How to address someone called Baron Stern of Brentford?

There are several options: Dr, because he has a D.Phil from Oxford, professor because that's what he is at the London School of Economics. Then there's the knighthood he received in June 2004, so one could try Sir Nicholas, except that he's a lord.

"Most people call me Nick. It's all too confusing and why should anybody outside Britain take the slightest bit of interest in the peculiarities of our titles?"

Indeed, especially when the real reason he's here is to tell the story - something he's been doing ever since he shocked the world in 2006 with a 700-page wake-up call known as the Stern Review.

The message about the dangers of climate change wasn't new, but the dry language of the balance sheet was. Rapid affordable action now (investing 1 per cent of global GDP per annum) would prevent huge losses (amounting to 20 per cent of global GDP) later. He since upgraded the investment needed to 2 per cent because he now feels he somewhat underestimated the risks.

With economics fused to climate change science, politicians and business leaders could now take over a debate that had been owned by environmentalists and scientists.

Probably more than any other, it's Stern we have to thank that terms like "offsets", "carbon tax" and "emission trading scheme" are now in common usage.

At first it's hard to credit that this owlish, grey-haired, soberly suited gentleman could have such impact. But behind the softly spoken words is a fastidious resolve and an unwavering rationality. "Setting out the evidence, setting out the argument is what really matters," is his refrain.

Some of Stern's doggedness undoubtedly comes from his upbringing. When he became a member of the House of Lords in 2007 there was a Baroness Stern in residence. The new lord had to choose a location for his title, hence Brentford in West London where he grew up.

His father was a German Jew who escaped to Britain after Kristallnacht in 1938, working there until the summer of 1940 when he was rounded up with other German and Austrian Jewish refugees in Britain and sent off to be interned in Australia for two years. Stern's grandmother was killed in the Holocaust.

He says his father wasn't bitter - "not in the sense of something corrosive that undermined him". What mattered, says Stern, were the values.

"It provided a deep commitment within the family against racism and fascism. Many of that generation were remarkable in their strength of looking forward. They asked the question: 'What should we do? Should we dwell on things and get bitter about past injustices? Or should we learn from that and fight the next one in the battle against racism and fascism?'."

The "very political breakfast, lunch and dinner table" in the Stern home has served him well. He is very much concerned with looking forward - to the end of this century to imagine what the world will be like for grandchildren.

The picture's not good - concentrations of greenhouse gas levels in the atmosphere that give us around a 50-50 chance of being 5C warmer relative to the mid 19th century.

That's a temperature increase the earth hasn't seen for 30 to 50 million years and something humankind, around for just 200,000 years, is unlikely to be able to adapt to. So Stern asks the same question his father did: "What should we do?"

To reduce the risk to a temperature rise of around 2C the arithmetic is fairly straightforward. Reduce worldwide emissions in the atmosphere from their current level of 50 billion tonnes of carbon-dioxide-equivalent to 35 billion tonnes by 2030 and to 20 billion tonnes by 2050.

It's also a story of investment in greener and more energy efficient technologies, and a transition to a low-carbon economy that can be the most dynamic period of growth in economic history.

Stern acknowledges it's a difficult story because the consequences can't be seen instantly.

"It's a test of rationality. It's a test of our powers to understand science and to explain."

The explaining is difficult too: "It's about relationships and probabilities. If you increase the concentrations of greenhouse gases, the probability of these kinds of temperature increases and the probability of climate changes rises. People try to say because the evidence is about probabilities it's uncertain. No it's not. It's evidence about uncertainty."

But the basic scientific conclusions about climate change are very robust and founded in more than 200 years of scientific endeavour. In short, greenhouse gases trap heat and humans are emitting ever more greenhouse gases.

"Those who choose to ignore evidence, I think, are reasonably described as unscientific or irrational."

So far, in the series of Sir Douglas Robb lectures he's given at Auckland University, he hasn't encountered New Zealand's vocal climate change deniers. But he's no stranger to people who disagree with with his point of view, sometimes in extreme ways.

"I had a bunch of people in California - seven or eight of them in different parts of the audience, all people in their 20s with shaven heads. That's their privilege. It's not their age or shaven heads that counts, but as it happens, that's who they were. And they would chant and be aggressive, but you just have to deal with that."

How? "You deal rationally with the issue. You say: 'Look here's the evidence. Here is how it's accumulated'. You can challenge the evidence - that's good science and good discussion. But chanting and slogans and making up facts and misinterpreting evidence isn't."

What bothers him most is the way deniers seize on "oscillations" or fluctuations in temperature and try to argue there is no trend. "If you've got an undergraduate student, trying to estimate the growth of the economy by joining a line between the peak of the last boom to the bottom of the last recession and think they are estimating the growth rate you would throw them out of class."

The same poor logic is used by deniers who try to argue temperature stopped rising 10 years ago - completely false when you look at the trend, ask about the overall average and apply knowledge about why decadal oscillations occur.

"Ordinary sensible people looking at evidence will know that you have to take an average over time and if you do that you see every decade over the last five to six decades has been hotter than the previous ones."

Stern's steadfast response to the deliberate sowing of doubt and spreading of confusion is rationality - patiently setting out the case in a balanced, clear way. "So many people who think they are Galileo are not."

A large measure of why the story isn't properly told he puts down to a failure of media and journalism. Yes, says Stern, the onslaught against the story is strong and organised, but it's irrational, unscientific and badly based. It says because you can't identify relationships with certainty, the best assumption is they don't exist. "That's a schoolboy error."

Stern says the media fails to look at the wider context - something that should have happened regarding the hacked "Climategate" emails from the University of East Anglia. "What journalists should have asked is that if everything that the university had done was obliterated, what difference would it make? The answer to that question would be hardly any difference at all."

Similarly, the IPCC paper incorrectly predicting Himalayan glaciers retreating should be seen in a wider view: "This idea that if you find a few papers that are wrong among a few thousand papers you've somehow undermined all the evidence is so intellectually dishonest, it's very important to expose that intellectual dishonesty for what it is."

But sometimes he says you have to accept you'll be deliberately misinterpreted. Like the Times headline last year: "Climate chief Lord Stern: give up meat to save the planet". He shrugs it off as the newspaper having a bad day. "The headline writer was perhaps short of sleep or short of time."

For the record he is not a vegetarian. "What I said was that diets which were strong on say beef or lamb had higher emissions than say chicken which has higher emissions that vegetarian diets." He says he was making a factual statement, not telling people to become vegetarians. He's quick to acknowledge too that pasture-fed livestock have have a much lower emission profile than grain-fed animals.

Stern doesn't see any point in feeling beleaguered or derailed by deniers. "It's only if you let yourself be derailed. Sometimes you get irritated, but you keep going.What else can you do? We can't lose our temper. We can't jump up and down. We just have to be patient and explain. Rationality beats irrationality."

Stern questions

To avoid a 50/50 chance of catastrophic temperature increases of 5C early next century we need to reduce worldwide greenhouse gas emissions in the atmosphere from the current 50 gigatonnes to:

* 35 gigatonnes by 2030, to
* 20 gigatonnes by 2050

(measured in carbon-dioxide-equivalents relative to 1990 levels)

Stern answers
* High emission reduction targets for rich countries
* Lesser targets for developing countries
* An international emissions trading regime
* Combating deforestation
* Technological advances to reduce emissions
* Financial assistance for developing countries to help adapt to climate change