When Greta Thunberg gives a speech, she wastes no time getting to the point.

¡°You come to young people for hope,¡± she the United Nations General Assembly in September. ¡°You say you understand the urgency...How dare you then pretend that this can be solved by ¡®business as usual¡¯?...Change is coming, whether you like it, or not.¡±

What Greta said is no less true of the most serious danger we face today: nuclear war.

To counter that, the world needs to take two major steps back from the brink of disaster, on which we have teetered for three-quarters of a century. The first of these steps is disarmament, and the second a ban on nuclear weapons.

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You may think these overly ambitious aims. But, to many, they represent the minimum that our predicament demands. Fortunately for the world, there are strong precedents for both these steps. Unfortunately, we have yet to heed those precedents.

Why are we faced with this need for change? It stems from the transformative power of modern science, which marks off the age of the atom from all preceding history. This need did not arise overnight, but did happen with remarkable suddenness.

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The word ¡°atom¡± is based on the Greek ¡°a-tomos¡±: ¡°not divisible¡±. Scientists, beginning with Ernest Rutherford, began questioning that thousand-year-old supposition. Rutherford and his students realised that they had in their experiments broken the atom apart. Then, a student of my father¡¯s, Leo Szilard, devised a way of harnessing the energy released in the nuclear break-up. His idea was to exploit the chemist¡¯s notion of chain reaction.

In 1935, when I was a child, Szilard sat in the garden of our house in Manchester, calculating, with my father, the temperature that would be reached in a nuclear chain reaction (which Szilard had patented, donating the patent to the British navy). The answer was thousands of millions of degrees centigrade. Ten years later, in 1945, two atomic bombs based on this idea were used in war, and 200,000 people were killed.

That was the start of our new era, as different from earlier times as the iron age was from the stone age. Since the ages of mankind only follow one another at thousand-year intervals, you should reflect on the extraordinary timing of your arrival on this planet. If, rather than being born in recent decades, you had been born in the previous thousands of years, great feats of imagination would not have been required of you. Today, they are needed for your, and your fellow beings¡¯, survival.

It is remarkable how speedily history has been transformed. Further developments will come as fast. As Thunberg stressed, we need now to believe in change. Not just scientific change, but also social. You will hear people say that you can¡¯t change human nature. But you can change human behaviour, as we must.

As early as 1945 leading scientists ¨C Szilard and Nobel laureate James Franck among them ¨C realised that world history had undergone a fundamental transformation. They stated this clearly, despite the fact that they were prevented from speaking publicly by secrecy.

What these scientists were saying, in advance of the first A-bomb, was the following (I quote from their report, given in secret): ¡°Unless an effective international control¡­is instituted, a race for nuclear armaments is certain to ensue following the first revelation of our possession of a nuclear weapon.¡±

They were right. Seventy years later, we still find ourselves in the midst of that arms race. As their report foresaw, such a race isn¡¯t likely to end spontaneously, but disastrously ¨C unless we deliberately curb armaments. And that we must do by law.?

The notion that war should be subject to law, is not new. It is as old as war. What is altogether new today is the scale on which we can kill, which leaves us no alternative but to curb war by law.

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This change became clearly evident through the years of the Second World War, when thousands of bombers were engaged in dispersing death indiscriminately. Opposition to this murderous development was at first muted. Today, when plans for war can no longer be distinguished from plans for genocide, the changed circumstance should be evident. Not for the first time, mankind has reached a historic moral divide.

Our task is to make people aware of this new reality. It is a noble task. We should rejoice that it falls to us. Rejoice or not, the obligation is ours. And it is clearly a task of education.

Let me say a word about my own education regarding the nuclear age. This was an inescapable topic when I was a young scientist at Princeton University, in the early 1950s. Today, it should be an inescapable topic for all scientists.

