Delegates from most United Nations member states are gathering in New York next month to negotiate a nuclear weapons ban, and 30 Nobel Laureates, a former U.S. Secretary of Defense and over 3,000 other scientists from 84 countries have signed an open letter in support. Why?

We scientists like to geek out about probabilities, megatons and impact calculations, so we see the nuclear situation differently than many politicians and pundits. From the public debate, one might think that the cold war threat is over and that the most likely way to be killed by a nuke is by being attacked by Iran, North Korea or terrorists, but that’s not what nerdy number crunching reveals. Those media-dominating scenarios could potentially kill millions of people—except that Iran has no nukes and North Korea lacks missiles capable of reliably delivering their dozen or so Hiroshima-scale bombs.

But scientific research has shown that a nuclear war between the superpowers might kill hundreds or potentially even thousands of times more people, and since it’s not a hundred times less likely to occur, the laws of statistics tell us that it’s the nuke scenario most likely to kill you.

Why is superpower nuclear war so risky? First of all, massive firepower: there are more than 14,000 nuclear weapons today, some of which are hundreds of times more powerful than North Korea’s and those dropped on Japan. Over 90 percent of these belong to Russia and the US, who keep thousands on hair-trigger alert, ready launch on minutes notice. A 1979 report by the US Government estimated that all-out war would kill 28-88 percent of Americans and 22-50 percent of Soviets (150-450 million people with today’s populations).

But this was before the risk of nuclear winter was discovered in the 1980’s.Researchers realized that regardless of whose cities burned, massive amounts of smoke could spread around the globe, blocking sunlight and transforming summers into winters, much like when asteroids or supervolcanoes caused mass extinctions in the past. A peer-reviewed analysis published by Robock et al (2007) showed cooling by about 20°C (36°F) in much of the core farming regions of the US, Europe, Russia and China (by 35°C in parts of Russia) for the first two summers, and about half that even a full decade later. Years of near-freezing summer temperatures would eliminate most of our food production. It is hard to predict exactly what would happen if thousands of Earth’s largest cities were reduced to rubble and global infrastructure collapsed, but whatever small fraction of all humans didn’t succumb to starvation, hypothermia or epidemics would probably need to cope with roving, armed gangs desperate for food.

Trinity Site fireball from the world's first nuclear explosion. Credit: Berlyn Brixner/Los Alamos National Laboratory Wikimedia

There are large uncertainties in Nuclear Winter predictions. For example, how much smoke is produced and how high up it rises would determine its severity and longevity. Given this uncertainty, there is no guarantee that most people would survive. It has therefore been argued that the traditional nuclear doctrine of Mutual Assured Destruction (MAD) be replaced by Self-Assured Destruction (SAD): even if one of the two superpowers were able to launch its full nuclear arsenal against the other without any retaliation whatsoever, nuclear winter might still assure the attacking country’s self-destruction. Recent research has suggested that even a limited nuclear exchange between India and Pakistan could cause enough cooling and agricultural disruption to endanger up to 2 billion people, mostly outside the warring countries.

The fact that nuclear powers are taking the liberty to endanger everyone else without asking their permission has led to growing consternation in the world’s non-nuclear nations. This has been exacerbated by a seemingly endless series of near-misses in which nuclear war has come close to starting by accident, and leaders of many non-nuclear nations feel less than thrilled by the idea of being destroyed by something as banal as a malfunctioning early warning-system in a nation that they are not threatening.

Such concerns prompted 185 non-nuclear nations to sign the 1970 Non-Proliferation-Treaty (NPT), promising to remain nuke-free in return for the nuclear nations phasing out theirs in accordance with NPT Article VI, whereby each party "undertakes to pursue negotiations in good faith on effective measures relating to cessation of the nuclear arms race at an early date and to nuclear disarmament, and on a Treaty on general and complete disarmament under strict and effective international control”. Nearly 50 years later, many of these "have-nots” have concluded that they were tricked, and that the "haves” have no intention of ever keeping their end of the bargain. Rather than disarming, the U.S. and Russia have recently announced massive investments in novel nuclear weapons. Russia has recently touted a cobalt-encased doomsday bomb reminiscent of the dark comedy "Dr. Strangelove,” and the U.S. plans to spend a trillion dollars replacing most of its nuclear weapons with new ones that are more effective for a first strike.

