In times of national crisis people naturally turn to authority figures for solutions, which is why recently Sir David Attenborough is being asked about the weather. He's being asked about it a lot. "This preoccupation with the weather is an English disease," he says. "We are always talking about the weather."

Sir David believes the washout summer may be down to climate change. As a credible explanation he points to research by the University of Sheffield which suggests melting Arctic ice has slowed the jet stream, causing it to break into loops which have ushered to the UK unseasonably cold and wet weather systems. And he is convinced humans are the main cause of this.

"There is no question that climate change is happening; the only arguable point is what part humans are playing in it," he says. "I would be absolutely astounded if population growth and industrialisation and all the stuff we are pumping into the atmosphere hadn't changed the climatic balance. Of course it has. There is no valid argument for denial."

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Over the 60 years Sir David has been a broadcaster, he has seen the planet change at a staggering rate. Wildlife paradises he visited in his early career have been decimated and he views the future with pessimism. "I'm not optimistic," he says. "The climate, the economic situation, rising birth rates; none of these things give me a lot of hope or reason to be optimistic."

The one ray of hope and possible solution Sir David does offer is a global slowdown in birth rate. At 86, he has become an unlikely poster boy for the population control movement.

"Population is one of my concerns. I'm not planning to contribute to it any more, but it is an interest."

During his lengthy career, the naturalist has watched humanity more than double from 2.5 billion in 1950 to nearly seven billion. He believes the profound effects of this rapid growth on humans and the environment are unsustainable and that the matter needs to be addressed urgently before nature takes its own action.

"We cannot continue to deny the problem. People have pushed aside the question of population sustainability and not considered it because it is too awkward, embarrassing and difficult. But we have to talk about it.

The only ray of hope I can see – and it's not much – is that wherever women are put in control of their lives, both politically and socially; where medical facilities allow them to deal with birth control and where their husbands allow them to make those decisions, birth rate falls. Women don't want to have 12 kids of whom nine will die."

He does not, however, advocate implementing population policies similar to China's controversial one child edict. "Draconian measures making it illegal to bear children with horrible punishments for infringement are not going to work. You have to convince the population that it is in their interests and make it possible for them to do something. The fact is, if we don't do something, nature will. "Quite simply, we will run out of food. People talk about doom-laden scenarios happening in the future: they are happening in Africa now. You can see it perfectly clearly. Periodic famines are due to too many people living on land that can't sustain them."

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For this reason, he explains, growing crops to create green biofuels is a waste of valuable resources. "Biofuels may be palliative in the short term in terms of greener energy. But in the long term we are going to run out of space to grow food, which is more important than finding alternative ways to power Rolls-Royces and superjets."

Few presenters can boast Sir David's breadth of experience. He is not keen on the phrase "national treasure", but it is an apt description of the man who began his career in 1952 as a producer in the factual department of the BBC's fledgling television department. In 1965 he became controller of BBC2 and commissioned The Old Grey Whistle Test and Monty Python's Flying Circus. His interest in natural history led him to resign the post to return to full-time programme making. His seminal Life series, which began with Life on Earth in 1979, set the benchmark for all other natural history TV documentaries.

He has produced 10 Queen's Christmas Broadcasts and has seen television evolve from black and white to colour, high definition and, most recently, – 3D a format in which his latest series, Kingdom of Plants, was shot. The three-part documentary, now available on 3D Blu-ray, uses the latest 3D technology and time-lapse filming to bring the plant world to life.

Sir David admits that 3D television has limitations. Cameras are cumbersome, and one of the reasons plants were chosen as subjects for the series was because "they can't run away when you lumber towards them with a camera".

Although he has a 3D set at home, he understands why the technology is not yet widely adopted. "There is no getting away from the fact that a lot of people don't like wearing the glasses," he says. "However, someone will think of a way to overcome this issue and when that happens it will be a great spur. Television is changing so rapidly now that in 10 or 15 years the type of television you watch will be unrecognisable. People will be seeing programmes in different forms on their iPads on their watches and on the inside of their eyeballs."

Despite his interest in broadcast technology, he is not on Facebook and has no time for tweeting. "I have enough to do in life," he laughs. "I am beavering away as hard as I can for most of my waking hours and the idea that I have to keep telling people what I am doing and catching up with what they are doing is absurd."

His next project will also be a 3D series produced by Atlantic and is set in the Galapagos Islands where Sir David was reunited with Lonesome George, the last known Pinta giant tortoise which died last month soon after the film crew left. "It was almost the last shot we got," says Sir David. "I crawled up alongside him and he looked at me. He was very old and creaky, just like me. I said a few words to him, he didn't reply. He was, in a scientific sense, already dead because a lonely male without a female has no future."

David, a grandfather, lost his wife, Jane, in 1997 and lives alone in the family home they shared in Richmond, Surrey. He admits he's in touch with his own mortality. "You can't ignore it when you have knees like mine."

He is bemused by the national debate over who will become his successor. "I read it was Chris Packham recently," he says. "I am surprised that people feel I need replacing because the number of programmes like Kingdom of Plants and the Life series where the presenter appears on screen for no more than 10 per cent of the time is limited. The style these days is presenter-led. I come from a period of history when the presenter seldomly appeared.

"I have nothing against that element of adventure, but if you want to know what controls the proportion of sex in crocodile eggs, you don't go to a chap who, when he sees a crocodile, spends his time jumping on it. It is a different programme to the type I make. If I had been starting out now, I would be behind the camera."

Attenborough on... Facebook

"I have enough to do in life. I am beavering away as hard as I can for most of my waking hours and the idea that I have to keep telling people what I am doing and catching up with what they are doing is absurd."

Attenborough on... his successor

" I read it was Chris Packham recently. I am surprised that people feel I need replacing because the style these days is presenter-led. I come from a period of history when the presenter seldomly appeared. I have nothing against that element of adventure, but if you want to know what controls the proportion of sex in crocodile eggs, you don't go to a chap who, when he sees a crocodile, spends his time jumping on it."

Attenborough on... climate change

"There is no question that climate change is happening; the only arguable point is what part humans are playing in it," he says. "Personally I would be absolutely astounded if population growth and industrialisation and all the stuff we are pumping into the atmosphere hadn't changed the climatic balance. Of course it has. There is no valid argument for denial."

Attenborough on... the late 'Lonesome George', the tortoise

"I crawled up alongside him and he looked at me. He was very old and creaky, just like me. I said a few words to him, he didn't reply. He was, in a scientific sense, already dead because a lonely male without a female has no future."