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Matthew Kroenig has witnessed firsthand the growing fear that nuclear war is imminent.

A professor at Georgetown University, he’s taught an undergraduate course on nuclear weapons and world politics for the past decade. He always asks the same question on the last day: How many of his students think they’ll see nuclear weapons used in their lifetime?

For many years, no more than one student would raise their hand. That made sense, he told me, because in those days, “talking about nuclear war was like talking about dinosaurs — it’s just something from the past that won’t be something in our future.”

But the past couple of years have been different. When he asked that question again this spring, roughly 60 percent of his students raised their hands. What’s more, he agrees with them. “If I had to bet at least one nuclear weapon would be used in my lifetime,” says the 40-year-old Kroenig, “my bet would be yes.”

Kroenig and his students are not alone. A January 2018 World Economic Forum survey of 1,000 leaders from government, business, and other industries identified nuclear war as a top threat.

The widespread concern is understandable. Last year, it seemed a nuclear conflict between the US and North Korea was on the horizon. India and Pakistan, two nuclear-armed enemies, could restart their decades-long squabble at any time. And the US and Russia — the world’s foremost nuclear powers — have had warheads pointed at each other since the earliest days of the Cold War.

President Donald Trump’s presence in the Oval Office has increased worries of a potential nuclear war. In January, a poll showed about 52 percent of Americans — many of them Democrats — worried that the president would launch a nuclear attack without reason.

So what is the risk of a nuclear war, really? After speaking with more than a dozen experts familiar with the horrors of nuclear conflict, the answer is that the chances are small — very small.

But that may not be too comforting, says Alexandra Bell, a nuclear expert at the Center for Arms Control and Non-Proliferation. “The chance is not zero because nuclear weapons exist,” she says. And the damage would be incalculable; all it takes is just one strike to conceivably kill hundreds of thousands of people within minutes and perhaps millions more in the following days, weeks, and years.

What’s more, that first strike could trigger a series of events, leading to a widespread famine caused by a rapidly cooling climate that could potentially end civilization as we know it.

Below, then, is a guide to who has nuclear weapons, how they might be used, where they could drop in the future, what happens if they do — and if humanity could survive it.

Two countries have nearly all the world’s nuclear weapons

Nations typically want nuclear weapons for two reasons: self-defense — why would anyone attack a country that could respond with the world’s most destructive bombs? — and global prestige.

Not every government can afford them because nukes take billions of dollars to build, maintain, and launch properly. The proliferation process is also risky, MIT nuclear expert Vipin Narang told me, because seeking a nuke makes a country a potential target. A nuclear bomb-seeking country is typically vulnerable to attack.

Today, only nine countries own the entirety of the roughly 14,500 nuclear weapons on Earth. That’s down from the peak of about 70,300 in 1986, according to an estimate by Hans Kristensen and Robert Norris of the Federation of American Scientists.

Two countries account for the rise and fall in the global nuclear stockpile: Russia and the United States. They currently possess 93 percent of all nuclear weapons, with Moscow holding 6,850 and Washington another 6,450 (which is smaller than the 40,000 that Russia, then known as the Soviet Union, had in the 1980s and the roughly 30,000 the US had in the mid-1960s through mid-70s).

During the Cold War, each side built up its arsenal in a bid to protect itself from the other. Having the ability to attack any major city or strategic military position with a massive bomb, the thinking went, would make the cost of war so high that no one would want to fight.

But two developments in particular led to the precipitous drop, Alex Wellerstein, a nuclear historian at the Stevens Institute of Technology, told me. First, Russia and the US signed a slew of treaties from the 1970s onward to reduce and cap parts of their nuclear programs. Second, both sides learned to hit targets with extreme precision. That negated the need for so many bombs to obliterate a target.

The US and Russia, though, still maintain thousands of nuclear weapons while the other seven countries — the UK, France, China, Israel, India, Pakistan, and North Korea — have no more than a few hundred. Still, every country has more than enough weapons to cause suffering on a scale never seen in human history.

Six easy steps to nuclear war

The question, then, is not just who might actually use the weapons they own, but how? It turns out it’s a lot easier to launch than you might want to believe.

The way leaders could launch their nuclear weapons vary.

