THIS week UN secretary-general Antonio Guterres warned that the world is “one miscalculation away from nuclear annihilation.” 

Speaking in New York at the opening of the long-delayed conference to review the nuclear Non-Proliferation Treaty, his words have to be a wake-up call: to leaders that pursue policies inexorably driving towards nuclear war — and to populations that are not yet taking action to stop these terrible dangers.

Guterres has no doubt about the seriousness of the situation, that we are at a time of nuclear danger “not seen since the height of the cold war.” 

He warned against countries seeking “false security” by spending vast sums on “doomsday weapons” and said that so far we have been extraordinarily lucky that nuclear weapons have not been used again, since 1945. But, as he rightly stated: “Luck is not a strategy. Nor is it a shield from geopolitical tensions boiling over into nuclear conflict.”

Indeed we cannot rely on luck to protect us from the risk of nuclear war. On this day of all days, the anniversary of the bombing of Hiroshima, we must remember what nuclear use means, and try and understand what nuclear war would look like today.

When the US used two atom bombs in 1945, one on Hiroshima and one on Nagasaki, an estimated 340,000 people died as a result, from the immediate consequences of the blast itself, but also the terrible subsequent deaths from radiation. 

Truly that was a crime against humanity. We hear those numbers every year, but what actually happened to those people in those two Japanese cities in August 1945? That will help us understand what will happen to us if governments continue along their current nuclear path.

The heart of a nuclear explosion reaches a temperature of several million degrees centigrade. This results in a heat flash over a wide area, vaporising all human tissue. 

Beyond this central area, people are killed by heat and blast waves, with buildings collapsing and bursting into flame. The firestorm creates hurricane-force winds spreading and intensifying the fire.

The most powerful testimony comes from those who witnessed the aftermath. These words are from Dr Shuntaro Hida, who was visiting a patient outside Hiroshima when the bomb was dropped. He saw the explosion over the city and returned immediately to help survivors.

“I looked at the road before me. Denuded, burnt and bloody, numberless survivors stood in my path. They were massed together; some crawling on their knees or on all fours, some stood with difficulty or leant on another’s shoulder. 

“No-one showed any sign that forced me to recognise him or her as a human being. Nearly all the buildings in the school complex had been destroyed, leaving only one structure that faced a hill at the back of the school grounds. 

“The area was filled with debris. Yet, the cruellest sight was the number of raw bodies that lay one upon the other. Although the road was already packed with victims, the terribly wounded, bloody and burnt kept crawling in, one after another. 

“They had become a pile of flesh at the entrance to the school. The lower layers must have been corpses because they emanated a peculiarly nasty smell characteristic of the dead that mingled now with burnt, bloody flesh.”

Many who survived the immediate blast died shortly afterwards from fatal burns. Others died because of the complete breakdown of rescue and medical services which had themselves been destroyed. Then radiation kicks in, with symptoms of nausea, vomiting, bloody diarrhoea and hair loss. 

Most of these victims died within a week. With radiation, there is no place to run to, no place to hide; if you escape the blast you cannot shut the door on radiation. It poisons and destroys, it brings sicknesses, cancers, birth deformities and death. This is the least we can expect from nuclear use.

Because as if that isn’t bad enough, the Hiroshima bomb was actually a small nuclear bomb in today’s terms. Today’s nuclear weapons are many, many times the power of the Hiroshima bomb. 

And there is no way to be prepared for a nuclear attack. We have to stop it happening. 

That is our most urgent task because it is in this time of escalating war, with nuclear arsenals on both sides — that we have to do everything possible to prevent nuclear use.

And of course the recent policies of nuclear weapons states are not making it easy. For some decades we had seen gradual reductions in nuclear weapons, but now we are seeing modernisation programmes on all sides — like Britain’s Trident replacement. In some cases we are even seeing increases — like Boris Johnson’s nuclear arsenal increase last year. 

But worst of all is the sanitising of the idea of nuclear use. Donald Trump had a lot to answer for this: he not only talked of so-called “usable” nuclear weapons, he also produced them and deployed them in his last year of office. 

So now the idea that they will never be used — the mutually assured destruction theory of the cold war — has gone. We hear of tactical nuclear weapons, as if you could use a small one on a battlefield and everything would be fine elsewhere. This is complete nonsense — and criminally dangerous nonsense.

This week in New York, the British government representative to the Treaty on the Non-Proliferation of Nuclear Weapons conference typically repeated Britain’s “commitment” to the treaty, and bragged about the big reductions in Britain’s nuclear arsenal since the cold war. But not a word about the arsenal increases announced by Boris Johnson last year, or US nukes coming back to Britain. 

But misrepresentation and double-speak from politicians cannot deflect us from the struggle for peace, for nuclear disarmament. 

I appeal to you all, in memory of those that were slaughtered at Hiroshima and Nagasaki — and to preserve a world for the coming generations — please join this struggle. We need you now.