Martyl Langsdorf’s clock has yet to strike midnight.

In 1953, with the United States and the Soviet Union testing hydrogen bombs and the cold war increasingly frigid, that ominous minute hand of hers stood just two ticks from the symbolically catastrophic 12. By 1991, after the signing of the Strategic Arms Reduction Treaty, it retreated to a relatively reassuring 11:43 p.m.

But the Doomsday Clock, which Mrs. Langsdorf drew for the June 1947 cover of the Bulletin of Atomic Scientists as a way to evoke the potential devastation of nuclear weapons, did not stay in reverse. Before Mrs. Langsdorf died on March 26, at 96, the board of the Bulletin, which adjusts the minute hand according to its annual assessments of threats to humanity, had set the clock to 11:55 p.m.

“The challenges to rid the world of nuclear weapons, harness nuclear power, and meet the nearly inexorable climate disruptions from global warming are complex and interconnected,” the board wrote when it moved the minute hand most recently, in 2012. “In the face of such complex problems, it is difficult to see where the capacity lies to address these challenges.”