A nuclear weapon is a certain thing—atomic or hydrogen, fission or fusion, bomb or missile, so many megatons—but nothing could be more uncertain than the consequences of using one. Nine nations have nuclear weapons; only the United States has ever used one, and that was in 1945. Our nuclear-weapons policy rests on a seven-decade-long history of events that have never happened: acts of aggression that were not committed, wars that were not waged, an apocalypse that has not come to pass. Strategists attribute the non-occurrence—the deterrence—of these events to the weapons themselves, to bombs on airplanes, missiles in silos, launchers on submarines. The power of deterrence, however, is a claim that cannot be proved. If, while a police car is parked in front of your house, your house is not robbed, you might suspect that a robbery would have taken place had the police not been there, but you can’t know that for sure. Nuclear-weapons policy is a body of speculation that relies on fearful acts of faith. Doctrinally, it has something in common with a belief in Hell.

This belief is about to be tested. The United States and its only nuclear rival have been reducing their arsenals since the end of the Cold War. In 1985, the United States and the Soviet Union held a combined stockpile of more than sixty thousand warheads; today, the U.S. and Russia have fewer than fifteen thousand between them. Dangers remain: a computer error, a malfunction in a silo, a rogue state, nuclear terrorism. In 2007, the former Secretaries of State Henry Kissinger and George P. Shultz, the former Defense Secretary William J. Perry, and the former senator Sam Nunn warned in an essay published in the Wall Street Journal that “the world is now on the precipice of a new and dangerous nuclear era . . . that will be more precarious, psychologically disorienting, and economically even more costly than was Cold War deterrence.” The time had come, they argued, for the eradication of nuclear weapons. Global Zero, an international nuclear-abolition organization, was formed the next year. In 2009, in a speech in Prague, Barack Obama pledged “America’s commitment to seek the peace and security of a world without nuclear weapons.” That speech helped earn him the Nobel Peace Prize. The reality did not match the rhetoric. Congress exempted nuclear weapons from mandatory cuts in the military budget, and, in exchange for support for the New START treaty, which reduced deployed warheads by two-thirds, the President pledged eighty-five billion dollars to modernize the nation’s aging nuclear arsenal. “I think we can safely say the President’s Prague vision is dead,” the Alabama congressman Mike Rogers said in 2015, at a weapons conference sponsored by Lockheed Martin, General Dynamics, and Northrop Grumman. “And I’ll leave it to the Nobel committee to ask for its prize back.”

The new President’s vision is unclear. “We have to be extremely vigilant and extremely careful when it comes to nuclear,” Donald Trump said during the campaign. But he also refused to rule out conducting a first strike, even on Europe (“Europe is a big place”); suggested that it might be a good thing for more countries to acquire nuclear weapons; and argued that it was pointless to manufacture weapons that could never be used, asking, “Then why are we making them?” In December, Vladimir Putin told military leaders in Moscow that he intended to bolster Russia’s nuclear arsenal. “Let it be an arms race,” Trump said in response. “We will outmatch them at every pass and outlast them all.”

Trump has often contradicted himself on the subject of nuclear weapons, but one of the more interesting things he’s said about them is that they are far more dangerous to the planet than global warming is. It’s a revealing comparison. The damage from a nuclear explosion does not respect national boundaries, and this adjustment in scale, from the national to the global, was the key argument put forward by advocates of disarmament. That argument has been won: Trump’s tweets aside, there is a bipartisan consensus in favor of significant arms reductions. Bipartisan agreement about the future of the planet falls apart not over the bomb but over the climate. Historically, though, they’re inseparable: the weapons and the weather are twisted together, a wire across time, the long fuse to an ongoing debate about the credibility of science, the fate of the Earth, and the nature of uncertainty.

In 1981, when Jimmy Carter delivered his farewell address, part of it was written by Carl Sagan. The Senate had proved unwilling to ratify a treaty that had come out of a second round of Strategic Arms Limitation Talks; Carter wanted to take a moment to reckon with that loss, for the sake of the planet. He turned to Sagan, whose thirteen-part documentary, “Cosmos,” first broadcast in 1980, was the most-watched PBS series ever. “Nuclear weapons are an expression of one side of our human character,” Carter said, in words written by Sagan. “But there’s another side. The same rocket technology that delivers nuclear warheads has also taken us peacefully into space. From that perspective, we see our Earth as it really is—a small and fragile and beautiful blue globe, the only home we have.”

Sagan was an astronomer, but early in his career he’d worked on a classified nuclear-weapons project. This was not unusual. Since the Second World War, the military has funded the preponderance of research in the field of physics, and, as historians have now established, a close second was its funding of the earth sciences. Although the environmental movement may not have started until the nineteen-sixties, the research that lies behind it began in the fifties, in the U.S. military. Indeed, the very term “environmental science” was coined in the fifties by military scientists; it was another decade before civilian scientists used the term.

Beginning on the day black rain fell on Hiroshima, nuclear weapons shaped environmental science. In 1949, the U.S. Weather Bureau launched Project Gabriel, a classified meteorological study of weapons and weather. The next year, the Department of Defense, in a study titled “The Effects of Atomic Weapons,” coined the word “fallout.” Researchers considered making the quantity, spread, and duration of fallout the standard measure of the force of a nuclear explosion, but found that approach to be too dependent on the weather. (Instead, they chose blast radius.) They measured and modelled the best weather conditions for explosions and the effects of those explosions on the natural world; they invented and refined tools to detect atmospheric weapons tests conducted by the Soviets; and they investigated the possibility of using nuclear weapons to alter the weather and even the climate of adversaries. Sagan, after finishing his Ph.D. at the University of Chicago, in 1960, worked on a secret military project code-named A119, which had begun in 1958, a year after Sputnik. Sagan was charged with calculating “the expansion of an exploding gas/dust cloud rarifying into the space around the Moon.” The idea was to assess whether a mushroom cloud would be visible from Earth, and therefore able to serve as an illustration of the United States’ military might.

Government-funded environmental scientists began noticing something curious: nuclear explosions deplete the ozone layer, which protects the Earth’s atmosphere. This finding related to observations made by scientists who were not working for the military. In the wake of Rachel Carson’s “Silent Spring,” published in 1962, the U.S. government formed a number of advisory and oversight organizations, including the Environmental Pollution Panel of the President’s Science Advisory Committee. The panel’s 1965 report, “Restoring the Quality of Our Environment,” included an appendix on “Atmospheric Carbon Dioxide,” laying out, with much alarm, the consequences of “the invisible pollutant” for the planet as a whole. In 1968, S. Fred Singer, an atmospheric physicist who had worked on satellites and was now a Deputy Assistant Secretary of the Interior, organized a symposium on “Global Effects of Environmental Pollution.” Four papers were presented at a panel on “Effects of Atmospheric Pollution on Climate.”