They are both environmentalists, but Zac Goldsmith and James Lovelock disagree on how to meet our energy needs. In the week a Government advisory body came down against a new generation of reactors, they debate the issue by email

Dear James,

The threat of climate change dramatically outweighs the dangers of nuclear power. So if it were the case that nuclear power is the only viable solution to climate change, I would embrace it. But given what we know, I would do so begrudgingly. Nuclear is so expensive that without massive government support, it wouldn't exist. It is so dangerous that the insurance industry will cover it only if the government agrees to cap its liabilities. And it is dirty. In this country, we face a staggering £70 billion clean-up bill.

Of course these costs are worth paying if nuclear represents a solution to climate change. I don't believe it does. First, with a programme of energy efficiency, we could easily achieve energy savings equivalent to double the energy currently generated by nuclear, at a fraction of the cost. Pound for pound, that's the best use of money, as the successes of companies like Dupont have shown.

Second, the nuclear lobby is prone to exaggerations. Even if we replaced existing nuclear reactors with gas and coal, we would raise carbon emissions by between just four and eight per cent. And, contrary to claims, nuclear isn't carbon neutral - every stage of the nuclear cycle, other than fission itself, produces carbon dioxide.

The alternatives exist, and we should embrace them.

ZG

Dear Zac,

The crux of our climate problem is an imminent heat age that could last as long as 100,000 years and nothing that we now do will stop it. The Green Movement's recommendations of sustainable development and renewable energy are well intentioned but a hundred years too late.

By unremitting growth and development, we have committed our once lush and lively planet to an arid and barren existence quite unable to support the billions now on Earth. Soon, the Arctic and perhaps these islands may be the only habitable places left and life on Earth will try to move here.

We cannot save the world but we can do something to save ourselves in the United Kingdom. For this, we need secure indigenous supplies of food and energy. Our only immediately available energy comes from coal and nuclear, our gas and oil will soon be gone and we can not rely on supplies from abroad.

We need urgently to recommission, not decommission, our nuclear power stations. The stories of vast costs and of dangers from nuclear wastes are unreal and no more than fearful imaginings born from Cold War propaganda.

Jim

Dear Jim,

We can argue indefinitely about the costs of nuclear - but to simply dismiss them is cavalier. Indeed, the City itself has rejected nuclear, both here and in the US, because of the high costs. And the waste figure I quote is from our (pro-nuclear) Government.

But, that aside, it's worth repeating that even if we replace existing nuclear reactors and double the number, we would see an eight per cent reduction in carbon emissions - and not until 2035. It is a gain, but a miniscule one, and at a huge cost.

What's more, nuclear power is not "immediately available". It will take at least 10 years for new plants to become operational. That's not the case with energy efficiency, which can happen today. Nor is it the case with the combined heat and power (CHP) systems that already flourish in parts of Britain and Europe, and which are demonstrably cheaper, cleaner and safer.

ZG

Dear Zac,

The City rejects dull, safe long-term nuclear power when they have subsidies for renewables and enjoy the glorious returns from the ever-rising prices of carbon fuels as supply fails to meet demand. Nuclear electricity is now the safest and the cheapest; it is the UK's only reliable and secure indigenous source and there is ample uranium at negligible cost for many decades from now.

The French build nuclear power stations in four and a half years. With a pro-nuclear Government so could we. By recommissioning nuclear energy, we can give some of the older reactors a longer lease of life and replace obsolete reactors on the same sites. It would be far cheaper and quicker than new build. Do you think the Finns, with ample Russian gas at their frontier, are mad to democratically choose a nuclear new-build programme? Maybe they know something we don't.

Jim

Dear Jim,

In the past 20 years, OECD governments have provided nuclear energy with $160 billion for research and development alone. Add to that the waste and security bills, the cost of cleaning up uranium mines, and your comment about subsidies for renewables looks absurd.

Similarly, your assertion that nuclear is the "safest". Even British Nuclear Fuels has described the prospect of an airplane crashing into a nuclear plant as "unthinkable", a view echoed by the International Atomic Energy Authority.

Finally, without imported uranium, nuclear energy couldn't happen, so to describe it as an "indigenous" energy source is misleading. What's more, it's hard to know exactly how much uranium exists, but some of the world's best-known commodity traders are betting on a huge price rise on the back of already-dwindling reserves. Some analysts believe that uranium supplies will deplete on roughly the same timescale as oil and gas.