For example, North Korean leader Kim Jong Un could likely order one without any checks on his authority. Russian President Vladimir Putin, meanwhile, would put the country’s forces on high alert if it detected an incoming nuclear-tipped missile, Russian forces expert Pavel Podvig told me.

The Russian military could respond in kind if troops noted a loss of communication with Putin and it confirmed nuclear detonations elsewhere in the country, Podvig added. While we can’t say for certain what Putin would do, it is definitely possible that he would order a nuclear strike first if he felt he needed to.

Still, he says Moscow would only respond to being attacked. “Only when we become convinced that there is an incoming attack on the territory of Russia, and that happens within seconds, only after that we would launch a retaliatory strike,” Putin said during a conference in Sochi on October 18.

And if Trump decided to attack, say, North Korea with a nuclear bomb, it would be hard to stop him from doing so because he has complete authority over the launching process.

“The president can order a nuclear strike in about the time it takes to write a tweet,” Joe Cirincione, the president of the Ploughshares Fund, a foundation that works to stop the spread of nuclear weapons, told Vox’s Lindsay Maizland in August 2017.

Here’s how the American system works:

1) The president decides a nuclear strike is necessary

It’s unlikely that the United States would turn to nuclear weapons as a first resort in a conflict. There are plenty of nonnuclear options available — such as launching airstrikes to try to take out an adversary’s nuclear arsenal.

But the United States has consistently refused to adopt a “no first use” policy — a policy not to be the first one in a conflict to use a nuclear weapon, and to use them only if the other side uses them first. That means Trump could theoretically decide to launch a nuclear strike before an adversary’s nukes go off in America.

In the heat of battle, the US military might detect an incoming nuclear attack from North Korea and the president could decide to respond with a similar strike.

Either way, the president is the one who ultimately decides to put the process of launching a nuclear strike in motion — but he still has a few steps to complete.

2) A US military officer opens the “football”

Once the president has decided the situation requires a nuclear strike, the military officer who is always by the president’s side opens the “football.” The leather-clad case contains an outline of the nuclear options available to the president — including possible targets, like military installations or cities, that the US’s roughly 800 nuclear weapons ready to launch within minutes can hit — and instructions for contacting US military commanders and giving them orders to launch the missiles with warheads on them.

3) Trump talks with military and civilian advisers

The president is the sole decision-maker, but he would consult with civilian and military advisers before he issues the order to launch a nuclear weapon.

A key person Trump must talk to is the Pentagon’s deputy director of operations in charge of the National Military Command Center, or “war room,” the heart of the Defense Department that directs nuclear command and control.

The president can include whomever else he wants in the conversation. He would almost certainly consult Gen. John Hyten, commander of US Strategic Command, since Hyten is responsible for knowing what the US can hit with its nuclear weapons. But Trump would likely also include Defense Secretary James Mattis, National Security Adviser John Bolton, and Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff, in that conversation as well.

The chat also doesn’t have to be held in the White House’s Situation Room; it can happen anywhere over a secured phone line.

If any of the advisers felt such an attack would be illegal — like if Trump simply wanted to nuke Pyongyang despite no apparent threat — they could advise the president against going ahead with the strike.

Last November, Hyten publicly said he wouldn’t accept an illegal order from Trump to launch a nuclear attack. “He’ll tell me what to do, and if it’s illegal, guess what’s going to happen?” Hyten told an audience at the Halifax International Security Forum last year. “I’m gonna say, ‘Mr. President, that’s illegal.’”

He continued by outlining what the military could consider an illegal order: if a nuclear attack isn’t proportional to the actual threat, for instance, or if the attack would cause unnecessary suffering. However, what does and doesn’t constitute a “legal” order is still up for debate and was the focus of a congressional hearing last November.

Either way, if Hyten refused to follow the order, Trump could fire him and replace him with someone who would carry it out.

4) The president gives the official order to strike

After the conversation, a senior officer in the “war room” has to formally verify that the command is coming from the president. The officers recite a code — “Bravo Charlie,” for example — and the president must then respond with a code printed on the “biscuit,” the card with the codes on it.

Then members of the “war room” communicate with the people who will initiate and launch the attack. Depending on the plan chosen by the president, the command will go to US crews operating the submarines carrying nuclear missiles, warplanes that can drop nuclear bombs, or troops overseeing intercontinental ballistic missiles on land.

