Johann Hari: Instead of buying more nuclear weapons, why don't we scale down our arsenal?

If nukes make us safer, then how can we object when the people of Iran and India say the same?

So Gordon Brown has announced the recommissioning of Trident - the delivery system for our very own Weapons of Mass Destruction - and without a pause the debate has immediately sunk into the incessant babble about the Labour leadership. Does this mean Gordon will be challenged from the left? Is this a rapprochement with Tony? No doubt in the middle of a nuclear war itself, Nick Robinson would stand in the shadow of a mushroom cloud, his remaining strands of hair falling from his head along with flecks of skin, and ask excitedly how this will affect the Blair-Brown relationship.

But this decision is far too important to be left to Westminster Village trivia. It affects the single greatest threat to the future of the human species, along with global warming - the spread of nuclear weapons. It is only four years now since India and Pakistan were so close to nuclear war that Britain had to order its citizens to evacuate the sub-continent. We are about to see a nuclear standoff between Israel and Iran in the heart of the most volatile region in the world. This is the reality of the Second Nuclear Age, a time when mini-cold wars are proliferating across the world's hot spots, each offering its own protracted Cuban Missile Crises.

That's why our politicians are right that the question of Britain's nukes is a "vital matter of national security" - it's just that they fail to see how. Let's start by asking the most basic question. What are the threats to Britain's security, and can Trident help? The most obvious danger to your physical safety and mine is from 7/7-style jihadi groups. Before these non-state actors, nuclear weapons stand limp and useless. It is hardly an option to deploy Trident against Leeds, the home of Mohammed Siddiq Khan.

The second risk - real but very slight - is from a hostile state, Iran or North Korea or some as-yet-unanticipated foe, at some point in the future trying to threaten London. There is a way to deter this tiny risk without having ready-to-fire nuclear weapons, which we'll get to in a moment.

That leaves us with the third risk, by far the greatest. It is of a nuclear exchange somewhere else in the world severely damaging the global environment. Every bomb held by a nuclear state today is between eight and 70 times more powerful than the bombs that burned Hiroshima and Nagasaki. The scientific evidence suggests that if more than a handful are ever used anywhere, Britain's climate will be dangerously disrupted and could create a nuclear winter. So it is obviously a matter of urgent national security to reduce nuclear tension everywhere.

This is tough if we are conspicuously clinging to - and even improving - our own nuclear weapons systems, claiming they are "essential for our national security". If we use this argument, how can we object when the people of Iran and India and Pakistan say the same? If nukes make us safer, why not, say, Egypt, Taiwan or Brazil? What makes us special, other than raw power? Brown has offered an unwitting argument for universal proliferation. As Mohamed ElBaradei, the Nobel Prize-winning head of the International Atomic Energy Authority (IAEA), says, "It is very hard to preach the virtues of non-smoking when you have a cigarette dangling from your lips and you are about to buy a new pack."

You do not have to believe in some (currently) utopian plan for total disarmament to say we must swing the momentum across the world from a rapid build-up to a gradual scaling-down. I recently spoke to Robert McNamara, who was Kennedy's defence secretary at the time of the Cuban Missile Crisis. He explained how this momentum could be achieved in the American context.

"I was placed on a panel where I was arguing for disarmament and the other guy was arguing in defence of nuclear weapons. I said to him, 'At the moment the US has 2000 nuclear warheads on hair-trigger alert to be fired in 15 minutes. That's enough to destroy the world 16 times over. Why don't we cut back to, say, 200, enough to destroy the world just once? Wouldn't that be a good start?' And he agreed. So let's cut back to that, then we can have a debate about the zero option."

Few people have pointed out that there is a similar way to achieve that momentum in the British context. It would allow us to retain a deterrent against any future aggressive state, and reduce the far greater danger from proliferation at the same time. In nuclear circles, it is called "the Japanese option". At the moment, Japan has a virtual nuclear arsenal. Dr Andrew Dorman of King's College London explains what this means: "Japan currently has a civil nuclear programme and advanced rocket technology. Estimates range from six months to two years for how long it would take Japan to build a nuclear capability. Likewise, Britain could retain its design teams and maintain the capacity to build and reconstruct its nuclear force, but not actually have one day to day."

The Japanese option would guarantee that if we ever needed a nuclear deterrent again - and a risk is not going to emerge overnight - we could very quickly assemble one. But we would have saved a fortune in cash and, more importantly, we would have sent a very strong signal that having dozens of weapons of mass destruction on standby is not our idea of security.

The Nuclear Non-Proliferation Treaty, our best hope for a less radioactive world, would begin to breath a little on its intensive-care bed.

If Britain went Japanese, this might also help us in the long term with the problem of Iran's desire for nuclear weapons. At the moment, the Iranian problem is being widely misrepresented as a problem with Ahmadinejad specifically. He is indeed a repellent fundamentalist (with the emphasis on the "mentalist"), but when it comes to acquiring nukes, he is simply following the wishes of his people.

The polls show that a fat majority of Iranians want a nuke, including the liberals the world looks to for an alternative. And in a world where even centre-lefties like Brown say it is essential for a nation's security, who can blame them?

So even when Ahmadinejad and the mullahs are eventually toppled in an internal democratic revolution - as I hope they are - the Iranian desire for nuclear weapons will still be there unless we change the wider, global nuclear context.

In a world where everyone is replenishing and polishing their nuclear warheads, the Iranian people want one of their own, and they are not alone. Where major nuclear powers are slowly scaling down their nuclear arsenals, that could begin to change. We have nothing to lose by trying it, and plenty to gain. But last week, Gordon Brown threw away any chance we have of finding out - and his decision made Britain far less safe.

j.hari@independent.co.uk