As so often happens, one discovery was in the process of triggering another. At the time of my arrival in Princeton, there was talk of an altogether new weapon, the H-bomb, more powerful by a further factor of a thousand than the A-bomb. The new weapon was to be based not on nuclear fission, but on the fusion of hydrogen nuclei ¨C the reaction that powers the sun. The scientists in the know agonised over this awful development.

That same year, 1950, President Truman determined to proceed with the H-bomb, despite virtually unanimous opposition from his most senior scientific advisors. Importantly, the source of their opposition was not technical, but moral.

The basis for the scientists¡¯ opposition was set out in an article by a leading nuclear physicist, my friend Hans Bethe. Despite all attempts at censorship, it was published in Scientific American in early 1950. ¡°Can we who insist on morality and humanity,¡± he wrote, ¡°introduce this weapon of total annihilation into the world?...Shall we convince the Russians of the value of the individual, by killing millions?¡±

Bethe, I remind you, was speaking before even a single H-bomb existed. For the second time in a few years, the scientists were being asked to look ahead, and to think beyond their area of expertise. Half a century later, we still struggle to comprehend the truth of Bethe¡¯s warning.

In retrospect you may be amazed that Truman ignored the advice of his scientists. Truman made his decision to proceed from atomic bombs to H-bombs, impelled by the terrible logic of the arms race. The Soviet Union had just tested an A-bomb and was designing an H-bomb. To the US, having been the first to make an A-bomb in 1945, it seemed imperative to be the first with the H-bomb, too. And so it was, in 1952. But this advantage, as should have been anticipated, was short-lived. Two years later, the Soviet Union tested its own H-bomb. Within a few years H-bombs were being built on both sides, in their thousands.

How, you will ask, could nations be persuaded to engage in such folly? The most prominent reason was the desire to keep ahead of their opponents. Moreover, since each party was obliged, in such a crucial matter, to leave a margin of error, that competition would be unending. Accordingly, in 1952, for the second time in the space of a few years, the world was launched on a quest for vastly increased destructive power. This time, to our shame, the development was in peace, rather than in war.

It would have been much saner to negotiate an agreement for mutual restraint with the single nuclear opponent at the time, the Russians. The option was discussed, but the voice of reason was too faint to be heard. What was missing, because of secrecy, was the clamour of popular demand.

The scientists, then as now, have a special responsibility to raise the alarm. Their voices were heard in the several appeals, most clearly in the Russell-Einstein manifesto of 1955, which called for international meetings of scientists, with clear public recommendations. Cyrus Eaton, a Canadian industrialist, offered to finance such meetings, starting in his hometown of Pugwash, Nova Scotia. In 1960, one of what became known as the Pugwash Conferences on Science and World Affairs was held in Russia for the first time. I was in attendance.

The participants agreed that, as an unmistakable step toward disarmament, the sides should have only a ¡°minimum deterrent¡±. They even suggested that this minimum comprise a single bomb located under Washington DC, and another under Moscow. This macabre proposal is worth recalling in today¡¯s world of thousands of H-bombs.

But the number of nuclear weapons, far from stopping at a minimum level that might be justified by mutual deterrence, reached 30,000 on either side by the end of the Cold War in 1990. At that time, Moscow, as we now know, was targeted by 400 nuclear weapons, so that it could be destroyed a hundred times over.

This madness was followed by significant restraint. The ¡°New Start¡± agreement of 2011 at present limits the two countries to 1,550 deployed (plus some 6,000 stored) nuclear weapons each, with, importantly, provision for intrusive inspection. The agreement lapses in February 2021, and negotiations for its renewal need urgently to be under way. But, tragically, there is no evidence of negotiations.

Reason is not dead. But the restraint that remains is a consequence of repeated close calls. This is a game of roulette too dangerous to continue. We would do well to recall the closest of close calls, the Cuban Missile Crisis. The crisis occurred two years after the Moscow Pugwash meeting. For 13 days, the world found itself at the brink of all-out nuclear war.

