One of the parents I used to see in the playground at my son’s primary school was a Rastafarian postman of Jamaican origin named Roger. It was a secular school, and Roger once told me, in reverent tones, ‘I don’t need any church: my religion is David Attenborough.’ He never missed a programme.

The cult of Attenborough is as close to a religion as you can get. A religion in its purest sense is a form of communication, tolerance and the spreading of awareness – somehow he unites all these things. He is a force for unification. He doesn’t preach, but David Attenborough makes you care.

His appeal is diverse, and lately it has been more diverse than usual: in 2013 he made a documentary with Björk, and this year he conducted an interview with Barack Obama and did an impromptu narration, on BBC Radio 1, of the video for Adele’s Hello. The truth is that Attenborough has unprecedented appeal (he even gets recognised in the North Pole), particularly for the young.

Attenborough in a submersible, off the Queensland coast Credit: BBC

He has brought a love of wildlife to all, and must be the most widely heard voice there is on crucial global issues such as conservation and climate change; while other, more qualified experts, are often derided or overlooked. It’s a huge responsibility. How does he feel about that?

‘Embarrassment, mainly,’ he says in his unmistakable soothing voice, the aural equivalent of a Werther’s Original. ‘The awful thing is that people think you know far more than you do. For example, I often get asked if I’ve actually seen climate change and I have to say, look, I could find you examples of dramatic climate change, and I could find the converse, but it’s very dangerous to just pick one particular circumstance.

'You have to take the bigger view; you have to respect the findings of people who spend their lives surveying this sort of thing, and make a responsible, scientific summary of where we are.’

What is it about him that draws people in so completely? Well, there is the voice, obviously, which distils within it his appeal: poetic, enticing, it contains a wealth of knowledge, curiosity and humour. The voice has changed over the years, certainly.

"Narration is like verbal carpentry – it’s making a sentence the right length and no longer or no shorter" David Attenborough

Its pitch has lowered and it has become more husky and ragged, but it has lost none of its enthusiasm, its musicality, whether he is exclaiming over the dancing prowess of the mantis shrimp or the sexual habits of coral. ‘Bundles of sperm and egg bulge from the coral polyps…’ he purrs in his new series about the Great Barrier Reef, tickling the words under their bellies. ‘Great ribbons of coral spawn drift over the surface of the sea…’

George Fenton, who composed the music for many of Attenborough’s major series, would treat his narration like the main character in a drama when he was scoring each episode, working the music around the inflections of his voice.

‘His inflection is so precise, so deft, that it’s like having a singer, because it tells me exactly what the juxtaposition of his line reading and the image is, so I can inflect in a more subtle way,’ he says. ‘The only dramatic arc in the story comes from his voice and the music.’

‘The whole business about narration interests me because it’s what I’ve spent my life doing,’ says Attenborough now. (He has recently been critical of the trend for getting actors to narrate wildlife documentaries.) ‘I’m not sure that anybody else is so completely into it.

'Actually, it’s like verbal carpentry – it’s making a sentence the right length and no longer or no shorter, and getting rid of unnecessary adjectives and similes – that sort of thing. I’m relieved if it doesn’t come out as being studied; if it comes out as if I’m speaking relatively normally. Of course, you know that’s not really how people speak.’

The natural habitat of Sir David Attenborough is an imposing house in Richmond, with interesting trees in the garden. The door is opened by his daughter Susan, formerly a head teacher, who now runs his affairs. (His son Robert is a lecturer in bioanthropology in Australia; his wife Jane died in 1997.)

Our interview takes place in a new wing of the house, in a huge library with a piano. Attenborough looks alert and relaxed in a fetching blue linen shirt. He doesn’t suffer fools gladly, which is to say he can be rather sharp. But once he is engaged, you are captivated; he has lost none of his acuity and talks eloquently, with no hesitation or forgetfulness.

He looks a lot better than when I last saw him, shortly before the release of the Frozen Planet series (2011), when his mobility was severely hampered by knee problems and he could barely walk.

He shows off his new knees now, kicking them up and down as if he is doing the cancan. They have given him a new lease of life, he says. Why didn’t he do it before? ‘Cowardice.’