5) Launch crews prepare to attack

The launch crews receive the plan and prepare for attack. This involves unlocking various safes, entering a series of codes, and turning keys to launch the missiles. Crews must “execute the order, not question it,” Cirincione told Maizland.

6) Missiles fly toward the enemy

It could take as little as five minutes for intercontinental ballistic missiles to launch from the time the president officially orders a strike. Missiles launched from submarines take about 15 minutes.

And then the president waits to see if they hit their target.

The three main risks of nuclear war — and one wild card

Those that have nuclear weapons, many have argued, will never use them. The destruction and human devastation is so unimaginable that it’s hard to believe a world leader will launch them again, they say. But no one can guarantee they won’t be used at least once more — and that possibility keeps most nuclear experts up at night.

They disagree wildly as to what the next nuclear use might look like or how it might happen, but they almost unanimously cite the same three risks.

1) US vs. North Korea war

The potential nuclear conflict between the United States and North Korea worries most experts — and likely most people on Earth.

That makes sense: Trump and Kim, the North Korean premier, spent most of 2017 threatening to bomb each other with nuclear weapons. Kim actually gained a missile capable enough of reaching the entirety of the United States, although questions remain about whether it could make it all the way with a warhead on top and detonate.

Still, there remains a genuine fear — perhaps slightly allayed now following Washington and Pyongyang’s diplomatic thaw — that the leaders might escalate their public squabble into a nuclear conflict.

In February, Yochi Dreazen wrote for Vox that “a full-blown war with North Korea wouldn’t be as bad as you think. It would be much, much worse,” in part because “millions — plural — would die.”

As Dreazen recounts, the US would likely have to send in around 200,000 troops to destroy Kim’s nuclear arsenal. Seoul, South Korea’s capital, would soon — if not already — lie in ruins due to North Korea’s large artillery capabilities.

None of that may even be the worst part:

Bruce Klingner, a 20-year veteran of the CIA who spent years studying North Korea, told me that Iraqi leader Saddam Hussein had stood by in 2002 as the US methodically built up the forces it used to invade the country — and oust Hussein — the following year. He said there was little chance that Kim would follow in Hussein’s footsteps and patiently allow the Pentagon to deploy the troops and equipment it would need for a full-on war with North Korea. “The conventional wisdom used to be that North Korea would use only nuclear weapons as part of a last gasp, twilight of the gods, pull the temple down upon themselves kind of move,” said Klingner, who now works for the conservative Heritage Foundation. “But we have to prepare for the real possibility that Kim would use nuclear weapons in the early stages of a conflict, not the latter ones.”

In effect, any attempt to overthrow the Kim regime would prompt North Korea to launch nukes at the United States. Washington would almost certainly respond in kind, leading to one of the worst wars in world history.

2) US vs. Russia war

Few experts discounted the idea that the US and Russia could yet engage in a nuclear war despite a decades-long standoff. After all, they’ve come close a few times.

Here are just two examples: In September 1983, a missile attack system made it seem like the US had launched weapons at the Soviet Union. One man, Soviet Lt. Col. Stanislav Petrov, decided it was a false alarm and didn’t report the alert. Had he done so, Moscow likely would’ve responded with an actual nuclear strike.

Two months later, a too-real NATO war game — Able Archer 83 — made the Soviets believe Western forces were preparing for an actual attack. Moscow put its nuclear arsenal on high alert, but ultimately, neither side came to nuclear blows.

Today, two main reasons explain why a US-Russia nuclear fight is a major concern.

The first is the most obvious: Moscow just has so many nuclear weapons. Russia is the only country that could match the US bomb-for-bomb in any conflict. The longer Moscow has its weapons, the thinking goes, the higher the chance it uses them on the US — or vice versa.

The second reason is the most troublesome: Washington and Moscow may be on a collision course. Russia is expanding further into Europe and encroaching on NATO territory. There’s even fear that Putin might authorize an invasion of a Baltic country that once was a part of the Soviet Union but is now in NATO. If that happens, the US would be treaty-bound to defend the Baltic country, almost assuredly setting up a shooting war with Moscow.

Experts disagree on what would happen next. Some, including the Trump administration, claim Russia would use nuclear weapons early in a fight as a way to “escalate to deescalate” — do something so brash at the start of a conflict that it has to end before it gets even worse. Others say Russia would use the weapons only if its forces are on the brink of defeat.