The danger came suddenly, as can happen when nations are poised at the brink of war. On 16 October?1962, a stunned public heard President Kennedy announce on television that the USSR was installing nuclear-armed missiles in Cuba. Kennedy responded that the launch of a single missile from Cuba against the US would lead to ¡°a full retaliatory response upon the Soviet Union¡±. This meant unrestricted nuclear war, and the deaths of hundreds of millions. At stake was the fate of civilisations.

During those 13 days, we largely stopped work in our lab. What was the point? Because of my involvement with Pugwash, I was asked by the Canadian Broadcasting Corporation, to comment. I arrived with a manuscript, which the show¡¯s producer examined. He remarked, ¡°I¡¯m glad that you are giving this talk, not me¡±.

I knew that what I was planning to say would be controversial. I felt it important to insist, contrary to accepted opinion, that the world had not been transformed by Khrushchev¡¯s action. We in the West had been totally vulnerable to Soviet nuclear weapons before his ill-judged act and were now, in addition, vulnerable to weapons in Cuba. But a colossal deterrent force still remained in the hands of the West: US missiles and the US air force maintained a threat of total devastation against the USSR. It would be reckless, therefore, for the US to invade Cuba to ¡°restore the balance¡±. The balance did not need restoring. We had best not start something we could not stop.

But this was not at all what people wanted to hear. Nor, indeed, was it the whole story.

Khrushchev had chosen to secretly ship nuclear missiles from Russia to Cuba. He thought he had good reason to do that. The Russian missiles that he sent closely resembled those that the US had the previous year installed in Turkey, a few minutes¡¯ flight from Russia. Khrushchev supposed Kennedy would see his move as restoring the balance. After all, Kennedy had been reported as saying that ¡°geography doesn¡¯t make much difference today. What does it matter if you got blown up by a missile based in Cuba, or the Soviet Union?¡±

It turned out it mattered a lot. Politics is not mathematics. Had Kennedy not responded forcefully to Khrushchev, he would have been seen to be weak, tempting Khrushchev to take still greater risks probably in Berlin, where the sides tensely confronted one another.

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The two principals in the Cuban Missile crisis, it should not need to be said, were extremely anxious to avoid war. Yet purely defensive actions taken by the US against Soviet submarines approaching Cuba seemed to Russia to carry a message of US readiness to launch nuclear war. For those submarines, entirely unknown to the US, were nuclear-armed. This was an unintended confrontation of nukes by nukes. The sides were inching closer to the brink.

At the same time, Kennedy was under heavy pressure from his military to send troops to attack the Russian missiles in nearby Cuba. Something he did not know was that the missiles in Cuba were nuclear-armed. Something else he did not know was that the local commanders in Cuba had been empowered to fire them.

It was the world¡¯s good fortune that Kennedy had the strength of character to hold back his generals. An attack, we now realise, would have resulted in nuclear war.

By the 13th day, it was clear that Khrushchev was ordering his missiles out of Cuba. This was the end of the crisis and also of Khrushchev¡¯s career ¨C a matter of importance to him.?

The 1962 crisis may seem remote. Today, we have nine independent nuclear powers, not two. But we can all recall the Korean missile crisis of 2017-18. Instead of a nuclear power rashly shifting weapons across the board, what we were experiencing was the advent of a new nuclear power, North Korea.

Prominent among North Korea¡¯s opponents were nations that had long had nuclear weapons, and zealously guarded their privileged status. They blithely assured the North Koreans that the existing nuclear weapons would not constitute a threat to their country, if only North Korea would end its reckless behaviour and disarm. The argument was sufficiently weak that the aspiring nuclear power lied year after year about its intentions, and, at the same time, raced to achieve the goal of nuclear invulnerability to a US first strike. Incredibly, on the backs of its destitute population, North Korea managed to achieve that ¨C by building mobile ICBMs that could not be reliably targeted, supplemented today by submarine-launched ICBMs similarly invulnerable to attack.