He saw a surgeon 10 years ago who told him he wouldn’t be able to provide anything better than he already had, and that he should wait. The knees then deteriorated and Attenborough lost his nerve. Finally, last year, he addressed the matter. He can’t stand looking at old footage of himself now. ‘I’m embarrassed – I think, who is that old bloke hobbling into shot?’

Clownfish are among the thousands of creatures that inhabit the waters of the Great Barrier Reef Credit: Alamy

Being 89 did not deter him from making his ambitious new series, The Great Barrier Reef, a natural wonder he has returned to nearly 60 years after first visiting it for his BBC Zoo Quest series in 1957. In episode one, we see him standing on the beach of the reef looking out to sea, then cut back to old black and white footage of exactly the same shot: a young, handsome David with a serious haircut, and the same thoughtful brow.

‘People say, what was the most magical moment in your career as a naturalist?’ the voice intones, ‘and I always reply, “The first time I put on a mask and went below the surface and moved in three dimensions with just the flick of a fin, and suddenly saw all these amazing multi-coloured things living in communities right there.”’

His initiation into scuba diving, he tells me, is indelibly printed on his mind. ‘You suddenly realise you can move in any direction. You’re not harnessed by gravity any more. You’re free. It’s bliss. An extraordinary experience, like going into space. There’s no equivalent anywhere else in the natural world of such splendour: all of these things moving through an architecture of coral.

"The underlying problem, of course, is beyond the reef: it is what the world is doing to the atmosphere" David Attenborough

'You never know what you’re going to see when you turn the corner – it’s far more obviously exciting and visually thrilling than, say, thetropical rainforest, which is the nearest biological parallel. In the rainforest they’re all hiding, so you have to be quite a good naturalist to really see what splendours are there. But on the reef they’re all on display. It’s like the Christmas windows at Harrods.’

Diving does have its inconveniences. ‘Unless you’re a real pro, which I’m not, underwater swimming can cause a lot of problems: breathing, clearing your head, clearing your ears, the pressure on your lungs – all that palaver.’ But in the new series he had unprecedented access. Using the 56m (184ft) exploration vessel Alucia as a base, he climbed into a submersible vessel that can dive an incredible 300m (984ft).

Not that there’s much to see at that depth. ‘No, you only need to go down 30ft on the reef to find sharks and turtles and schools of fish and all sorts of wonderful things. It’s a joy.’ The excitement in his voice is palpable as he describes jostling coral colonies, clicking seahorses, damselfish, baby sharks, and clownfish bathing in anemones that would be poisonous to other fish – a lovely view of the marine world going about its daily business.

They dived at night, which is when most of the action takes place. The submersible is piloted by an ex-naval submariner, Buck Taylor. ‘It’s a plastic bubble where you breathe normally and the temperature doesn’t vary, and you’re with a highly professional chap who’s done several hundred dives – it’s enchantment.’ There are just the three of them, including the cameraman, and the arm of the submersible can be directed to collect things into a basket.

Attenborough in 1957 Credit: BBC

The Great Barrier Reef is so big that it can be seen from outer space. It is the largest living structure on the planet: 1,429 miles long and 140 miles across, with about 900 islands and 3,000 coral reefs. The reef is special for many reasons, including the shape of its floor – a shallow shelf that drops down steeply 2,000m (6,562ft) or more – and it is an ecosystem like no other: life here can evolve 50 per cent faster than in other marine environments. It contains 450 or more different species of hard coral.

But a lot of coral has already been lost, and if the ocean warms by the accepted lethal measurement of 2C, then most of it will die. ‘The underlying problem, of course, is beyond the reef: it is what the world is doing to the atmosphere.

The big problem that faces all maritime nations with coastlines is acidity [due to increased carbon dioxide] in the oceans, and an increase in temperature. And coral is sensitive to that. But it will be interesting to see what happens with deep-sea corals, where temperature is not an issue, or not a lethal issue anyway – it may be that corals will travel to different depths.’

Bushy red soft coral at the Great Barrier Reef Credit: Alamy

Earlier this year, he found himself discussing this issue with the president of the United States. He was summoned to the White House, and the resulting interview was a slightly odd affair, with a star-struck Barack Obama looking charmed, and Attenborough, slightly awkward in a tie, robustly suggesting that what an achievement it would be for America to embrace the challenge of harnessing energy from natural sources and finding a way of storing it, and the economic opportunities that would ensue.

