But soon enough, scientist skeptics weighed in. The temperature drop would not be as precipitous as Dr. Sagan and Company had forecast, they said. Nor would the quantities of combustible material — plastics, wood, petroleum, vegetation — be as vast as first thought. For some scientists, “nuclear winter” was a good deal less probable than a milder “nuclear autumn.” The end was not nigh. By 1990, even those who had issued the earlier doomsday warnings took a step back.

One of them was Richard P. Turco, a physicist who had coined the phrase “nuclear winter.” Not that menace no longer loomed, he asserted. “Essentially, what we say is that the basic physics we proposed turned out to be correct, although the magnitude of the effects has been moderated somewhat,” Dr. Turco said in 1990.

Actually, he said, he never believed humankind was likely to be wiped out. “That was a speculation of others, including Carl Sagan,” he said. “My personal opinion is that the human race wouldn’t become extinct, but civilization as we know it certainly would.”

Some might see an element of the surreal in a debate about whether the long-range effects of a full-scale nuclear war would be (a) indescribably horrible or (b) cosmically ruinous. That said, the specter of nuclear winter helped spur major reductions in the superpowers’ nuclear arsenals, by making the utter folly of nuclear warfare plainer than ever. Any country that dared to launch an attack would inevitably wind up under the same toxic gauze as everyone else, and thus commit national suicide.

The worldwide inventory of nuclear weapons is now believed to be about one-fourth what it was in the early 1980s. Last year, the total stood at an estimated 15,850, with more than 90 percent of them in the hands of the United States and Russia, according to the Stockholm International Peace Research Institute, which tracks the armaments.

But there are also now more nuclear-armed countries than in the 1980s — nine of them, by the institute’s count: the United States, Russia, Britain, France, China, India, Pakistan, Israel and North Korea. Not all are paragons of rational behavior. And not every scientist is convinced that the world is out of danger.

It would not take an American-Russian conflagration to inflict enormous environmental damage, said Alan Robock, a climatologist at Rutgers University.

“A ‘small’ nuclear war between India and Pakistan, with each using 50 Hiroshima-size bombs (far less than 1 percent of the current arsenal), if dropped on megacity targets in each country would produce climate change unprecedented in recorded human history,” Dr. Robock wrote in 2011 in the journal Nature. Temperatures, he continued, “would be lower than during the ‘Little Ice Age’ (1400-1850), during which famine killed millions.”

Back to geoengineering. Pumping chemicals into the upper atmosphere would amount to a mild — one can only hope extremely mild — version of a nuclear winter effect. Ideas along this line have been around for a while, but putting them into practice has proved elusive. For starters, who gets to choose what method, if any, should be employed? Do all countries have a say? How much would the project cost?

And how does anyone keep the immutable rule known as the Law of Unintended Consequences from kicking in? “Anything built by humans and operated by humans can fail,” Dr. Robock told Retro Report. “So would you trust our only planet to this?”

A concern often expressed about geoengineering is that it might undermine efforts to achieve the fundamental goal of keeping greenhouse gases out of the atmosphere. Would people change their carbon-spewing ways if they believed some quick fix existed?

Similarly, in the hypothesizing over the extent of climate change in the aftermath of a nuclear war, might not a more useful focus be how to keep the bombs from falling in the first place? Thoughts turn to “WarGames,” a 1983 film in which a supercomputer, thinking that it is merely caught up in an exercise, nearly touches off global nuclear warfare.

“A strange game,” the computer finally concludes, to everyone’s relief. “The only winning move is not to play.”

CLYDE HABERMAN, a regular contributor to Retro Report, has been a reporter, columnist and editorial writer for The New York Times, where he spent nearly 13 years based in Tokyo, Rome and Jerusalem. Subscribe to our newsletter here and follow us on Twitter @RetroReport.

This article first appeared in The New York Times.