But Olga Oliker and Andrey Baklitskiy, experts on Russia’s nuclear strategy, wrote at War on the Rocks in February that Moscow’s “military doctrine clearly states that nuclear weapons will be used only in response to an adversary using nuclear or other weapons of mass destruction,” or if the country’s survival is in doubt. In other words, they say Russia would only use nukes in retaliation or to avoid certain extinction.

Washington, of course, would likely respond with its own nuclear strikes after Moscow dropped its bombs. At that point, they’d be in a full-blown nuclear war with the potential to destroy each other and much of the world (more on that below).

3) India vs. Pakistan war

India and Pakistan have gone to war four times since 1947, when Britain partitioned what had been a single colony into Hindu-majority India and Muslim-majority Pakistan. The worry today, though, is that a fifth conflict could go nuclear.

After decades of testing, India officially became a nuclear power in 1998. Islamabad, which had started a uranium enrichment program in the 1970s, soon joined New Delhi in the nuclear club.

Two of their fights — the 1999 Kargil War and the 2001-’02 Twin Peaks Crisis — happened with fully functioning nuclear arsenals, but ultimately, neither country chose to use them.

But the opportunity keeps presenting itself. Each side claims the other has violated an ongoing ceasefire in the contested, but India-administered, Kashmir region. The region continues to be roiled by violence; for instance, six people were killed in separate instances on September 27.

The dispute over Kashmir is a key reason for current India-Pakistan tensions — and has the potential to spiral out of control.

Some fear that India and Pakistan may reach for the proverbial nuclear button sooner rather than later. Here’s just one reason why, according to an April report by Tom Hundley for Vox:

The Pakistan navy is likely to soon place nuclear-tipped cruise missiles on up to three of its five French-built diesel-electric submarines. ... Even more disturbing, Pakistani military authorities say they are considering the possibility of putting nuclear-tipped cruise missiles on surface vessels. ... Pakistan says its decision to add nuclear weapons to its navy is a direct response to India’s August 2016 deployment of its first nuclear submarine, the Arihant. A second, even more advanced Indian nuclear submarine, the Arighat, began sea trials last November, and four more boats are scheduled to join the fleet by 2025. That will give India a complete “nuclear triad,” which means the country will have the ability to deliver a nuclear strike by land-based missiles, by warplanes, and by submarines.

In effect, India and Pakistan are in a nuclear arms race, and historical enemies will soon patrol dangerous waters in close proximity with nuclear weapons aboard their vessels.

While there’s no real indication a fifth India-Pakistan war is on the horizon, it’s possible one flare-up puts both countries on the path to a nuclear crisis.

Wild card: Trump’s temperament

Cirincione, the head of the Ploughshares Fund, told me the risk of nuclear war is increasing because of one factor: Trump.

“He is the greatest nuclear risk in the world, more than any person, any group, or any nation,” he said. “The policies he is pursuing are making most of our nuclear risks worse, and he is tearing down the global institutions that have reduced and restrained nuclear risks over the last few decades.”

Here’s what he means: The administration’s Nuclear Posture Review, released in February, lowered the threshold for dropping a bomb on an enemy. Basically, the US said that it would launch low-yield nuclear weapons — smaller, less deadly bombs — in response to nonnuclear strikes, such as a major cyberattack. That was in contrast with previous US administrations, which said they would respond with a nuke only in the event of the most egregious threats against the US, like the possible use of a biological weapon.

The document also calls for more, smaller weapons on submarines and other platforms to attack enemies. Many experts worry that having tinier nukes makes them more usable, thereby increasing the chance of a skirmish turning into a full-blown nuclear war. (Think, for example, of the US-China trade war escalating to the point that Trump thinks his only option is to launch a smaller nuke, or how Trump could respond to Beijing after a devastating cyberattack on US infrastructure.)

Plus, increasing the arsenal in this way would partially undo decades of the US’s work to stop nuclear proliferation around the world.

Some experts, like Georgetown’s Kroenig, say having smaller tactical weapons is actually a good idea. Our current arsenal, which prioritizes older and bigger nukes, leads adversaries to think we would never use it. Having smaller bombs that America might use, then, makes the chance of a nuclear conflict less likely. “It gives us more options to threaten that limited response,” Kroenig told me. “We raise the bar with these lower-yield weapons.”