‘The overwhelming impression was what a nice man Obama was,’ says Attenborough. ‘And how open he was, and how welcoming and charming. It was a huge privilege.

One thing that wasn’t recorded was that on the way to the Oval Office, walking through the Rose Garden, he said to me [here he does a creditable impression of Obama] about his interest in the natural world stemming from growing up in Hawaii, and I found myself saying, “Well, in that case, you’ll be familiar with the humuhumunukunukuapua’a,” which is a little reef triggerfish – and I used to keep them in an aquarium in this house.’

The natural world continues to surprise Attenborough. He says it will never run out of surprises. In his 90th year, he doesn’t plan that far ahead, but let’s say his dance card is full for a while yet. There are places he hasn’t been to and a few where he’d still like to go, but mostly there’s a reason why he hasn’t been to them – for example, ‘the centre of the Gobi desert because there’s absolutely nothing there’.

"People say, what was the most magical moment in your career as a naturalist? I always reply, 'The first time I put on a mask and went below the surface'" David Attenborough

His approach to writing hasn’t changed much: for his major series – Life on Earth, The Life of Mammals – Attenborough would conceive, research and write the programmes, but in the last few years he has done less of that. He was actively involved in all aspects of The Great Barrier Reef, whereas for the recent BBC series The Hunt, produced by his long-term collaborator Alastair Fothergill, he did only the narration.

‘We have to let the pictures tell our story,’ says Fothergill, ‘so we shape the narration during the edit – ensuring there are gaps for it to fit into, and considering the placing and role of the score at the same time. By the time it leaves the cutting room it is 80 per cent there. Then the producers and I work on it to get it 90 per cent there before we send it to David, who always receives it at least two weeks before the recording.

'As we have worked with David so much, we know how to write for his style of delivery. He makes changes and the script bounces back between us. This gives him a chance to really understand the twists and turns of the narrative. When he comes to record his narration it really helps him give the wonderful performance he does.’

Attenborough on Heron Island, Great Barrier Reef Credit: BBC

Lately Attenborough has become more outspoken on climate change (and population control – he is a member of Population Matters, a charity advocating sustainable human populations) but it is an issue he rarely talked about, not least because of the BBC’s obligation to remain non-partisan.

But also because he wasn’t sure. ‘My inclination was to believe in it, but I shrank from speaking about it too loudly because if you’re on the air as much as I have been in the past 50 years you have to recognise that you’re in a very privileged position and you have to be careful about what you say. And if you’re going to talk about something this consequential, you have to be damned sure that you’re right and wait for the scientific consensus to be really firm.’

‘He is very aware of the responsibility,’ Fothergill says, ‘though for a long time he has done a lot for conservation behind the scenes. He used to say he was not an environmental expert and others were already beating that drum, but when we made Planet Earth in 2006 he presented a powerful two-parter on global climate change. He has always recognised the need to show new generations the wonders of the natural world – if they can’t see it, how can they care?’

A mantis shrimp, one of the creatures found at the Great Barrier Reef Credit: BBC

Recently he dominated the news agenda from Paris at the global climate conference, asserting unequivocally on BBC Radio 4’s Today that ‘the signs are absolutely clear that the world is warming’.

It’s not all bad, he says now, offering up as an example that the population of mountain gorillas is on the up. ‘There are many more now than when we first filmed them in the 1970s. Every now and again, you see a thing like that and your little heart says, well at least we did something.’ He admits it is probably because of the gorillas’ financial value – ‘the most important earners of foreign currency to Rwanda by a long way.’

Attenborough is aware of the danger that people might suffer from climate-change fatigue. ‘Sure they do. So do I. Wouldn’t it be nice if you didn’t have to worry about it? But you can’t afford to be fatigued.’

"If you’re going to talk about something this consequential, you have to be damned sure that you’re right" David Attenborough

He’s a realist. Things are not going to get better, he says, ‘but if we’re lucky, and we work very, very hard, they’re going to get less worse. If we work very, very hard the population might not increase quite as fast as it has done. If we work very, very hard the forest might not be obliterated at quite the rate it is at the moment. If we work very, very hard maybe we’ll prevent the ocean temperature from rising the dreaded two degrees.’

Give the natural world half a chance, he has always maintained, and it will bounce back. ‘But we’re doing our damnedest to prevent that now.’

The Great Barrier Reef starts on December 30, at 9pm on BBC One