Adding insult to injury, India, Pakistan and Israel have been allowed to join the nuclear club without major repercussions. "The probability of a nuclear calamity is higher today, I believe, that it was during the cold war," said former U.S. Secretary of Defense William J. Perry, who signed the open letter.

This disillusionment from the “have-nots” prompted 123 of them to launch an initiative in the United Nations General Assembly, where the nuclear nations lack veto power. In late 2016, they voted to launch the aforementioned UN negotiations that may produce a nuclear weapons ban treaty this summer. But a ban obviously wouldn’t persuade the nuclear ``haves” to eliminate their nukes the next morning, so what’s the point of it?

The way I see it, most governments are frustrated that a small group of countries with a minority of the world's population insist on retaining the right to ruin life on Earth for everyone else with nuclear weapons. Such “might makes right” policy has precedent. In South Africa, for example, the minority in control of the unethical Apartheid system didn't give it up spontaneously, but because they were pressured into doing so by the majority. Similarly, the minority in control of unethical nuclear weapons won't give them up spontaneously on their own initiative, but only if they're pressured into doing so by the majority of the world's nations and citizens. The key point of the ban is to provide such pressure by stigmatizing nuclear weapons.

Nuclear ban supporters draw inspiration from the 1997 Ottawa treaty banning landmines. Although the superpowers still refuse to sign it, it created enough stigma that many people now associate mines not with national security, but with images of children who have had limbs blown off while playing in peace-time. This stigma caused leading arms manufactures to half production in response to investor pressure and dwindling demand. In 2014, the Pentagon announced that it was halting landmine use outside of the Korean peninsula. Today, the global landmine market has nearly collapsed, with merely a single manufacturer (South Korean Hanwa) remaining.

The "have-not” negotiators hope that a nuclear ban treaty will similarly stigmatize nuclear weapons, persuading us all that we’re less safe with more nukes—even if they are our own. If this happens, it will increase the likelihood that the ``haves” trim their nuclear arsenals down to the minimum size needed for effective deterrence, reverting from SAD back to MAD and making us all safer.

Here is the text of the letter. A list of some of the notable signatories follows.

An Open Letter from Scientists in Support of the UN Nuclear Weapons Negotiations

Nuclear arms are the only weapons of mass destruction not yet prohibited by an international convention, even though they are the most destructive and indiscriminate weapons ever created. We scientists bear a special responsibility for nuclear weapons, since it was scientists who invented them and discovered that their effects are even more horrific than first thought. Individual explosions can obliterate cities, radioactive fallout can contaminate regions, and a high-altitude electromagnetic pulse may cause mayhem by frying electrical grids and electronics across a continent. The most horrible hazard is a nuclear-induced winter, in which the fires and smoke from as few as a thousand detonations might darken the atmosphere enough to trigger a global mini ice age with year-round winter-like conditions. This could cause a complete collapse of the global food system and apocalyptic unrest, potentially killing most people on Earth – even if the nuclear war involved only a small fraction of the roughly 14,000 nuclear weapons that today’s nine nuclear powers control. As Ronald Reagan said: “A nuclear war cannot be won and must never be fought.”

Unfortunately, such a war is more likely than one may hope, because it can start by mistake, miscalculation or terrorist provocation. There is a steady stream of accidents and false alarms that could trigger all-out war, and relying on never-ending luck is not a sustainable strategy. Many nuclear powers have larger nuclear arsenals than needed for deterrence, yet prioritize making them more lethal over reducing them and the risk that they get used.

But there is also cause for optimism. On March 27 2017, an unprecedented process begins at the United Nations: most of the world’s nations convene to negotiate a ban on nuclear arms, to stigmatize them like biological and chemical weapons, with the ultimate goal of a world free of these weapons of mass destruction. We support this, and urge our national governments to do the same, because nuclear weapons threaten not merely those who have them, but all people on Earth.