But the Trump risk may have less to do with what kinds of bombs he has and more to do with his temperament. Take his tweet from January 2 toward the end of his spat with Kim Jong Un, the North Korean leader:

North Korean Leader Kim Jong Un just stated that the “Nuclear Button is on his desk at all times.” Will someone from his depleted and food starved regime please inform him that I too have a Nuclear Button, but it is a much bigger & more powerful one than his, and my Button works! — Donald J. Trump (@realDonaldTrump) January 3, 2018

While tensions with North Korea were high early on in Trump’s presidency, he has yet to face a situation, like his predecessors did, where it seemed nuclear war was likely.

The 13-day Cuban missile crisis in October 1962, where the Soviet Union had secretly placed missiles in Cuba — just 90 miles from the US mainland — comes to mind. Members of President John F. Kennedy’s team, especially his military advisers, called for airstrikes on Cuba and even an invasion.

But Kennedy decided to set up a blockade of the island and try to work out a diplomatic settlement with the Soviets, in part because a military confrontation might turn nuclear. Ultimately, the situation ended when they agreed on a deal: The Soviets would withdraw the missiles from the island, and the US would take out its missiles in Turkey. Before that conclusion, both sides came as close to nuclear war as ever.

How would Trump handle himself in a similar situation? Would he resist the urges of some in his military brass to strike an enemy — perhaps with a lower-yield nuke — or would he simply tweet out a threat in a hair-trigger moment?

The fact is we don’t know — but what we do know about Trump makes his demeanor in such a situation a potential, even if very small, nuclear risk.

Here’s what happens in a nuclear attack

The theory around whether someone might drop a nuclear bomb takes away from the most serious matter in these discussions: the human and physical toll. Simply put, a nuclear strike of any magnitude would unleash suffering on a scale not seen since World War II. And with the advances in nuclear technology since then, it’s possible the devastation of the next nuclear strike would be far, far worse.

It’s hard to picture what the effect of a modern-day nuclear attack would actually look like. But Wellerstein, the nuclear historian, created a website called Nukemap that allows users to “drop” a specific bomb — say, the roughly 140-kiloton explosive North Korea tested in September 2017 — on any target.

So I did just that, detonating that North Korean device on the Capitol building in the heart of Washington, DC — and, well, see for yourself:

Roughly 220,000 people would die from this one attack alone, according to the Nukemap estimate, while another 450,000 would sustain injuries. By comparison, America’s two nuclear attacks on Japan in 1945 killed and injured a total of around 200,000 people (granted, Hiroshima and Nagasaki had smaller populations than the Washington metro area).

It’s very likely that North Korea wouldn’t launch just one bomb, but multiple at DC and likely some at New York City, the West Coast, and possibly US military bases in Guam and/or Hawaii.

But for simplicity’s sake, let’s focus on the effects of this one horrible attack.

The center yellow circle is the fireball radius — that is, the mushroom cloud — which would extend out about 0.25 square miles. Those within the green circle, approximately a 1.2-square-mile area, would face the heaviest dose of radiation. “Without medical treatment, there can be expected between 50% and 90% mortality from acute effects alone. Dying takes between several hours and several weeks,” according to the website.

Radiation poisoning is a horrible way to die. Here are just some of the symptoms people sick with radiation get:

Nausea and vomiting

Spontaneous bleeding

Diarrhea, sometimes bloody

Severely burnt skin that may peel off

The dark grey circle in the middle is where a shock wave does a lot of damage. In that 17-square-mile area, the bomb would flatten residential buildings, certainly killing people in or near them. Debris and fire would be everywhere.

People in the bigger yellow circle, a 33.5-square-mile area, would receive third-degree burns. “There’s a bright flash of light,” Brian Toon, a scientist and expert on nuclear disasters at the University of Colorado Boulder, told me about when the bomb goes off. Those exposed to the light, which would stretch for miles, would get those burns if their skin were exposed. The light would also “easily ignite fires with flammable objects like leaves, twigs, paper, or your clothing,” he added.

The victims may not feel much pain, however, because the burn will destroy pain nerves. Still, some will suffer major scarring or have the inability to use certain limbs, and others might require amputation, according to Wellerstein’s site.

