During a play about the suffering caused by foot-and-mouth disease, one man sat under cover of the theatre’s darkness and let the tears run down his cheeks.

It was years since Sir David King, the Government’s Chief Scientific Adviser, had made the unpopular decision to carry out a cull of millions of animals to stop the outbreak. But this was the first time that he had let himself feel the human cost.

“The play was about what it does to a family. And I was quite tearful. It really got to me,” said Sir David, who is standing down on New Year’s Eve after seven years in one of the most high-pressure jobs in government.

The incident did not cause Sir David a fit of regret or self-doubt. Instead, characteristically, he treated his tearful breakdown as a conundrum to be puzzled over. What did it say about the battle between emotion and reason that dominates his job? When should he use emotion to grab attention – such as his famous pronouncement that global warming was more of a threat than terrorism? And when should he use cold, hard reason to fight the emotional pleas of protesters – on the MMR vaccine, badger culling or foot-and-mouth?

“On foot-and-mouth, when I was in a helicopter being taken around the country, I was only concerned to bring the disease under control with the smallest number of animals culled,” he said.

“But it was interesting that I hadn’t come close to understanding the trauma these people were experiencing.”

Would it have been dangerous if, at the time, he had let it cloud his judgment? “I think so. If my feelings at the time had overcome the sort of rational processes I was going through, that might have been wrong.”

His detractors use this kind of language as evidence that he is arrogant. Last week Richard Horton, the Editor of The Lancet, accused him of “throwing tantrums” in response to criticism and “taking his faith in science into the realms of totalitarian paranoia”.

His supporters put it another way – that he is passionate, persuasive and sticks his neck out when it matters.

In person, he veers to the latter. Yes, he is relaxed because this is his last interview in his post, but for a scientist, let alone a civil servant, he is no geeky robot. He wears fashionable glasses and talks with wit, warmth, and – especially on climate change – great passion.

This a subject on which he is outspoken to the last. He is scathing – almost recklessly so for someone with a senior government role – about the American performance at the UN global climate talks in Bali last week. “It was embarrassing to watch the Americans resist until the very last moment any attempts at setting an agenda which will actually manage the problem. In the end, they gave way. So what we were watching was the total opposite of leadership.”

Still, he worries that Bali was a rerun of Kyoto in 1997, all talk, little action. “We’ve lost ten years, ten crucial years. We can never take them back. We cannot avoid dangerous climate change now. In 1997 we could have. I’m really worried now that people are talking about it and there’s a sense in which talking is all you have to do. Action is required.”

When Sir David says action he means it. Success is going to be measured in ten or 20 years time, not only in “a dramatic change in every form of energy that we’re using”, but in “every part of government that’s involved, including urban planning”.

If that is what is needed, how does he view such gestures as Gordon Brown and David Cameron putting solar panels or a windmill on their roofs? He smiles and raises a sceptical eyebrow. “Symbolic gestures of course need to be examined carefully. I don’t want to criticise either Cameron or Brown for demonstrating their intent, but we’ve got a long way to go.”

Individuals can do their bit, of course: he recently said that “young women should stop admiring young men in Ferraris”, a shorthand for the kind of cultural shift that he thinks is already under way. He is doing a little more: at 68 he is starting a new career as head of a University of Oxford institute encouraging business solutions to environmental problems.

Life is “complicated”. His family home, where he brought up four children, remains in Cambridge, where he was a professor of chemistry. His wife, Jane, who was a highly successful partner in a city law firm, is giving up her career to teach in Zambia for 18 months.

Their home is not festooned with solar panels and windmills – he’s not convinced that the technology is advanced enough to make either worthwhile. In summer he uses “direct solar heating”, using the Sun to warm the water for his shower. Aside from that he just does the “usual things” to his house: insulation, energy-efficient light bulbs. This prompts another provocative attack on the Government.

“I’ve got to do up my flat in Oxford. I walked into a shop to buy some lamps. You couldn’t stay in the region where they were selling them very long just because of the heat being generated. Why should you be able to walk into a shop and buy a tungsten filament bulb? I think it’s time to completely phase them out.”

