'Live the way you want to, but look after the natural world, and don't waste life' - David Attenborough

But where once such figures were held up as a sign of the dedication of the film-makers involved, now - in this era of acute climate consciousness - they raise a different question: is it worth racking up such a heavy toll in carbon to show us the damage humans are already doing to the world?

The 93-year-old broadcaster himself took two flights during the filming of Seven Worlds, One Planet, to Kenya and Iceland, but insists each was justified both in terms of carbon and cost.

"I don't think people should fly just for the hell of it," he says. "Apart from anything else, the BBC would be absurd to spend money on flights that are not essential or important."

When we meet in a bar adjoining a multiplex in central London following the premiere of the first episode of the new series, surrounding streets are under Extinction Rebellion blockade. Attenborough has arguably done more than anyone to push conservation to the top of the political agenda. Has he considered joining their ranks? "For young people, it's their world," he says, even though many of those being arrested boast the same silver head of hair. "Lots of them haven't got political power, so they're making their voices heard, and who can blame them? My role is a different role. I'm an old guy in his 90s. I can provide information about certain things, but they have their own voice and don't need to look at me."

But look to him they do. In recent years, Attenborough has gained messiah status among the young. Demand for tickets to an early screening of Seven Worlds, One Planet earlier this month officially outstripped that of Glastonbury. Earlier this year, at Glastonbury, Attenborough made a surprise appearance on stage to promote the new series and was cheered like a rock god. That he is still travelling at all is a sign of his remarkably robust health. He has outlived many of those closest to him: his beloved wife and mother of his two children, Jane, who died of a brain haemorrhage in 1997, and brothers Richard and John.

He told me, in a previous interview to mark his 90th birthday, that he thinks about death every day and studies his own mental and physical decline with the keen interest of a naturalist. Now he jokes: "I don't celebrate my birthdays any more, but try and draw a veil over it instead."

As Attenborough cracks open a KitKat, his favourite treat, I ask what he makes of this hero status among the young.

"I am absolutely persuaded there has been a major change in the attitude worldwide to the natural world," he says. "These programmes get huge audiences and they are not just entertainment. It's stuff that is beautiful, lovely and enriching."

He believes young people are hardwired with an appreciation of nature that only becomes lost as we grow older. He recalls walking with his godson in the Oxfordshire countryside and seeing the youngster's delight at discovering a slug on a stone.

"He said 'What a treasure!' and, of course, he's right," Attenborough says. "If you lose that interest, you've lost one of the most precious things going. If you haven't got that as a continuing thread in your own mental life, you are much the poorer for it."

On stage, Attenborough has just been feted by various BBC bigwigs, including director-general Tony Hall, and answered questions via satellite link from children in India and South Africa. He was also asked by a five-year-old boy in the audience what he could do to save the world.

"You can do more and more the longer you live," he replied. "But the best motto to think of is don't waste things: paper, electricity, food. Live the way you want to, but look after the natural world and don't waste life."

Later, as if to illustrate the point, he gleefully shows me his mobile phone: a flipscreen model which must be near two decades old.

The film-makers insist they have taken great care to reduce their environmental impact. Jonny Keeling, executive producer on the series, says flights were kept to a minimum, with a local crew used on every continent except Antarctica - because nobody lives there. Instead, the crew left their equipment on the ice for more than a year to avoid extra return flights.

The series is also heavily reliant on aerial shots recorded by drones rather than helicopters, which Keeling says has resulted in a "huge amount of carbon saved". He adds they are also looking at offsetting the carbon generated during filming. The series, four years in the making, contains the array of natural wonders audiences have come to expect: from the largest gathering of great whales ever recorded to - Attenborough's favourite - the golden-haired snub-nosed monkey.

As with his recent blockbusters such as Blue Planet II, which prompted an outcry over the proliferation of plastic in the oceans, the damage humans are wreaking on the planet takes centre stage.

"The evidence is there now and the facts are absolutely clear," Attenborough says. "Putting aside whether mankind or not is responsible for this change, and I think it is, we know what we can do whatever the cause, and we know how we can stop the accumulation of carbon dioxide and we know that will have an important effect."

He doesn't name climate change denier-in-chief Donald Trump, but expresses "great sadness and sorrow" at the US refusing efforts at co-operation to set global targets to reduce emissions. He also names Australia, home to his beloved Great Barrier Reef, for falling short in its efforts.

Nobody could accuse Attenborough of doing the same. Beyond this series, he is committed to further projects spanning the next 18 months.

"When you are my age, 18 months is quite significant," he says. "If they [the BBC] had any sense, they wouldn't commit me to recording commentaries four years hence."

He insists, munching another KitKat, that he doesn't particularly watch his diet in order to stay young. In 2017, he said he had lost his appetite for meat, but he still eats fish.

Eventually, he suspects, it will be the voice that will give way, and a BBC executive will give him a tap on the shoulder. When the time comes, there will probably not be many volunteers.

In recent years, the timbre of his commentary has certainly changed, but it has gained a greater poignancy, too. Listen to old commentaries from previous decades and the voice today possesses an unsurprising fragility, one that matches the vulnerable beauty he remains intent on revealing to us.

"Our influence is everywhere," he says. "We had it in our hands and we made a tragic desperate mess of it so far. But at last humans are coming together and realising we are all on the same planet."

Seven Worlds, One Planet begins on BBC One tomorrow at 6.15pm