The biggest circle encompasses the near entirety of the air-blast zone: a 134-square-mile area. People can still die, or at least receive severe injuries, in that location. The blast would break windows, and those standing near the glass might be killed by shards, or at least shed blood from myriad cuts.

Those who survive the bombing and its effects will have to walk through burning rubble and pass lifeless, charred bodies to reach safety. Some of them will ultimately survive, but others will succumb to sustained injuries or radiation. The wind, meanwhile, will carry the irradiated debris and objects — known as fallout because they drop from the sky — far outside the blast zone and sicken countless others.

As for Washington, it will likely take decades and billions of dollars not only to rebuild the city but clean it of radiation entirely.

It’s worth reiterating that all of the above are estimates for one strike on one location. An actual nuclear war would have much wider and more devastating consequences. And if that war spiraled out of control, the effects after the conflict would be much worse than the attacks themselves — and change the course of human history.

“Almost everybody on the planet would die”

It’s possible you have an idea of what a post-nuclear hellscape looks like. After all, disaster movies are obsessed with that kind of world. But scientists and other nuclear experts care deeply about this issue too — and their research shows the movies may be too optimistic.

Alan Robock, an environmental sciences professor at Rutgers University, has spent decades trying to understand what a nuclear war would do to the planet. The sum of his work, along with other colleagues’, is based on economic, scientific, and agricultural models.

Here’s what he found: The most devastating long-term effects of a nuclear war actually come down to the black smoke, along with the dust and particulates in the air, that attacks produce.

In a nuclear war, cities and industrial areas would be targeted, thereby producing tons of smoke as they burn. Some of that smoke would make it into the stratosphere — above the weather — where it would stay for years because there’s no rain to wash it out. That smoke would expand around the world as it heats up, blocking out sunlight over much of Earth.

As a result, the world would experience colder temperatures and less precipitation, depleting much of the globe’s agricultural output. That, potentially, would lead to widespread famine in a matter of years.

The impact on the world, however, depends on the amount of rising smoke. While scientists’ models and estimates vary, it’s believed that around 5 million to 50 millions tons of black smoke could lead to a so-called “nuclear autumn,” while 50 million to 150 millions tons of black smoke might plunge the world into a “nuclear winter.”

If the latter scenario came to pass, Robock told me, “almost everybody on the planet would die.”

Let’s take each in turn.

1) “Nuclear autumn”

A nuclear fight between New Delhi and Islamabad could cause a “nuclear autumn.”

“Even a ‘small’ nuclear war between India and Pakistan, with each country detonating 50 Hiroshima-size atom bombs,” Robock and Toon, the University of Colorado Boulder professor, wrote in 2016, “could produce so much smoke that temperatures would fall below those of the Little Ice Age of the fourteenth to nineteenth centuries, shortening the growing season around the world and threatening the global food supply.”

Here’s why: an India-Pakistan nuclear fight of that size could emit at least 5 million to 6 million tons of black smoke into the stratosphere.

At that point, American and Chinese agricultural production, particularly in corn and wheat, would drop by about 20 to 40 percent in the first five years. It’s possible that the cooling would last at least a decade, plunging temperatures to levels “colder than any experienced on Earth in the past 1,000 years,” Robock and Toon wrote.

Ira Helfand, a board director at the anti-nuclear war Physicians for Social Responsibility, calls this scenario a “nuclear autumn.”

As many as 2 billion people would be at risk of starvation even in that “limited” range, he estimates, most of them in Southeast Asia, Latin America, North America, and Europe. “The death of 2 billion people wouldn’t be the end of the human race,” he told me, “but it would be the end of modern civilization as we know it.”

The effects could get worse. The lack of food would drive up prices for what sustenance remains. Surely there would be worldwide skirmishes — and perhaps wars — over remaining resources. The situation could get so bad that we might see another nuclear war as states try to seize control of more food and water, Helfand fears.

That’s a scary scenario — but it could be even more horrifying still.

2) “Nuclear winter”

The absolute doomsday scenario is a “nuclear winter.” For that to happen, the US and Russia would have to use about 2,000 nukes each and destroy major cities and targets, Toon told me. Each country would effectively take out the other — and likely bring down most of humanity as well.

