David Attenborough's latest TV series, Frozen Planet, is being heralded as his take on climate change. Now 85, he explains why - finally - he's speaking out on the issue, and shares the joys of a long life spent filming sex and death in the wild

'I'm not a propagandist, I'm not a polemicist; my primary interest is just looking at and trying to understand how animals work," says David Attenborough. We are talking in a gigantic BBC sitting room. Attenborough, wearing slacks, shirt and jacket, is a trifle unkempt at 85, but sharp as ever and kind, too, listening carefully as I ask what it felt like for the crew on his latest BBC series, Frozen Planet, to meet the Inuit people whose way of life is cracking up with the ice underneath them. I tell him I found this upsetting, but imagine he doesn't cry easily.

"No, I don't cry easily. Yes. [He pauses.] Yes, but there's inevitability about it. You can cry about death and very properly so, your own as well as anybody else's. But it's inevitable, so you'd better grapple with it and cope and be aware that not only is it inevitable, but it has always been inevitable, if you see what I mean."

The series, which is stunning, and took four years to make, has been heralded as Attenborough's take on climate change – though for most of it he is the narrator rather than author-presenter. But while it might look like a political statement for the BBC to invest a vast sum in seven hours of TV about the Arctic and Antarctic – for what better way to get people worrying about polar bears and ice caps than to send Attenborough to the north pole, for the first time in his life, in his 80s? – he insists that reporting on climate change was not the main purpose.

"These are fascinating, low-temperature ecosystems with wonderful, amazing things to be discovered. So most of the series is about that, but if you're going do it as thoroughly as we hoped to, you have to at some stage address the question of whether or not we are damaging it, or it's disappearing or changing. I won't say that we were reluctant to do so because that wouldn't be true, but it was not the prime motive."

And so, in the final programme, Attenborough appears on location, talking to camera in his own measured words about shrinking glaciers, warming oceans, and the threat posed by man-made global warming. "The polar bear is the easy one, it's a very charismatic animal that people can identify with," he says. "It's beautiful, and also savage; it's got a lot going for it. But it's only a white grizzly bear, really. All these big issues need a mascot and that's what the polar bear is. But climate change is going to affect us much more profoundly than the loss of the polar bear."

When the broader picture is so dark, how do they strike the right balance between comedy and tragedy, sex and death? These are seven films about survival, and there are some gruesome moments. In one extraordinarily powerful sequence filmed from land and helicopter in the Canadian Arctic, a wolf chases down a bison several times her size and scraps with it, one on one, until the poor bison lies down to die, exhausted. In another sequence, polar bears are filmed mating in what they think is a secluded spot.

"If you cut out all the savagery and so on, then you turn it into a fairy story," Attenborough says. "I mustn't sound too highfalutin, but that's a problem anybody has with a novel. How do you deal with the sex scenes without being lurid? So yes, you have that problem, and you have it in a vivid and obvious way. People who accuse us of putting in too much violence, [should see] what we leave on the cutting-room floor. My conscience troubles me more about reducing the pain and savagery that there is in the natural world than the reverse."

Do the horrible scenes stay with him?

"A lion ripping a gazelle fawn to pieces is not a pleasant sight, and the sound alone is awful. But I'm not haunted by it, no. I'm not haunted by anything much, I'm not a haunted kind of bloke. I don't haunt easy."

Though he has ruffled a few feathers recently with his advocacy of human population control, Attenborough remains one of the most popular men in Britain, causing even the least patriotic to reflect that there is still something appealing about a certain kind of Englishness. The sequence in Life on Earth in which he interacts with mountain gorillas remains one of the best-loved moments of television. He directed the Queen's speech, is used to meeting prime ministers (though cheekily "can't remember" if Blair is among them), and personifies a modest, no-nonsense, unflappable type often associated with the second world war.

He speaks of his work on Frozen Planet, and previous series in which he performed the same role, as a craft, "word carpentry", altering a first draft given to him by the director so that the final script will sound like his voice. "I'm an essential evil, the commentary is an essential evil," he says. "What you do is try to construct a sequence that tells a story with no words at all – none. So the art of commentary writing is not to say too many words. When I look back I always think there are too many words."

Attenborough is modest but knows his value, telling me the reason he has never appeared in an advertisement is because he knows his audience believes what he says. If he were to try to sell them something, for money, he would cheapen his brand.

And he does not see himself as an expert, brushing off his two-year Cambridge degree in natural sciences, shortened for national service ("I mean I'm uneducated, I was never a real scientist"), and saying he is "very flattered" when people categorise his programmes as scientific. "They're what 19th-century parsons did. They're watching dear little butterflies emerge from the pupa and that sort of stuff. Obviously, I think that's quite important so I don't want to talk it down too much, but it isn't profound; it isn't atomic physics."

