cited the rise of “fake news” as a concern: Last month, the defense minister of Pakistan mistook a fake news story about Israel as real and tweeted a saber-rattling nuclear promise to that country.

But all these additional existential concerns have in some ways diluted the Bulletin’s original reason for existence: concern about the threat of nuclear warfare. In the 1980s, the clock stood at three minutes to midnight—yet in 1983, the world nearly tumbled into nuclear war twice (though the public had no idea it was happening). Last year, former Secretary of Defense William Perry warned that the clock should stand at five minutes to midnight for nuclear war—but only one minute to midnight for the threat of nuclear terrorism.

At last year’s press conference, he warned that the Doomsday Clock now issued a  “more dangerous, more ominous forecast than two thirds of the years during the Cold War.”

On Thursday, the panel of scientists warned that all those structural dangers were still major threats, but that they had been intensified by thoughtless rhetoric and a rising tide of nationalism around the world. They exhorted Trump and Putin to use their friendly relationship to reduce nuclear-weapons stocks, and not to plunge into the nuclear-modernization programs that both their countries are planning.

Such programs might even lead to a resumption of nuclear tests, said Krauss.

“President Trump and President Putin, who claim to have great respect for each other, can choose to act together as statesmen—or they can act as petulant children, risking our future,” he said. “Facts are stubborn things. They must be taken into account if the future of humanity is to be preserved.”