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If you are of a nervous disposition, please turn over now, because this piece is extremely disturbing. The first interviewee in our series on the future of energy and the environment was Sir David King, the government's chief scientific adviser, who was at bottom an optimist. Our final expert is James Lovelock, an independent scientist, researcher, inventor and author who believes that about 5 billion people are about to die from war, disease and famine.

Lovelock argues that global warming is so far advanced that extreme climatic change is inevitable. "It won't make any difference if we stop producing carbon dioxide tomorrow," he says. And we are going to notice dramatic effects very soon. "I think it will be pretty bad by the middle of this century. By then all the floating polar ice will be completely gone and you'll be able to sail to the North Pole. That is a huge effect and adds enormously to the amount of heat been received by the Earth as all that white ice used to reflect solar heat. It's a pretty grim picture, there's no question about it."

Portrait by Julian Anderson

But why should anyone listen to the dire warnings of this silver-haired 86 year old? Well, apart from anything else, Lovelock is sprightly and eloquent for his age, and speaks passionately about his science. Indeed, he is not sitting around at home but is on an intensive world tour of lectures to spread his message and promote his new book, which goes by the appropriately dramatic title The Revenge of Gaia.

Furthermore Lovelock is highly respected for his body of work, which includes the invention of the electron capture detector - a device that can detect minute quantities of pollution - the microwave oven and, most significantly of all, the Gaia theory. Widely accepted within the environmental community, this states that the earth is a self-regulating mechanism where ecosystems and the elements needed for survival, such as the atmosphere, have evolved to work in harmony together to sustain life. The problem is, mankind's activities have upset this equilibrium to such an extent that Gaia's response is to chew us up and spit us out.

Lovelock has warned about the consequences of global warming for many years, but his concern about the state of the planet took on a new urgency after a visit to the Met Office's Hadley Centre for Climate Prediction and Research in Exeter. "I get on well with the scientists there and they invited my wife Sandy and I to go along and see what they were doing. We went from one group of people to another and everyone had a very gloomy story to tell. But they hadn't factored in the whole picture."

Lovelock says scientists tend to work in relative isolation on tightly focused pieces of research. As a scientist whose Gaia theory is all about the big picture, he put these individual stories together and realised that the human race was on the brink of disaster. "The problem is we have all been in denial about it for much too long, and didn't realise how serious it was," he says. "All talk of sustainable development is complete moonshine. What we need is a sustainable retreat."

All talk of sustainable development is complete moonshine. What we need is a sustainable retreat

According to Lovelock, rapid temperature rises will turn most of the food-producing countries of the world into desert and scrub before the end of the century. "The four horsemen of the apocalypse will ride again - famine, pestilence, war and death," he warns. "We are talking about huge deaths. We would be lucky if a billion survive; it is more likely to be half a billion." Most survivors will try and get to the Arctic where conditions for survival should be just about right, he adds.

According to Lovelock, survivors will also try and get to the UK, which is where a note of slight optimism is sounded. "My hope is that these islands will survive it because of the Gulf Stream," he says, referring to the warm ocean current that keeps us some 8°C warmer than we should be at this latitude. Global warming will switch this current off, but the fall in temperature will be balanced by global warming. "It won't happen smoothly, but on the whole the British Isles will do a lot better than the rest of the world. We are a very desirable bit of real estate, but we'd better run it decently and properly."

Lovelock compares the UK's current situation to the Second World War. "You have to pull together and do everything you can to stop it," he says. "We can't stop climate change, but we can protect ourselves and save civilisation in these islands to a considerable extent."

He describes rising sea levels as a "nuisance", in that they won't have an immediate impact, but storm surges such as that which wiped out New Orleans are a real possibility. Like King, he says we should be preparing better flood defences to protect London and areas of productive farmland - we will need every scrap of productive agricultural land.

Portrait by Julian Anderson

A lot of new homes will also be needed, partly to house those displaced by floods but also because energy efficiency will become essential. "All our houses are inefficient. We should really be thinking of replacing our housing stock with appropriate housing for the new world to come, which is going to be very strange," says Lovelock. "We have got to start thinking about high-density living, as we will need every scrap of land for food production. ºÃÉ«ÏÈÉúTV houses with gardens will be utterly out." These new communities will resemble dense medieval cities because the rarity of cars will make wide roads redundant. Houses will have to be built from locally sourced materials as the cost of energy will prohibit long-distance transportation.

Lovelock's view that it is too late to stop global warming means he has an unusual take on energy supply. He dismisses renewable energy and has a particular disregard for wind power, describing it as a "pointless exercise". "Nobody mentions that the wind has changed as a result of climate change," he says. "It's no longer blowing great big westerly gales, but it's mostly coming from the north and east - gently - and the wind farms are producing almost no energy. It's an expensive folly."

I have offered many times to take the full output of any nuclear power station in the country and put it on my land. I would have my grandchildren sit on it

Solar power is also dismissed as too expensive, unless there is a technical breakthrough, and Lovelock adds that it only produces energy when we don't need it. As for biofuels, these are totally out, because all available land will be needed for food. "Energy saving is more important than renewables," he says. "Proper construction of houses that are well insulated so they don't leak energy is the first requirement and should be a priority."

In terms of generating new power, he agrees with King: nuclear is the answer. "I'm so glad the prime minister has gone nuclear because it's the only way of keeping business going," he says. His support of nuclear power has infuriated many in the green movement, which venerated Lovelock for the Gaia theory. Nevertheless, Lovelock wants new-generation nuclear - and fast. "We often hear that it will take 10 years to build a nuclear power station," he says. "This is rubbish. The first ones that were built in this country took three-and-a-half years and some are still working. It's the constipated civil service that slows everything down. It's got to be woken up and shaken up."

The main reason behind why a man who thinks it is too late to stop global warming wants nuclear power would surprise many. Lovelock describes it as potentially the cheapest source of energy there is - particularly given the certain increase in energy prices in the future. He says it is made expensive by planning and excessive health and safety regulations. "It's such a safe industry - nobody has died in it since it started in Britain, and nobody has been injured by radiation as far as I know."

He describes the £70bn earmarked for decommissioning as a "waste of money". Those who raise their eyebrows at this should bear in mind that Lovelock is prepared to put his money where his mouth is. "I've offered many times to take the full output of any nuclear power station in the country and put it on the land I own because it's not unsafe - I know what it is because I have worked with radiation all my life. It could go in a concrete pit. It's perfectly safe - I would have my grandchildren sit on it."

According to Lovelock, 30-40% of our energy needs should be provided by nuclear energy. He says it could be even higher - if politics didn't prevent it. The rest of our energy will have to come from coal. "Global warming is going to happen whether we burn coal or not and the UK's contribution is tiny. For us to burn coal to generate electricity would probably add 0.5% to the total of the world's CO2," he says. He adds that nations such as China and India are burning vast amounts of coal and nothing we do will stop them.

Lovelock does have some good news. After all, he points out, someone has to build all these flood defences, new homes and power stations. "This is an enormous opportunity for the construction industry," he says. "It sounds awful to say it but you have an exciting future - there are enormous challenges but there is a lot of money to be made too." However he does admit to wondering quite what this rich construction industry will spend its profits on when the four horsemen ride into town …