Attenborough was a Midlands grammar school boy, and middle son of a college principal who rejected his Baptist background because of "profound family drama, involving pregnancy and so on. I have no knowledge of my Attenborough relatives at all."

From the start his style as a presenter combined something of the amateur enthusiast, brimming with boyish curiosity for his subject, with an utterly professional approach to broadcasting, and a technical virtuosity that has developed to the point where a big series like Frozen Planet would be incomplete without a raft of dramatically unprecedented footage.

"The filming of killer whales tipping ice floes and knocking seals off was an unbelievable achievement, unbelievable," he tells me with enormous pride. "Vanessa [Berlowitz, the series producer] devised this system in which you had a small inflatable boat and mounted it on a tripod with this extraordinary giro-controlled stabiliser that was originally used for helicopters. And this thing that Scott had put down as a rumour, that killer whales were threatening sailors, and that a lot of people had discounted, she for the first time worked out how it was done, and as a consequence she filmed it not just once but 22 times, and so you've got this sequence which is just amazing!"

But Attenborough's sense of himself as a non-expert, combined with the trust invested in him by his vast audience, has also held him back. It is the reason he was reluctant to speak publicly about climate change, although privately convinced of the evidence for 15 years. "I'm not a chemist, I don't know about the chemistry of the upper atmosphere, so that is why I kept out of the argument for as long as I did. But eventually enough people say two and two makes four for you to say yes, it's four."

The other reason was BBC impartiality, drummed into him as a young producer and reinforced during a stint running BBC2. He says it took him a long time to grow out of this, and as recently as a decade ago, when he delivered a piece-to-camera on Easter Island that warned of the grave dangers of resource depletion, he still "felt a little uneasy saying it, really. I remember brooding and worrying about it, I thought that's rather dangerous. [He guffaws]. But nobody noticed!"

The BBC has surely never played its cards better than by holding him as close as it has. He is unswervingly loyal, and one of the corporation's most precious resources. So has his salary kept pace? "I have no idea! I can tell you I don't get what Jonathan Ross gets. I don't know how much Stephen Fry gets for doing a programme about the brain."

When I suggest the world of natural history filming still looks very male, judging from the Freeze Frame shorts about the making of Frozen Planet that give viewers a chance to meet the crew and ogle the technology on which their licence fee is being spent, he swiftly disagrees. "No! Vanessa was one of the key people! The BBC in terms of women's employment has been far ahead. Every boss I had was a woman. It angers me when people think the BBC is male-dominated, it's not, and it never has been. The Madagascar series was almost entirely women, I think I was the only bloke there."

When we meet our photographer I am surprised to see a different side to his personality. He's more abrasive, though not unfriendly, perhaps more how he might behave with his crew. "The light is better up there? Baloney, absolute baloney." How was the north pole? "As the Arabs say about sand, once you've seen some of it, you've seen all of it."

He walks with a limp, but is in good health. His two brothers Richard (Lord Attenborough) and John are not so well, and his wife Jane died in 1997. But his daughter Susan keeps him company in Richmond, where he is in the process of turning a building that used to be a pub into an extension of his house. And he is busy, soon heading off to make another programme in Borneo.

He flirted with anthropology as a postgraduate, but couldn't stand the theoretical approach then in vogue. His son Robert became an anthropologist, and moved to Australia. Was he a hard act to follow, as a father? "Well, he hasn't followed my act at all. He is a very self-effacing academic, his talents which are considerable, aren't histrionic at all."

Robert has two daughters, both now in England, so it is to them Attenborough refers when he says: "If my grandchildren were to look at me and say, 'You were aware species were disappearing and you did nothing, you said nothing', that I think is culpable. I don't know how much more they expect me to be doing, I'd better ask them."

In Life on Earth, his first series that told the story of evolution in 12 hours of groundbreaking television, he referred to Darwin as being "enthralled almost to the point of ecstasy" by his discoveries. Does he recognise that feeling?

"Again, it's a bit highfalutin but there are occasions, yes. The process of making natural history films is to try to prevent the animal knowing you are there, so you get glimpses of a non-human world, and that is a transporting thing. A displaying blue bird of paradise is one of the most mind-blowing things you can imagine, but I suppose if I had to pick one I would say I remember getting up before dawn and going to a hide we had built by a billabong in northern Australia.

"Going there in the pitch dark and just watching dawn, watching the animals coming to this billabong in front of you, seeing the birds arrive and the kangaroos coming out and then seeing the crocs gliding across the top, and pythons snaking through the water and then these wonderful ibis and magpie geese and the sun coming up and the whole thing, I mean you suddenly saw a kind of prelapsarian, paradisical, Rousseauesque, Breughel-like world of the garden of Eden. Hmm ... "

Frozen Planet starts on Wednesday on BBC1 at 9pm