In its vain attempt to thwart this development, the US had threatened nuclear war at intervals for the 30 years, starting in the 1990s, that it took North Korea to achieve its objective of becoming a nuclear superpower with a guaranteed second strike capability. This spectacle of the helplessness of the most powerful nation on earth in the face of one of the weakest has not had the attention it deserves.

It has two sources. The first is that the existing nuclear weapons states have no special right to possess nuclear weapons. Indeed, they should have no such moral right. Secondly, although powerful, these existing nuclear powers are vulnerable, being bound together by fragile networks of nuclear obligations. The guarantees of nuclear retaliation that they have given to their allies ensure that a small nuclear match can set aflame a large tinderbox. It is our great good fortune ¨C nothing more ¨C that has spared us from nuclear war.

How close did we come to nuclear war in the recent North Korean crisis?

The impasse was only resolved, as in the case of the Cuban crisis, by one side backing down. But this time it was the US that retreated, accepting the fact that a new country had joined the Nuclear Club. The risks of nuclear war, were, however, real. The foreign minister of North Korea was stating a fact when he declared at the UN that ¡°the slightest miscalculation could have led to all-out war¡±.

This account, alarming as it is, glosses over an important further source of danger. It stems from the fact that the two principal actors in this story, President Trump and North Korean ruler Kim Jong Un, were individually empowered to initiate nuclear war. This appalling circumstance comes about through the belief, in a dictatorship or a democracy, that nuclear war should not be subject to delay. The button, as the leaders repeatedly termed it, must always be at hand.

We have for too long acceded to this perilous situation, rendered still more dangerous by secret delegations of power that subsequently come to light when and if history is written.

These observations are fraught with lessons. The first is that the risk of nuclear war from miscalculation, even when all parties are alert to the danger, is intolerably high. The second is that restraint can be counted on to bring unforeseen rewards.

In regard to the second point, we realise today that in addition to the millions killed directly by a nuclear war, millions more will die in subsequent years from famine, as crops fail due to the ¡°nuclear winter¡± caused by the blocking of the sun¡¯s rays by soot from fires. We cannot calculate the precise numbers, but it is not an experiment we should ever dare conduct.

The hazards do not end there. Today, as never before, miscommunication heightens the danger. Miscommunication comes not only by chance but by intent, since it has become a weapon. An escalating arms race in cyberspace is forever compounding the risk of war.

We take heart from the fact that despite increasing risks, for three-quarters of a century there has been no nuclear war. Most mistakenly attribute this to the solidity of deterrence. But deterrence is less stable than it appears. If deterrence were indeed stable, why would nuclear powers compete so vigorously to strengthen their armaments, as they are doing? What is the lack that calls constantly for weapons modernisation and diversification? Why is it that the supposed equilibrium of deterrence is accompanied by its opposite, a race for dominance?

The reason is as simple as it is alarming. It is that the parties who claim to be deterred are not. They have yet to hear the voices of Ronald Reagan and Mikhail Gorbachev ?¡°A nuclear war cannot be won, and must never be fought.¡± Instead, although they seldom advertise it, they nurture the ambition to be nuclear victors.

This, too, is the reason they keep their ¡°strategic¡± arsenals ready for launching in a matter of minutes. Their objective of this ¡°launch-on-warning¡± is to first degrade the opponent¡¯s attack by a disarming first strike, followed by missile defence of their own territory.

This is not the only way in which the concept of deterrence is being eroded. A second development in the same direction is to reduce the explosive power of nuclear weapons so that they can be seen as suitable for achieving limited objectives. This ¡°tactical¡± use of nuclear weapons was proposed decades ago and was rejected when military exercises convinced the parties that what was being protected by such means would have been destroyed.?