William J. Perry, mathematician, US Secretary of Defense 1994-97, AAAS fellow

Peter Ware Higgs, University of Edinburgh, Emeritus Professor of Theoretical Physics, 2013 Nobel Laureate in Physics

Leon N. Cooper, Brown University, Professor of Science, 1972 Physics Nobel Laureate

Sheldon Glashow, Boston University, Professor of Physics & Mathematics, 1979 Physics Nobel Laureate

Wolfgang Ketterle, MIT, Professor of Physics, 2001 Nobel Laureate in Physics

Edvard I. Moser, Norwegian University of Science and Technology, Professor of Neuroscience, 2014 Nobel Laureate in Physiology/Medicine

May-Britt Moser, Norwegian University of Science and Technology, Professor of Neuroscience, 2014 Nobel Laureate in Physiology/Medicine

David Gross, Kavil Institute For Theoretical Physics, Professor of Theoretical Physics, 2004 Nobel Laureate in Physics

Leland Hartwell, Arizona State University, Professor, 2001 Nobel Laureate in Physiology/Medicine

Jerome I. Friedman MIT, Emeritus Professor of Physics, 1990 Nobel Laureate in Physics 1990

Paul Greengard, The Rockefeller University, Professor of Neuroscience, 2000 Nobel Laureate Physiology/Medicine, Member, National Academy of Sciences

Roy J. Glauber, Harvard University, Professor of Physics, Emeritus, 2005 Nobel Laureate in Physics

Richard J. Roberts, New England Biolabs, Chief Scientific Officer, 1993 Nobel Laureate in Physiology/Medicine

David Politzer, Caltech, Professor of Physics, 2004 Nobel Laureate in Physics

Frank Wilczek, MIT, Professor of Physics, 2004 Nobel Laureate in Physics

Jack Steinberger, CERN, Physicist, 1988 Nobel Laureate in Physics

J. Michael Bishop, UCSF, Professor Emeritus of Microbiology and Immunology, 1989 Nobel Laureate in Physiology/Medicine

Eric Kandel, Columbia University, University Professor, 2000 Nobel Laureate in Physiology/Medicine

Martin Chalfie, Columbia University, University Professor, 2008 Nobel Laureate in Chemistry

George F. Smoot, University of California at Berkeley, Professor of Physics, Director, 2006 Nobel Laureate in Physics

David J. Weinland, 2012 Nobel Laureate in Physics

Dudley Herschbach, Harvard, 1986 Nobel Laureate in Chemistry, Emeritus Prof. of Chemistry, 1986 Chemistry Nobel Laureate

Joseph Hooton Taylor, Jr, Princeton University, James S McDonnell Distinguished University Professor of Physics, Emeritus, 1993 Nobel Laureate in Physics

H. Robert Horvitz, MIT, Professor of Biology, 2002 Nobel Prize in Medicine, 2002 Nobel Laureate in Physiology or Medicine

Serge Haroche, Collège de France, Paris, Professor Emeritus, Nobel Prize in Physics 2012, 2012 Nobel Laureate in Physics

Claude Cohen-Tannoudji, 1997 Physics Nobel Laureate, Professor of Physics

John C. Mather, Senior Astrophysicist, NASA 2006 Nobel Laureate in Physics

John L Hall, University of Colorado, Boulder CO USA, Professor, 2005 Nobel Laureate in Physics, Republic of France Légion d'Honneur (2004)

Robert W. Wilson, Harvard-Smithsonian Center for Astrophysics, Senior Scientist, 1978 Nobel Laureate in Physics

Klaus von Klitzing, Director, Max Planck Institute for Solid State Research, Stuttgart, Germany, Professor of Physics, Nobel Prize in Physics 1985

John Polanyi University of Toronto, Professor of Chemistry, 1986 Nobel Laureate in Chemistry

Stephen Hawking, Director of research at Dept. of Applied Mathematics and Theoretical Physics at Cambridge, 2012 Fundamental Physics Prize Laureate for his work on quantum gravity

Edward Witten, Institute for Advanced Study, Professor of Physics, 1990 Fields Medalist, U.S. National Medal of Science, Kyoto Prize, Breakthrough Prize, NAS member

Sir Michael Atiyah, Edinburgh University & Trinity College Cambridge, Professor of Mathematics, 1966 Fields Medalist

Curtis T. McMullen, Harvard University, Cabot Professor of Mathematics, 1998 Fields Medalist, NAS Member