Nevertheless, we agree on the need to reduce emissions, and to provide energy security. So I have two questions: First, if energy efficiency can deliver energy savings that greatly exceed the energy currently provided by nuclear - and more cheaply -why do you not support such a programme as a first priority?

Second, given that there are plenty of examples of combined heat and power systems reliably catering for large urban areas - at greatly reduced financial and environmental cost - we know already that alternatives exist. We also know that decentralised energy is less wasteful, and importantly, in light of your dire warnings, more adaptable. Why, then, do you not encourage their rapid adoption?

ZG

Dear Zac,

Yes, the OECD countries have spent billions on nuclear projects, and we are about to squander £60 billion to turn our nuclear power stations into children's playgrounds. Why not keep them running and provide emission-free electricity? Nuclear is now the cheapest energy source and will save us billions in electricity bills and taxes. Any government foolish enough to decommission will face an angry electorate when the electricity bills hit home. Certainly save energy, but it needs leadership to enforce.

Germany has devastated its countryside with a surfeit of wind farms yet they produce only 16 per cent of their rated electricity output, and Germany now emits more CO2 than it did before they were installed. As a good city needs its parks, so we on this crowded island need the countryside. Incredible that Greens, of all people, should wish to make it an industrial site filled with giant wind turbines. The history of world power production shows nuclear to be the safest - 10 times safer than hydroelectricity and hundreds of times safer than carbon fuels.

Anti-nuclear argument is based on irrational fear, and the prospect of an aeroplane flying into a power station is unthinkable only because it is a waste of time to speculate on it. Apart from plant workers and passengers in the plane, no one would be killed. As a long time Green, I am appalled by the way that our movement has become a theocracy that sustains itself by fear; we should have learnt from the churches who mistakenly tried to keep their flocks faithful by preaching fear of hell fire. Such fears become unsustainable and are misleading now that we face truly great dangers from global heating; for these, renewables are about as practical as an umbrella in a tornado.

J

Dear Jim,

The difficulty is that we don't speak the same language. For me, a nuclear accident would render large tracts of land uninhabitable, and would be disastrous. For you, it wouldn't fundamentally undermine the biosphere, and is therefore not worth worrying about. We are approaching these issues from different perspectives. Perhaps that's why I've failed to extract answers from you to many of the points I've raised.

For instance, if the impact of nuclear on emissions is minimal, how can it represent anything more than a marginal solution to climate change? If it is based on an imported resource that is limited, why do you describe it as sustainable and indigenous? If uncontroversial alternatives, like CHP are proven, why not pursue them?

I share your fear that we are running out of time. But that is not a reason to abandon best practice in favour of a half-remedy that comes with much unwanted baggage. Our politicians are able -- indeed some are eager -- to embark, for instance, on a programme of energy efficiency. And it's not courage that is motivating them. Energy efficiency requires no sacrifice on the part of the electorate, it would save us all a great deal of money. It's one of those rare win-win solutions.

Incidentally, I don't disagree with some of your views on giant wind monocultures. Wind is a useful option, but not at the expense of our countryside.

ZG

Dear Zac,

So we have to agree to disagree. Perhaps we did not try hard enough to find common ground -- we should have because it exists. We know that energy saving works; it did so in the Second World War by tight rationing. But could any of today's political parties, when in government, have the will to enforce it? We both, I think, view with suspicion technological fixes to cool the global climate, and this is something the United States is likely to try.

Where we most seem to differ is over the state of the Earth. You believe that somehow, by good green action, we could turn back the clock and restore the world to its pre-industrial state. I wish that you were right and that we could, but I see the Earth as far beyond recovery and now moving ineluctably to hot and arid landscape.

To me, the urgent task before our government is to plan and spend now to lessen the catastrophe when storms and rising sea levels flood London and other coastal cities. We will need also to protect by embankments the low lying productive farmland of East Anglia that is our best source of food. Food and fuel imports may no longer be available at prices that we can afford.

J

• Zac Goldsmith is the editor of The Ecologist magazine and is also helping to organise an energy review for the Conservative Party

• James Lovelock is an environmental scientist and the author of The Revenge of Gaia: Why the Earth Is Fighting Back - and How We Can Still Save Humanity (Allen Lane)