That year, Eugene Rabinowitch, a former Manhattan Project scientist who co-founded the bulletin, wrote: “The achievement of a thermonuclear explosion by the Soviet Union, following on the heels of the development of ‘thermonuclear devices’ in America, means that the time, dreaded by scientists since 1945, when each major nation will hold the power of destroying, at will, the urban civilization of any other nation, is close at hand.”

The clock has been adjusted many times since it debuted in 1947. Since 2010 — years before Mr. Trump’s presidency — the needle has moved ever closer to midnight: 5 minutes in 2012, 3 minutes in 2015, and two and a half minutes last year.

Along with nuclear proliferation and climate change — which first factored into the setting of the clock in 2007 — the scientists said they were alarmed by the speed of technological change. They called on world leaders to manage the advances so that the benefits are reaped and the dangers countered.

They cited, among other threats, the hacking of computer systems that control financial and energy infrastructure; the development of autonomous weaponry that can make “kill” decisions without human supervision; and the possible misuse of synthetic biology, including the revolutionary Crispr-Cas9 gene-editing tool.

The clock does not lack for critics. For example, some say that warning people of danger actually induces political paralysis. Others question the judgments of the expert panel that oversees the clock — the bulletin’s science and security board — including the finding that the safest moment was in 1991, right after the Cold War had ended.

The bulletin’s scientists did not seem unduly alarmed in 1962, the year of the Cuban Missile Crisis, which, along with the early 1980s, was one of the moments when the United States and the Soviet Union came closest to catastrophic blows.

“One of the things about the clock is that it doesn’t change in response to individual events,” Lawrence M. Krauss, a cosmologist at Arizona State University and a member of the board, said in a phone interview on Thursday. “It’s really hard to compare, in an absolute sense, today to 1953. More important is whether the clock is closer to or farther from midnight. Is this year more dangerous than last?”