The British government elected yesterday will have to take two radioactive decisions by 2008. These choices will address the biggest existential threats to the future of the human species: man- made climate change and nuclear weapons. Your great-grandchildren will live with the consequences when Tony Blair, Gordon Brown and Michael Howard are nothing but skeletons and a pile of yellowing press cuttings.

The first decision: Should we build a string of nuclear power stations? Just five years ago, environmentalists wouldn't have paused for a second before answering: no! no! no! The green movement was smelted in the fight against nuclear power, and even now the facts are startling: every man reading this, for example, has plutonium in his testicles thanks to nuclear power.

But now there is an even-greater ecological danger than a Sellafield or Chernobyl: man-made global warming. A handful of environmentalists like James Lovelock have broken ranks to declare that waiting at the end of the climate change rainbow there sits a nuclear power station. They give off no greenhouse gases " none " so Lovelock argues that their dangers 'are insignificant compared with the intolerable heat waves and rising sea levels of global warming'.

I am uncomfortable with his unquestioning evangelism for nuclear power. Leaving future generations with tonnes of nuclear waste that remain toxic for 10,000 years is hardly problem-free, even if you forget about the risk of accidents or fanatics getting hold of nuclear material. One environmentalist says: 'Embracing nuclear power in an attempt to avoid global warming is like taking up heroin to avoid an addiction to crack.'

But are they equally bad? To a sunny optimist like me, it is painful to admit we now face three options and they are all lousy. Option one: we can carry on with the current level of carbon emissions and try " somehow " to deal with soaring temperatures and the unravelling of the world's ecosystems. Option two: we can have a massive shift to nuclear energy, and deal with the cancer and the nuclear waste. Option three: we can drastically scale back our carbon emissions through government diktat, with the inevitable impact this will have on mobility, economic growth and employment.

I wish renewable energy sources like wind, wave and solar power made up a fourth option, but we have to be honest: while essential they will not meet most of our energy needs any time soon. I reluctantly think the least-bad (but still terrible) option is to build nuclear power-stations for a generation while we wait for these clean sources to become sufficiently advanced to meet our needs.

But this leads to the second, equally large nuclear decision - and a difficult problem. If we accept the spread of civil use of nuclear science as a regrettable lesser evil, are we also making the spread of nuclear weapons inevitable? Many of the countries that are trying to acquire nuclear weapons publicly claim to be legally developing nuclear power, but have in fact been enriching uranium on the side.

Britain is hardly blameless. For the first time since the end of the Cold War, nukes are about to be rhetorically wheeled into the chamber of the House of Commons. The government now has to decide whether to replace the Trident nuclear submarine with a new pounds 10bn set of weapons. To WMD or not to WMD? That is the question.

The government and the defence establishment will try to frame this as a question of national virility. Do you want your country to be able to 'punch above its weight'?

But there is a wider context. This is an age of massive nuclear proliferation, with regional Cold Wars developing all over the world: India vs Pakistan " a rivalry so volatile that Britain evacuated her citizens from the region three years ago " and soon Israel vs Iran, and North Korea vs Japan and South Korea. If these eyeball-to-eyeball confrontations continue, sooner or later there will be a nuclear exchange.

During the past few years, the British government has had a failing, flailing strategy: try to bribe or bomb new countries out of acquiring WMD. But the strategy of renewing and upgrading our own WMD while lecturing the rest of the world not to get them simply doesn't work. The pressure to acquire weapons from the Iranian public and Iranian democrats is even stronger than among the unelected mullahs. If anything, our hypocrisy seems to be making them more determined to get nukes.

There is another way. Right now, all the pressure on the government about nukes comes from conservatives who want to upgrade and retain every single warhead. It is time to offer a counterbalance by reclaiming the Campaign for Nuclear Disarmament from unilateralist Trots and make it a mainstream, multilateralist organisation again.

It might sound nave to propose reviving disarmament, but it is even more delusional to believe we can carry on with dozens of countries acquiring nuclear weapons without confronting catastrophe sooner or later. As a signatory to the Nuclear Non-Proliferation Treaty (NPT), Britain is supposed to already be committed to 'pursue nuclear disarmament'. The treaty is currently being reviewed in a month-long meeting in New York, where Kofi Annan has described existing nuclear arsenals as 'a real danger' and said disarmament was 'vital'.

So how could Britain begin to meet these commitments, as most of the other 180 signatories are demanding? We could begin with an acknowledgement of reality: Britain is extremely unlikely to ever use our nuclear warheads. Against the only security threat we face they are totally useless. Even in the extremely unlikely worst-case scenario that al-Qa'ida acquired and used a nuclear device, what good would our deterrent be? Who would we nuke in response?

There is only one good use for our nukes: as bargaining chips to begin a process of phased multilateral disarmament that would gradually reduce the world's nuclear stand-offs. Britain " one of the founder members of the nuclear club " could offer to begin slowly dismantling a few of our 48 warheads in exchange for Iran remaining non-nuclear and the other nuclear powers making similar incremental reductions. The drift towards building nukes might " just might " begin to be replaced with a momentum towards disarmament if we blink first. The NPT provides the architecture to do this " but until we force our government, it will cling to these totems of power.

Do we have to wait for another Hiroshima before we act?

Long after the ephemera of day-to-day Westminster politics and the bustle of the election has been forgotten, these are the nuclear decisions on which our politicians will be judged by the history books " if there are people around to write them.

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