According to Robock and others, the roughly 150 million tons of black smoke rising from burning cities and other areas would spread around to most of the planet over a period of weeks. That would plunge surface temperatures by about 17 to 20 degrees Fahrenheit for the first few years, and then come back up just by 5 degrees Fahrenheit for the following decade.

The Northern Hemisphere would suffer the coldest temperatures, but the world would feel the impact. “[T]his would be a climate change unprecedented in speed and amplitude in the history of the human race,” they wrote.

Global precipitation would also drop by around 45 percent. Between that and the cold, almost nothing would grow, ensuring those who didn’t die in the nuclear firefight soon would of starvation. And if that didn’t do it, the depleted ozone layer — a side effect of a major nuclear war — would allow large amounts of ultraviolet light to make it to the surface. That would harm nearly every ecosystem and make it harder for some humans to go outside. “A Caucasian person couldn’t go outside for a few minutes before getting a sunburn,” Toon told me.

Some experts, however, disagree with the conclusions of Robock and his colleagues’ work. In 1990, five scientists who coined the term “nuclear winter” said their original findings were overblown and that a large-scale nuclear war wouldn’t extinguish humanity. And in February 2018, Jon Reisner and others in a government-backed study wrote that the impact of smoke in the atmosphere would be bad, but not as dire as Robock’s crew have predicted.

Still, the point remains the same: A nuclear war would almost certainly affect hundreds of millions or billions of people not directly caught in the fighting. Its effects would reverberate, sometimes literally, around the planet.

That’s why some don’t ever want to run the risk of a nuclear conflict — and are trying to do something about it.

What to do about nuclear weapons?

There’s only one surefire way to stop the future use of nuclear weapons: remove them entirely.

Former senior US leaders have made this case for years. Four of America’s elder statesmen — former Secretaries of State George Shultz and Henry Kissinger, former Secretary of Defense William Perry, and former Sen. Sam Nunn — wrote in 2007 in the Wall Street Journal that they wanted to see “a world free of nuclear weapons.” Having nukes in the Cold War made sense, they said, but now they’re “increasingly hazardous and decreasingly effective.”

And current health and humanitarian officials worry about nuclear use’s impact on the world.

“Even a limited use of nuclear weapons would have devastating, long-lasting and irreparable humanitarian consequences,” Kathleen Lawland, the arms unit chief for the International Committee of the Red Cross, said at the UN on October 17. “The only safeguard against nuclear catastrophe is nuclear disarmament. It is a humanitarian imperative.”

Worries over nuclear weapons have led many to push for a nonnuclear world. Beatrice Fihn, whose International Campaign to Abolish Nuclear Weapons won the Nobel Peace Prize in 2017, is one such person. She and her team helped get 69 countries to adopt the Treaty on the Prohibition of Nuclear Weapons at the United Nations, although none of the countries that have nukes signed on to the measure.

It will take 50 countries to ratify the treaty for it to become international law; so far, only 19 have done so. And while Fihn hopes she will see another 31 countries ratify the treaty, she thinks it’s already having an effect.

“The treaty is going to change a norm and will change expectations of behavior,” she told me. It will put pressure on countries not to pursue nuclear weapons and other weapons of mass destruction, she continued, because it acts kind of like a “no smoking” sign that makes it harder for smokers to light up.

The problem is it’s unclear, and rather unlikely, that the world will destroy all the nuclear weapons on earth.

The nine countries that have them consider them useful for their protection. North Korea’s Kim, for example, believes he needs nukes to ensure his regime’s survival because they deter an invasion from a foreign country like the US. And Elbridge Colby, who until earlier this year was a top Pentagon official, in October wrote in Foreign Affairs that the US should consider nuclear weapons as a key tool to fend off global challenges from Russia and China.

What’s more, while Russia and the US have reduced their arsenals significantly over the years, neither side has seriously pushed for complete disarmament.

That means the chance that a nuclear bomb is dropped sometime in the future — and perhaps in our lifetimes — is more than zero. If that frightens you, it should.

Correction, October 24, 2018: An original version of this article misstated the temperature change in the second decade of a nuclear winter, based on Robock et al.’s work. It got warmer by 5 degrees Fahrenheit, not colder by 5 degrees Fahrenheit. Kudos to Brian Hawkins for pointing it out.