So what is the path forward? I return to the steps I mentioned at the outset. First, we must demand that the nuclear weapons states move to the situation that most imagine, quite wrongly, already exists, namely deterrence based on shared vulnerability. A clear-sighted Robert Oppenheimer likened this to a pair of scorpions in a bottle, each able to kill the other but only at the cost of its own life.

We shall know deterrence actually exists when only a few nuclear weapons remain. We shall also know deterrence is the aim when these weapons are no longer kept on alert, being instead separated from their warheads. This is not a preposterous demand. China has for years limited itself to a deterrent of a few hundred nuclear weapons, with its missiles and warheads visibly separated.

This step must be followed by a second more demanding one. We must set aside the perilous notion (expressed by Winston Churchill) that ¡°peace can be the sturdy child of terror¡±. The risks of such a course, as should be evident, are intolerable. Instead, we must resolve, at last, to implement a solemn agreement made by almost all the nuclear and non-nuclear states as long as half a century ago. It is an agreement to outlaw nuclear weapons. This lies at the heart of the Non-Proliferation Treaty of 1968, the central arms control agreement of the age.

The objective spelled out in Article 6 commits the five then existing nuclear powers to complete nuclear disarmament. This was a logical necessity since the five had no basis whatsoever for arguing that their nuclear weapons, in contrast to others¡¯, served the cause of peace.

In the intervening years, those five have been joined by a further four, and all nine can be seen to be extending their nuclear arsenals. Yet in the New START agreement, limiting the number of nuclear warheads deployed by the superpowers, the central assertion is that more weapons would be accompanied by less security. This is an important step on the path to acknowledging that the continued existence of these weapons poses an unacceptable threat to humankind.

Importantly, at the UN General Assembly on 7 July 2017, an overwhelming majority of nations, 122 of the UN¡¯s 190, voted to frame a global ¡°Nuclear Weapons Prohibition Treaty¡±. Once ratified by 50 UN states, this treaty would prohibit nations developing, testing, possessing, using or threatening to use nuclear weapons. So far, 33 have ratified.

Is this nuclear weapon ban beyond the realm of the possible? Surely, not. In 1968, the entire region of Latin America declared itself a nuclear weapon-free zone, under the Treaty of Tlatelolco. That prohibition remains. The framers of that agreement, in common with today¡¯s proponents of a nuclear weapons prohibition, were acting out of moral repugnance against weapons of indiscriminate slaughter. That repugnance, as it should be, is already present in international law, giving rise to such measures as the existing bans on chemical and biological weapons, cluster bombs and anti-personnel mines.

But where, it will be asked, does such legal commentary on moral matters get us? The answer is that, little by little such laws inevitably come to govern behaviour. That has proved to be the case in regard to such issues as slavery, apartheid and genocide.

It goes without saying that the existing nuclear powers and their allies scorn the proposed nuclear weapons ban as undermining the international order. The government of my country, Canada, deems it ¡°unrealistic¡± and ¡°premature¡±. Better premature, perhaps, than too late. As for ¡°unrealistic¡±, we pass through unreality on the way to new realities. Consider the fact that Canada, a member of a nuclear alliance, has for half a century refused to have nuclear weapons on its soil. Canadians take pride in that protest against a reality so clearly in need of change.

What is the circumstance that most compellingly calls for change? It is our present enslavement to barbaric instruments of war. To a few they bring wealth, to most impoverishment, and to all, peril. Essential to the earlier escape from slavery was moral revulsion; this cannot help but be the case again.

President Kennedy¡¯s life was cut short by a crazed assassin, yet he left us with a guide to our common future. ¡°We all inhabit this small planet,¡± he remarked. ¡°We all breathe the same air. We all cherish our children's futures. And we are all mortal.¡±

John Polanyi is university professor in the University of Toronto¡¯s department of chemistry. He won the 1986 Nobel Prize in chemistry for his research in chemical kinetics. This is a slightly edited version of a speech he gave at the Nobel Foundation in Sweden on 8 December.

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