So why doesn’t the Government do it tomorrow. “Ask them.”

If his big achievement has been raising government consciousness – such as it is – on climate change, what will be the challenge for his successor, Professor John Beddington? Sir David suggests that it may be flu – or rather the prospect of a global pandemic caused by a bird flu that can spread between humans.

“If you’re managing 6.5 billion people on the planet, how many fatalities would there be with a pandemic around the world? Many millions, tens of millions. So the effort involved in trying to quench it has not yet matched the seriousness of the threat.

“What I think really hasn’t got across to people is the horror. The calculations indicate that within three months of an outbreak occurring anywhere in the world, we have a pandemic in every country in the world. And that’s simply because so many people are travelling by plane.”

Although Sir David thinks that this is a “low probability event, the impact is so high we really have to manage our defences”, his description of the apocalyptic catastrophe that would ensue in Britain, as society falls apart, is chilling. “We’re planning for a fatality rate of 300,000. So imagine what’s happening to our economy. Imagine the follow-through in terms of essential services. Now that’s bad enough in Britain, but now imagine this is happening all over the world. So what you have is a situation that the world hasn’t planned for.”

While Britain is quite well prepared – with a stockpile of antiviral drugs that has been doubled, partly at Sir David’s instigation – he worries that too little is being done to help countries such as Vietnam, Indonesia and China to prepare. This is vital for our own sakes: if they can contain a new virus, a pandemic may be stopped at source. “It’s not only us worrying about the state of people in other countries, it’s not good for any of us."

The World Health Organisation “simply hasn’t got appropriate resources to hand. Every country is keen to protect its own population, and only as a secondary thought thinks about the rest of the world. Now the impact on countries that can’t afford all these defences is going to be absolutely awful, but we just don’t have the support for the WHO.”

Is he celebrating Christmas religiously? “Do I believe in God and pray to a god? The answer is no. But I think that my views of the natural world and of human beings’ place in the natural world, however, is not unaligned with a lot of people who call themselves religious.” He said that he was not that distant from Richard Dawkins, the scientific scourge of the religious. “I worry about any abandonment of the Enlightenment, I worry about the encroachment of creationism.”

Instead, he chooses to sing a hymn of praise to planet Earth. “I’m a humanist, but more than that I worry about the biodiverse system. Let me have a pop at people who say we should seek another planet: we’re wrecking this one. There’s no understanding in that statement about what this ecological niche the planet Earth is, for us and for all of the living matter on it.”

His legacy in the job? Climate change, of course. “I feel slightly embarrassed, but I can quote someone else. Ian Conn, who’s a director of BP, made a speech in which he said that before Al Gore was Dave King.

“The point is that I was up there sticking my neck out on that issue certainly before Gore started making those speeches.”

Should he have got a Nobel prize, too? “That was suggested by Ian Conn, but I’m delighted that the inter-governmental panel got the acknowledgement it really deserves. And that Al Gore was acknowledged for the enormous work he’s done.”

Sir David does not want for self-belief. But on this occasion, we can forgive him a little emotion colouring the facts.

Sir David King

Age 68

Family Married with three sons and one daughter, all grown-up

Education University of the Witwatersrand, Johannesburg, South Africa

Research career Junior posts at Imperial College, London, and University of East Anglia; Brunner Professor of Physical Chemistry at the University of Liverpool, 1974-1988; Professor of Physical Chemistry at the University of Cambridge since 1988; head of the Cambridge Chemistry Department, 1993–2000

Government career Chief Scientific Adviser to the Government and head of the Government Office for Science since October 2000

Honours Elected as a Fellow of the Royal Society, 1991; knighted, 2003; elected as an Honorary Fellow of the Royal Academy of Engineering, 2006

Future Steps down as chief scientist on December 31, to be succeeded by Professor John Beddington. He will then join the University of Oxford, as director of its new Smith School of Enterprise and the Environment

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